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User’s Manual
QMS 100 Series
Gas Analyzer
1290-D Reamwood Avenue
Sunnyvale, CA 94089 U.S.A.
Phone: (408) 744-9040, Fax: (408) 744-9049
Email: info@thinkSRS.com ▪ www.thinkSRS.com
Copyright © 2012
All Rights Reserved
Version 3.2 (1/2012)
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Certification
Stanford Research Systems certifies that this product met its published specifications at the time
of shipment.
Warranty
This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.
Service
For warranty service or repair, this product must be returned to a Stanford Research Systems
authorized service facility. Some components may be serviceable directly from the supplier.
Contact Stanford Research Systems or an authorized representative before returning this product
for repair.
Trademarks
Ultra-Torr® and VCR® are registered trademarks of Swagelok Co.
Tygon® is a registered trademark of Norton Co.
All other brand and product names mentioned herein are used for identification purposes only,
and are trademarks or registered trademarks of the respective holders.
Information in this document is subject to change without notice.
Copyright © Stanford Research Systems, Inc., 1996, 2000, 2006, 2012. All rights reserved.
Stanford Research Systems, Inc.
1290-D Reamwood Avenue
Sunnyvale, California 94089
408 744 9040
Printed in USA
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Table of Contents
Safety ........................................................................................................................................................................... iv
Symbols .........................................................................................................................................................................v
Fast Start ...................................................................................................................................................................... vi
Specifications.............................................................................................................................................................. vii
Materials List ............................................................................................................................................................... ix
Calibration Log..............................................................................................................................................................x
Chapter 1. Introduction & Operation.........................................................................................................................1–1
Introduction ....................................................................................................................................................1–2
What’s Inside ?...............................................................................................................................................1–4
Operation........................................................................................................................................................1–7
Mass Spectrometry Basics............................................................................................................................1–16
Chapter 2. Windows Software................................................................................................................................... 2-1
Overview ........................................................................................................................................................ 2-3
System Requirements ..................................................................................................................................... 2-4
Getting Started................................................................................................................................................ 2-4
Features and Operation................................................................................................................................... 2-6
RGA Head and Scan Parameters.................................................................................................................. 2-13
Display Modes.............................................................................................................................................. 2-15
General Utilities ........................................................................................................................................... 2-17
Head Calibration and Security...................................................................................................................... 2-20
RGA On-line Help ....................................................................................................................................... 2-23
Chapter 3. Measurement Techniques......................................................................................................................... 3-1
Calibration...................................................................................................................................................... 3-2
Techniques ................................................................................................................................................... 3-16
Glossary
References
SRS QMS Gas Analyzer
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iv
Safety
WARNING
Hazardous voltages, capable of causing injury or death, are present in this
instrument. Use extreme caution whenever the instrument cover is removed.
Always unplug the unit while removing the cover.
Line Voltage
The QMS system is specified for line power of either 110 V / 60 Hz or 220 V / 50 Hz. The
diaphragm pump will only operate on the specified voltage. Operating at other voltages will
damage the motor. For 110 V operation use one 3 A fuse. For 220 V operation, two 1½ A fuses
must be used in the power entry module.
Exhaust
As shipped, the QMS system exhausts to the atmosphere. If the system is analyzing hazardous
gases, the user must make provisions to handle the exhaust from the system. A standard 1/4 inch
connection is provided for this purpose.
Ventilation
The QMS system requires forced air cooling to operate at a reasonable
temperature. Do not block the air inlet or exhaust on the back of the unit.
Components will fail without this cooling.
Lifting
The QMS system is heavy; use care when lifting. Two people are recommended
for lifting the system. The handle is provided for occasional use and small
distances.
Elastomer Seals
Silicone has been reported to react adversely and irreversibly with the glass contained in an
electron multiplier. In systems containing an electron multiplier, do not use silicone greases or
oils on seals; use only hydrocarbon based materials.
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SRS QMS Gas Analyzer
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Fast Start
• Connect the power cord to the QMS. Set the four switches on the control panel to off and turn
on the main power switch.
• Turn on the diaphragm pump switch and the turbo pump switch. The pumps should begin the
startup sequence, which takes several minutes. When both green lights on the display are bright
the system is ready to operate.
• Connect the serial cable between the RGA and an available COM port on the computer (typically
COM2).
• Install the QMS software on the computer by inserting the first disk and executing the SETUP
program.
• Start the QMS program. Under the Utilities menu, choose “RS232 Setup...”. In the dialog box
that appears, choose the COM Port that the QMS is connected to and then press the “Connect”
button. After a short initialization, the QMS is ready. To confirm communications, under the
“Head” menu choose “Get Head Info...”. A box will appear showing information about the QMS.
• Click the filament button (
) on the toolbar to activate the ionizer. Click the GO button on the
tool bar and a scan will begin from 1 to 50 amu. The mass spectrum will show a rough
background spectrum.
• Click the filament button (
) on the toolbar to deactivate the ionizer.
• Loosen the inlet fitting and remove the plug. Attach the capillary inlet and tighten the fitting
(finger tight; no tools are required).
• Turn off the Turbo Pump switch and wait for the green light to extinguish. Next, set the
Capillary Flow Valve to open. With the flow started, the Turbo Pump switch can be turned back
on. Initially, the pump will not start and the light will blink. The pump will start as soon as the
pressure falls below 5 mbar. When the pump and capillary flow are started, the first three lights
will be on and bright. In the software, select the Utilities|Pressure Reduction menu item and enter
the Pressure Reduction Factor for the capillary. Check the box to enable the factor.
• Set the sample flow switch on the control panel to open. Restart the filament and perform a scan.
The spectrum of air will be displayed, which can be compared to the example below.
mBar
1.0x10
1.0x10
1.0x10
3
2
1
Air Scan
0
1.0x10
-1
1.0x10
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73
m/z
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vii
Specifications
Inlet
Type capillary: available in stainless steel, PEEK, and glass
lined plastic
Flowrate 1 to 10 milliliter per minute at atmospheric pressure
Response time <400 ms
Pressure selectable from 10 mbar to 1 bar
Mass Spectrometer
Type quadrupole
Detector Faraday cup standard
electron multiplier optional
Range 1 to 200 atomic mass units (amu)
Resolution <0.5 amu at 10% of peak height
Detection limit <10 ppm with Faraday cup detector
<1 ppm with electron multiplier
Operating pressure
5 × 10
-6
mbar
Connections
Inlet 1/4 inch Ultra-Torr
®
fitting
Exhaust 1/4 inch Swagelok tube adapter
Computer RS-232C, 28800 baud, DB9 connector
Power 3 pin grounded cable
System
Pumps hybrid turbomolecular/drag pump, 70 liter/s,
ultimate pressure 2 × 10
-9
mbar
diaphragm pump with ultimate pressure less than 1 mbar
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viii
protection class IP44
Materials
(see full materials list for details)
construction: SS304 and SS316
insulators: alumina, ceramic
seals: Viton, buna-N, and nitrile butyl rubber
misc: aluminum, Tygon
General
Startup time 2 minutes from full stop
Max. Ambient Operating
35 °C
Temperature
Power requirement either 110 V / 60 Hz or 220 V / 50 Hz (not field
selectable) less than 600 W total
Dimensions
44 cm H × 20 cm W × 61 cm D ( 17 in H × 8 in W ×
24 in D )
Weight 34 kg (75 lb. )
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ix
Materials List
SRS receives many requests for information about corrosion compatibility. It is our policy not to
state the compatibility of our system with various corrosive environments. We simply cannot
test the myriad combinations of environments that our customers use.
We do provide a list of all the materials exposed to the gas being introduced into the system.
Our expectation is that users who need to measure corrosive environments already have some
type of system that creates, handles and contains the corrosive gases. Given that they have
designed and operate said system, they are the best people to decide the compatibility of the
materials in our system with the specific corrosive environment.
The QMS system contains the following materials:
Body
• 304 stainless steel - high vacuum tube
• 316 stainless steel - quarter inch tube and fittings
• molybdenum - electrical feedthrough
• ceramic - electrical feedthrough
• AgCuIn - braze material on feedthroughs
• alumina - contained in the RGA
• aluminum - body of diaphragm pump
Replaceable Components
• glass - if an electron multiplier is installed in the RGA
• chromium - surface of the electron multiplier
• IrO
·ThO2 - filament of RGA
2
Seals
• copper - seals in the CF high vacuum flanges
• 316SS - major component of VCR
• silver - a thin layer on the VCR
• Viton - o-ring seal in the KF flange
• buna-N - seal in the high conductivity valve
• neoprene - diaphragms in diaphragm pump
• nitrile butyl rubber(NBR) - diaphragm pump valves - backing line
• Tygon
®
- connections to diaphragm pump (can be substituted)
®
seals
®
seals to prevent gauling
SRS QMS Gas Analyzer
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x
Calibration Log
SRS serial number ___________
In the table below are the results of the factory calibration of the inlet and capillary. The factor
is entered in the pressure reduction factor dialog box (under the Utilities menu) in the RGA
software. Although the RGA software will store the value for you, a written record is
recommended.
performed
by
FACTORY
capillary
length P
high side test gas factor
ID
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Chapter 1.
Introduction & Operation
In This Chapter
Introduction......................................................................................................................................................................1–2
External Connections................................................................................................................................................1–3
Operating Orientation...............................................................................................................................................1–4
What’s Inside ?................................................................................................................................................................1–4
Gas Handling Subsystem.........................................................................................................................................1–4
Mass Spectrometer ....................................................................................................................................................1–6
Operation ...........................................................................................................................................................................1–7
Front Panel Operation...............................................................................................................................................1–8
Continuous Sample Mode......................................................................................................................................1–10
Startup Just the Pumps.....................................................................................................................................1–10
Startup the Sample Flow ...................................................................................................................................1–10
Idle.......................................................................................................................................................................1–11
Shutdown ...........................................................................................................................................................1–11
Batch Analyze Mode..............................................................................................................................................1–12
Microcontroller Error Checks.................................................................................................................................1–13
Operating Modes of the Spectrometer.................................................................................................................1–14
The RGA as a Mass Spectrometer ..................................................................................................................1–14
The RGA as a Single Gas Monitor ..................................................................................................................1–15
The RGA as a Total Pressure Gauge..............................................................................................................1–15
Mass Spectrometry Basics ...........................................................................................................................................1–16
How Mass Spectra are Interpreted.......................................................................................................................1–16
Partial Pressure Measurement...............................................................................................................................1–17
Partial Pressure Sensitivity Factors ......................................................................................................................1–18
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1–2 Introduction
Introduction
The QMS 100 series instruments are modern mass
spectrometers designed for the analysis of light gases.
The three systems, 100, 200 and 300, differ only in
the mass range they can detect. A quadrupole mass
spectrometer performs the task of analyzing the gas.
The spectrometer operates at high vacuum and
therefore, pumps are required to draw the gas into the
instrument and maintain the vacuum. State-of-the-art
pumps are used that allow the entire instrument to be
contained in a small transportable package. The inlet
continuously samples gas at low flow rates (several
milliliters per minute) making the instrument ideal for
on-line analysis. The inlet can be equipped to sample
at pressures from above atmospheric to as low as 10
mbar. Not only is data acquired continuously (as
opposed to batch sampling employed by gas
chromatographs) but the response is fast. A change
in composition at the inlet can be detected in less than
1/2 second. The system allows data to be collected
quickly; a complete spectrum can be acquired in
under one minute and individual masses can be
measured at rates up to 25 ms per point. This
modern system allows many new applications where
the traditional mass spectrometer was too large and
heavy.
The QMS can be considered as two main
subsystems: gas handling and the analyzer. The
analyzer is the quadrupole mass spectrometer, which
can only operate in high vacuum. The class of
quadrupole mass spectrometer employed belongs to
a class referred to as residual gas analyzers (RGA).
These spectrometers specialize in large dynamic range
measurements of light gases. The gas handling system
consists of pump and valves that deliver the sample
Figure 1-1. Front Panel View
gas to the analyzer. These components are controlled
from the front panel. Using the QMS does not require a detailed understanding of the quadrupole or the
principles of the pressure reducing inlet. This manual (User’s Manual) discusses the aspects of the
instrument that are relevant to someone who only operates and acquires data with the QMS. The
instrument contains a microcontroller that assures correct operation of the pumps and valves. The
software controls the spectrometer and provides many data acquisition modes, which should fulfill the
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Introduction 1–3
needs of most users. For users with specialized needs, the QMS can be controlled from user programs.
The Technical Reference Manual discusses details of the QMS, its programming, and service.
External Connections
The instrument has two gas connections: an inlet on
the front panel and an exhaust on the back panel.
The inlet connection is a 1/4 inch Ultra-Torr port,
which is a high vacuum o-ring seal. The capillary is
installed in this port. The port is sealed and released
by turning the knurled nut. To release a capillary,
loosen the nut 1/2 turn. When installing a capillary,
press the end fitting on the capillary completely into
the port. Turn the nut finger tight to make the seal.
The o-ring seal relies on clean smooth surfaces and
precision machining to achieve the vacuum seal.
Excessive force or grease are not required nor
helpful. Keep the fittings free from dirt and dust and
the port will maintain a quality seal. The o-ring can be
inspected for damage by completely removing the nut
and ferrule.
The standard capillary shipped with each instrument
consists of a 0.005 inch bore PEEK capillary
designed to work at atmospheric pressure. The end
fitting which attaches to the instrument is a 1/16 to 1/4
tube adapter containing a graphite ferrule. The use of
graphite ferrule on capillaries is recommended.
Compared to metal ferrules, they do no distort the
tube and conform to irregularities in the outside
diameter of the capillary. The inlet end of the
capillary is fitted with a Luer taper fitting, which is the
standard used for syringes. The fitting is not
permanently attached at the factory. For long term
use, the capillary should be sealed into the fitting. A
recess in the back of the fitting is provided for sealant.
Many filters and needles are available that can attach
to the Luer taper. A 0.22 µm filter is provided with
the capillary and should be used when possible. The
Figure 1-2. Back Panel View
filter slows the response time of measurements
slightly, but protects against clogging and keeps particles out of the QMS.
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1–4 What’s Inside ?
The system exhausts to the rear panel. All of the gas drawn into the inlet is exhausted through this port.
The port is a 1/4 inch Swagelok tube stub, which can be connected to a wide variety of tube fittings.
When sampling hazardous gases, the user must ensure that the exhaust gas is properly handled. The
pump behind the exhaust port cannot produce significant pressure above atmospheric; therefore,
provide a low resistance path when connecting extra exhaust tubing.
Three electrical connections are located on the back panel: a computer connection, power line, and
chassis ground. The computer is connected from its serial port to the 9 pin connector on the back
panel. The communications between the instrument and computer uses full hardware handshaking and
thereby requires all 9 wires in the cable. Beware that some 9 to 25 pin serial port adapters do not
contain all 9 connections. A standard three wire power cord is connected to the module on the back
panel. The module also contains the fuse(s). To replace the fuse, first remove the power cord. Next,
place a small screwdriver in the slot on the top of the module and pry open the cover. The red fuse tray
can then be accessed to replace the fuse. The ground stud is provided to meet CE requirements.
Operating Orientation
The QMS can be operated either standing or laid on its side. The instrument utilizes a pressure gauge
that will not operate correctly when the system is tipped front to back. The pumps contain no oil or
other liquids and therefore gravity has no effect on them. If the system is to be laid on its side, the left
side (viewed from front) should be down. If the handle and rubber feet are removed, the system will fit
on a shelf in a standard 19” equipment rack. The system can be stored or transported in any
orientation.
What’s Inside ?
The instrument can be described in terms of two main subsystems: gas handling and mass spectrometer.
These two operate independently of each other. The gas handling subsystem draws in the sample gas
and transports it to the spectrometer at reduced pressure. The spectrometer analyzes the gas provided
to it. The spectrometer belongs to a specific class commonly referred to as residual gas analyzer or
RGA. In this manual, we use RGA to refer to just the analyzer and QMS to refer to the entire
instrument. The following two sections provide a basic description of these subsystems, refer to the
Technical Reference Manual for a detailed description. For the curious, the Technical Reference
contains an internal photograph in Chapter 1.
Gas Handling Subsystem
Figure 1-3 shows a schematic of the gas handling system. The discussion in this manual assumes that
inlet gas is sampled at 1 bar using the standard capillary supplied with each instrument. The system
reduces the pressure of the gas approximately 9 decades from the inlet at 1000 mbar to 10-6 mbar at
the RGA. This large reduction factor is accomplished in two stages. A flow of several milliliters per
minute (STP) is drawn into the capillary. The capillary bore diameter and length are such that the
pressure at the capillary exit is 1-4 mbar. The standard capillary has an inside diameter of 125 µm and
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What’s Inside ? 1–5
a length of 0.9 m. Most of the capillary flow does not travel to the RGA, but is bypassed directly to the
diaphragm pump. A small part of this flow is drawn through a small aperture (60 µm diameter) into the
chamber where the RGA is located. The RGA chamber is maintained at approximately 10-6 mbar by a
hybrid turbomolecular/drag pump. The flow through the RGA chamber is recombined with the flow that
was bypassed around the chamber. The recombination of the two flow streams is the key to the
compactness of the QMS instrument; without it, the system would require two pumps.
Figure 1-3. Schematic of gas flow.
The bypass flow configuration is also critical to the fast response time achieved by the QMS instrument.
A single stage pressure reduction with no bypass flow is possible using leak valves. These valves use
precision knife edges to create microscopic gaps. Such an inlet would draw gas at only 5 µl per minute
from atmospheric pressure into the RGA chamber. This flowrate is so small that the system would take
hours just to drain the volume in the body of the valve and result in an unusable response time. The
bypass flow configuration gives an exceptional response time of less than 0.2 seconds from the tip of the
capillary to the RGA chamber. The flexible capillary allows the inlet to be connected to remote
locations easily. Capillaries are available in a wide variety of materials and sizes for different
applications. The Technical Reference Manual contains guidelines for choosing the diameter and length
of custom capillaries.
Two solenoid valves control the gas flows. They can be set in four different combinations, and the
Operation section describes the use of each. The entire gas handling system is supervised by a
microcontroller, which assures that the valves and pumps are operated correctly.
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1–6 What’s Inside ?
Mass Spectrometer
A residual gas analyzer (RGA) is mass spectrometer of small physical dimensions whose function is to
analyze the gases inside the vacuum chamber. The principle of operation is the same for all instruments:
A small fraction of the gas molecules are ionized to create positive ions, and the resulting ions are
separated, detected and measured according to their molecular masses. RGA’s are widely used to
quickly identify the molecules present in a gas, and when properly calibrated, can be used to determine
the concentrations or partial pressures of the components of a gas mixture.
The SRS RGA is a mass spectrometer consisting of a quadrupole probe, and an electronics control unit
(ECU) which mounts directly on the probe’s flange, and contains all the electronics necessary to
operate the probe. The computer software communicates only with the spectrometer. The RGA has
no direct knowledge that it is present in a QMS system; it simply analyzes what is present at the ionizer.
Figure 1-4. Quadrupole Head Components
The probe consists of three parts: the ionizer (electron impact), quadrupole mass filter and ion detector.
All of these parts reside in the vacuum space where the gas analysis measurements are made. The
ionizer converts the sample gas into an ionized gas that is collected by electric fields into the filter. The
quadrupole mass filter contains a precisely controlled high frequency electric field that allows only a
narrow mass range to pass through. The ions that pass through the filter are measured by the detector,
which is either the standard Faraday cup, or the optional electron multiplier detector.
The ECU is a densely packed box of electronics that attaches directly to the probe’s feedthru-flange. It
includes several regulated power supplies, a microprocessor, control firmware, and a standard RS232
communications port. The ECU contains a unique, temperature-compensated, logarithmic picoammeter that can measure currents from 10
-15
to 10-7 A. This large dynamic range exceeds the physical
limitations of RGA’s, which can achieve six orders of magnitude dynamic range during partial pressure
measurements. The high gain of the electron multiplier allows partial pressures better than 10
be detected. RGA’s operate in the “constant resolution” or “constant ∆m ” mode, with ∆M
-13
mbar to
preset
10%
to one amu at the factory. The operating pressure range is from UHV to 10-4 mbar. The RGA ionizer
will shut down if the total pressure rises above this limit.
Intelligent firmware, built into the RGA Head, completely controls the operation of the instrument, and
provides three basic modes of operation of the mass spectrometer:
• Analog scanning
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Operation 1–7
• Histogram scanning
• Single mass measurement
RGA Windows provides fast access to all the RGA functions without the need for any computer
programming; however, the instrument can also be programmed directly using the RGA Command Set
supported by its serial interface. Consult the RGA Programming chapter of the Technical Reference for
information on programming and a complete listing of the RGA Command Set.
Operation
The QMS system is controlled with the four switches on the front panel and the one main power switch.
The main power switch activates the cooling fan and provides power to the system. When the main
power is on, the RGA is active and can be communicated with through the serial port.
The basic operation of the system is simple:
• Connect the inlet to the gas to be analyzed.
• Start the pumps and gas flow.
• Use the RGA Software to analyze the gas.
The following sections provide all the information to operate the QMS instrument and a primer on mass
spectrometry. Chapter 2 provides a general description of the software and Chapter 3 describes
specific techniques and procedures. Infrequently, the system will require tuning, which is discussed in a
later chapter.
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1–8 Operation
Front Panel Operation
The two pumps and two valves are operated with the four switches on the small panel (see Figure 5).
The Pressure bar display is the pressure at the inlet port on the instrument front panel. The Current
bar display is an indication of the current drawn by the turbo pump and is useful as a system diagnostic.
Each square in the display is 0.1 A; typically 2-3 bars will be lit. The fault light is an indication that the
turbo pump bearings are overheated; this is a severe problem and the system should be shutdown if it
ever lights (the turbo pump controller will automatically stop the pump).
A microcontroller supervises the system; it makes sure that the components are turned on in the correct
order and prevents improper states. The four switches are requests to actuate the associated
component. The controller decides whether to perform the request or wait. As an example, the
controller waits for the turbo pump to reach full speed before opening the sample valve. The controller
sets the light adjacent to each switch to indicate the status of each component:
BLINKING - The requested action is not allowed presently. The action may be
performed when other requests are completed or when other switches are changed.
DIM - The action is in progress, e.g. pump is reaching full speed.
STEADY - The action is complete: the valve is open, or pump is operating
The four switches can be set in 16 possible combinations; only 8 of these are allowed by the
microcontroller. If the user sets a switch in a disallowed arrangement, the light next to that switch will
blink. Figure 6 shows the allowed states and the names of each. The microcontroller further limits the
transitions between states. All states are related to adjacent states by the changing of one switch.
Delays and checks are performed by the microcontroller to assure the system operates properly. These
constraints cause the system to only start one component at a time, and they limit the rate of change.
SRS QMS Gas Analyzer
Figure 1-5. Front control panel.
Page 19
Controller State Diagram
is
Operation 1–9
2B
T urbo Pump
2
0
Off
1
Rou gh Pum p
3
Bypass Flow
-1
Vent
2A
T est Bypass
2
1
2
E rr or co n ditions which cau s e
a retreat to previou s s tate:
5
B atch Analyze
Figure 1-6. Controller state diagram. The gray boxes show which side of the switch is pressed; off is the left.
4
Sample Flow
1
T u rb o pum p is not at s pe ed
2
P res s u re
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1–10 Operation
The microcontroller is inactive for about 5 seconds after main power is turned on. The user cannot turn
on any of the system components during this period. If the user does turn on one of the front panel
switches during this period, it will be rejected and the adjacent light will blink. To recover from this
state, turn off all four switches and begin again. This restriction is related to power failures, which are
discussed later in this chapter.
The microcontroller was programmed to support two modes of sampling: continuous and batch.
Continuous mode is suited for samples of unlimited volume that change with time. Batch mode is for
small volume samples of fixed composition. The QMS system is strongly oriented towards the
continuous sampling mode, which makes good use of ability of the instrument to perform high speed
continuous measurements. For situations where this flowrate is unacceptable, the batch mode may be
useful. The next two sections describe how to setup the instrument to perform these two types of
sampling.
Continuous Sample Mode
Continuous sampling requires that the system be set to state 4 in the state diagram (pumps on and all
valves open). As the diagram shows, there are two main paths to get from Off to Sample Flow. The
preferred path follows the diagram through state 2A. The user should in sequence turn on all four
switches in the order Mechanical Pump, Capillary Flow Valve, Turbo Pump, and Sample Inlet Valve.
The user does not have to wait for each action to be completed; just toggle the switches in the order
you wish the actions to occur. The controller will perform the sequence, following the state diagram
through state 2A. When these actions are complete, the system will be sampling and ready to record
data. This is the main mode of operation. Other actions are described in the following sections.
Startup Just the Pumps
To prepare the system, but not draw any sample gas, the two pumps can be started. In this state the
RGA can be operated, although the mass spectrum will only show the chamber background. This state
is useful to prepare the software, test custom programs, and to idle the system (discussed further
below). Press just the Mechanical Pump and Turbo Pump switches, which will bring the system to state
2B in Figure 6. The microcontroller will start the diaphragm pump first. After a few seconds, the light
will become bright and the turbo pump will start. The turbo pump accelerates to full speed in under 2
minutes. During the acceleration of the pump, the current will reach full scale. Once it reaches full
speed, the light will become bright.
Startup the Sample Flow
To start the sample flow from state 2B, the two valves need to be opened. The user can toggle the
Capillary Flow Valve and then the Sample Inlet Valve switches to continue to state 4. At state 2B,
there is a volume of trapped gas that is at atmospheric pressure between the exit of the capillary and the
entrance of the capillary flow valve. This volume must be removed slowly. The solenoid valves are
either fully open or fully closed, so the microcontroller follows a pulse program to open the valve. The
valve will open momentarily and then close. This will be repeated as many times as necessary to start
the capillary flow without stopping the turbo pump. Capillaries with large volumes will require several
steps; this will be accompanied by much clicking, which is normal. Once the capillary flow is
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Operation 1–11
established, the sample valve will open, and measurements can be made. When the capillary flow valve
is first opened, a pressure pulse will occur in the system that will invariable shutoff the RGA filament. If
you where previously making measurements at state 2B, the RGA filament should be turned off while
starting the sample flow. Restart the RGA after the sample flow has been established.
An alternative route is to turn off the turbo pump and restart the system by the normal path
(0,1,2A,3,4). This avoids the valve pulsing discussed above and is slightly quicker. To use this path,
turn of the Turbo Pump switch and wait for the light to go off. Then (in this order) turn on the Capillary
Flow Valve, Turbo Pump, and Sample Inlet Valve. As with the previous method, turn off the RGA
filament during these steps.
Idle
When samples are not being measured, the system can be idled at two levels. Idle states minimize the
load on the pumps, thereby extending their life. The system also can pump out the background more
quickly at idle. These statements do not mean the system is not designed to operate continuously; the
system will run 24 hours a day.
For short periods, idle the system by closing the Sample Inlet Valve. This lowers the pressure in the
high vacuum area of the system and helps to keep the background minimized. Stopping the sample flow
when measurements are not being made is most important when gases containing water are being
measured. Water permeates into the walls of the vacuum chamber and is only removed slowly. The
less exposure to water, the quicker the background will be lowered.
For long periods, idle the system by closing both valves. With the capillary flow closed, the work load
of the diaphragm pump is at its lowest, which extends the life of the diaphragms. The turbo pump can
also more effectively pump the high vacuum area with both flows stopped.
Shutdown
The diaphragm pump should not be stored under vacuum for long periods. Because of this, different
procedures are recommended when the system is shutdown for short or long periods. For short
periods simply turn off the main power. For shutdown periods longer than about 30 minutes, it is
advisable to vent the system. There are several acceptable methods for venting. All are accomplished
by opening the Capillary Flow Valve with all the other switches off (state -1 on the diagram):
• SLOW - While the turbo pump is still coasting to a stop, open the capillary flow valve with the
capillary still connected to the inlet. The system will be slowly vented, and the turbo will stop in
a few minutes. Once the pressure gauge shows atmospheric pressure, the system is vented.
• FAST - Wait for the turbo pump to coast to a stop. Remove the capillary and open the
capillary flow valve. Useful when filling the system with a dry gas for storage.
• FASTEST - When the turbo pump is still coasting to a stop, remove the capillary and open the
capillary flow valve. This instantly raises the system pressure to atmospheric. Unlike older
devices, the turbo pump will not be destroyed. Still, this is the least recommended path.
NOTE: Do not vent the system to pressures above atmospheric. If the capillary is
connected to a gas at pressures above 1 bar, monitor the pressure during venting. Stop
just as the pressure reaches atmospheric.
SRS QMS Gas Analyzer
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1–12 Operation
If the diaphragm pump is inadvertently stored under vacuum for extended periods, the internal pressures
will reach a state that prevents the pump from starting. If this has happened the user will hear a relay
click but the diaphragm pump will not start. This locked state is cured by venting the system; the
diaphragm pump will then start up. Storage under vacuum also temporarily degrades the ultimate
pressure of the diaphragm pump. Once the pump is operating, the performance will return over several
hours.
Overpressure Protection
The RGA cannot drive the ionizer filament if the pressure in the chamber is too high. When the RGA
senses high pressure (via the power required to drive the filament), it will shut off the filament. This is a
protection measure, and should not be relied on to turn off the filament. When shutting down the
system, turn off the QMS filament before the turbo pump is turned off.
Batch Analyze Mode
This state is an special mode designed for advanced users and requires constructing a custom inlet
apparatus. You can ignore this section and mode, unless you specifically require the batch feature. This
mode requires that the inlet pressure of the QMS is less than 5 mbar. If this condition is satisfied, the
microcontroller will allow the Sample Inlet Valve to be opened. At state 5, there is no bypass flow and
a small flow is draw from the sample. Using this mode requires that the capillary be replaced with a
custom apparatus (provided by user) which allows the sample to be admitted into a small reservoir that
will hold the sample during measurement. After the sample is admitted at atmospheric pressure, the
reservoir is closed off and pumped down to less than 5 mbar. An external pump can perform this, or it
is possible to use the QMS itself to pump the reservoir, by momentarily using state 3 to pump down to
vacuum and then changing back to state 2B. When designing experiments with this mode, users need to
be aware that the internal volume of the QMS is about 50 ml (volume between two valves and inlet
port). Flush cycles will be required to remove gas from previous samples.
SRS QMS Gas Analyzer
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Operation 1–13
Microcontroller Error Checks
The microcontroller is programmed to prevent the system from restarting after a power failure. If the
Mechanical Pump switch is turned on before line power is applied, the controller will halt and prevent
further action from taking place. It will stay halted until the Mechanical Pump switch is turned off, which
resets the system. When the system is halted after a power failure, you will see that the diaphragm
pump switch is on and the light is flashing. Under normal circumstances, the diaphragm pump switch is
never rejected, and therefore the light would be either dim or steady. Only a power failure can cause
the diaphragm pump switch to be rejected.
The microcontroller performs several error checks to assure correct operation:
• From 2B→3: The capillary flow will not be started until the turbo pump is at speed. If the
pressure at the inlet to the QMS is above 5 mbar, the microcontroller opens the capillary flow in
short pulses. The valve is pulsed repeatedly until the pressure is below the setpoint.
• At 3: If the turbo pump begins to slow down, it will be turned off and the system will retreat to
state 2A. The likely cause of a sudden slowdown of the pump is excessive pressure. The
system will wait at 2A until the pressure is okay and then try to restart the turbo pump.
• At 3: If the pressure is above 5 mbar, the capillary flow valve is shut off and the system retreats
to state 2A. It will stay there until the pressure is satisfactory.
• At 4 or 5: If the intermediate pressure exceeds 5 mbar, the sample valve will be closed. This
action prevents excessive pressures in the RGA chamber.
The microcontroller does not time out nor latch error conditions. This means that if the switches are set
to sample gas (State 4), the microcontroller will continuously try to reach that state.
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1–14 Operation
Operating Modes of the Spectrometer
The RGA is a mass spectrometer that analyzes residual gases by ionizing some of the gas molecules
(positive ions), separating the resulting ions according to their respective masses and measuring the ion
currents at each mass. Partial pressure measurements are determined with the help of previously
calculated sensitivity (i.e. calibration) factors by reference to the abundance of the individual mass
numbers attributed to each gas type.
During analysis, positive ions are formed within the ionizer and directed towards the spectrometer’s
quadrupole mass-filter. The mass filter determines which ions reach the detector at any given time. It
is operated by a combination of RF and DC voltages and the filtering action is based on the mass-tocharge dependency of the ion trajectories on the RF and DC fields. The magnitude and frequency of the
RF determine the mass-to-charge ratio of the ions that can pass through the filter without striking the
rods (i.e. with stable oscillations). The RF/DC ratio determines the filter selectivity. Ions that successfully
pass through the filter are focused towards the detector and the resulting analog current is measured by
the very sensitive electrometer.
A brief note on Mass Units in Mass Spectrometry:
Since molecules are so small, it is convenient to define a special type of mass units to
express the masses of individual ions. The atomic mass unit, amu, defined as 1/12 of
the mass of a single carbon atom, isotope 12 (i.e. 12C), is the unit of molecular mass
most commonly used in mass spectrometry (1 amu = 1.660 540 x 10
-27
kg). To a very
accurate approximation, the mass of a molecule in atomic mass units (amu) is equal to
its mass number M, defined as the sum of the number of protons and neutrons in the
molecule.
Mass spectrometers do not actually measure the molecular mass directly, but
rather the mass-to-charge ratio of the ions. The mass-to-charge ratio, M/Q, is
defined as the ratio of the mass number M of the ion to its charge Q, measured in units
of the electron charge e-. For example: doubly charged ions of argon isotope 36
(36Ar2+) and singly charged ions of water, 1H
16O1+
, have M/Q = 18, and cannot be
2
differentiated from each other with most mass spectrometers.
For singly charged ions, the mass to charge ratio is numerically equal to the mass of the
ion in atomic mass units (amu).
RGA users often use the term “mass of an ion” when they really mean the
mass-to-charge ratio. This convenient way of speaking is strictly valid for singly
charged ions only.
The RGA as a Mass Spectrometer
The RGA can perform both analog and histogram scans over its entire mass range. Residual gas analysis
relies on the interpretation of the spectral data generated by these two modes to completely
characterize, both qualitatively and quantitatively, a vacuum environment.
SRS QMS Gas Analyzer
Page 25
Operation 1–15
RGA Windows uses the two modes to generate the data for the Analog and Histogram Scan Modes.
Analog scanning is the most basic operation of the RGA as a quadrupole mass spectrometer. During
analog scanning the quadrupole mass spectrometer is stepped at fixed mass increments through a prespecified mass-range. The ion current is measured after each mass-increment step and transmitted to the
host computer over RS232. Analog scanning allows the detection of fractional masses and provides the
only direct view of the peak shapes and resolution of the instrument.
A Histogram (Bar Mode) Scan consists of a succession of individual peak-height measurements over
a pre-specified mass range. A single value is used to represent the peak heights at each integer mass
within the range. The peak height measurements are made with the Peak-locking scanning procedure
described in the next section. Histogram scanning is one of the most commonly used modes of operation
for the RGA. Its two main advantages are a faster scan rate than analog scans, and a reduced amount of
data being exchanged during the scan.
The RGA as a Single Gas Monitor
The RGA can measure individual peak heights at any integer mass within its mass range.
This mode of operation is used to generate data for leak testing measurements, and to track changes in
the concentrations of several different components of a mixture as a function of time. The outputs
provided by a set of single mass measurements are often used in process control programs to control
alarms, analog and digital outputs, and relays.
RGA Windows uses this mode to generate its data for the Table, Pressure vs. time, Annunciator and
Leak Detection modes.
Peak Locking procedure: During a Single Mass Measurement the RGA performs a Miniscan around
the mass requested, and the maximum current value measured is sent out over RS232. The
scanning procedure, referred to as Peak-Locking, is designed to measure peak currents for individual
masses in a mass spectrum without being affected by drifts in the mass-axis calibration. The Miniscan
covers a 0.6 amu range centered at the mass requested, and selects the maximum current from 7
individual measurements performed at 0.1 amu mass increments.
The RGA as a Total Pressure Gauge
The RGA can measure total pressures.
The RGA might be thought of as a Total Pressure Ionization gauge with a mass analyzer interposed
between the ionizer and the detector. Thus, by disabling the mass-filtering action of the analyzer section,
it is possible to detect the total ion current from the ionizer and perform total pressure measurements. A
total pressure sensitivity factor, stored in the non-volatile memory of the RGA, is used by RGA
Windows to convert the total current measurements into total pressures.
Important: The sensitivity factor for the total pressure measurements is highly mass dependent. Some
additional mass discrimination takes place in the filter that results in the mass dependence of the RGA
readings being different from that of the Bayard Alpert gauges. Expect to see deviations between the
two gauges as the composition of the residual gas changes.
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1–16 Mass Spectrometry Basics
Mass Spectrometry Basics
The RGA can perform both qualitative and quantitative analysis of the gases in a vacuum system.
Obtaining spectra with the RGA is very simple. Interpreting the spectra, that is, understanding what the
spectra is trying to tell you about your vacuum system requires some work. The following sections will
introduce some basic concepts of Spectral Analysis emphasizing the main aspects of Residual Gas
Analysis. For additional information on the subject of Residual Gas Analysis refer to:
J. Drinkwine and D. Lichtman, Partial Pressure Analyzers and Analysis, AVS
Monograph Series published by the Education Committee of the American Vacuum
Society
Basford et. al., J. Vac. Sci. Technol. A 11(3) (1993) A22-40 “Recommended
Practice for the Calibration of Mass Spectrometers for Partial Pressure Analysis.
Update to AVS Standard 2.3”.
For information on multiple linear regression analysis consult:
William H. Press, et. al., 1992, Numerical Recipes in C, The Art of Scientific
Computing, Second Edition, Cambridge Univ. Press, section 15.4, page 671.
Bevington, P.R., 1969, Data Reduction and Error Analysis for the Physical Sciences,
New York, McGraw-Hill, Chapters 8-9.
How Mass Spectra are Interpreted
A mass spectrum, taken in a real system, will almost always contain signals from a mixture of various
gases. Careful and complete interpretation of the spectrum (i.e. a complete spectral analysis) should
reveal the identity, as well as the concentrations, of the various components which have produced the
spectrum.
The first step in the spectral analysis process is to correctly identify the mass-to-charge ratio of all the
peaks in the mass spectrum. A well calibrated mass scale is essential to this task. See the RGA Tuning
Chapter for a detailed description of the mass scale calibration procedure.
Once all the peaks have been labeled, the next step is to identify the residual gases that have produced
the spectrum. A knowledge of the recent history of your system may provide very valuable clues as to
the possible gases that may be residuals in the vacuum chamber. A familiarity with the standard spectra
of commonly expected gases will generally help to determine the major and minor components in the
system. Any peak in the spectrum may consist of contributions from molecular ions and/or fragment
ions, or multiply ionized species. The qualitative spectral analysis is completed when all the
peaks in the spectrum have been “uniquely assigned” to the components of a gas mixture, in
complete agreement with the known fragmentation patterns of the components.
In cases where only the major components are of interest, some of the minor peaks of the spectrum will
remain unassigned. If only a few species are being monitored, only the peaks corresponding to the
substances of interest need to be assigned and monitored.
SRS QMS Gas Analyzer
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Mass Spectrometry Basics 1–17
Notes on Fragmentation Patterns: The electron impact type of ionizer used in modern RGA’s almost
always causes more than one kind of ion to be produced from a single type of gas molecule. Multiple
ionization, molecular fragmentation and changes in the isotopic composition of the molecule are
responsible for the effect. All ions formed contribute to the mass spectrum of the molecule and define its
fragmentation pattern. The identification and interpretation of mass spectra must begin with a
knowledge and understanding of the standard fragment patterns of atoms and molecules that may exist
in the system. The standard fragment patterns of most molecules commonly encountered in residual gas
analysis are well established and listed in the general RGA Literature. A very complete library can also
be accessed through the Library Search Utility of the RGA Windows software. The Gas Library has a
standard text file format, and can easily be read, extended or modified by the user to fit his individual
needs.
Partial Pressure Measurement
Once the different components of a mixture have been identified it is possible to use the RGA to obtain
quantitative values for the various partial pressures. This section describes the basic steps needed to
perform quantitative measurements with the instrument. The formalism presented assumes multiple gas
analysis, but is equally valid for single gas measurements. Please consult the suggested references for
details and examples of these procedures.
The entire mathematical formalism used to derive the partial pressures of a mixture based on a single
mass spectrum is based on one assumption:
The total spectrum is a linear combination of the spectra of the different species that are present in the
mixture. In other words, the total spectrum is equal to the sum of the individual peaks that would be
observed if each constituent were alone in the system.
In mathematical terms, the assumption stated above can be written as the following linear equation:
where:
HM = Σg h
Mg
(1)
g is an integer variable that represents the gases present (i.e. assign an integer to each gas starting with
one)
M is an integer variable that represents the mass numbers for the entire mass range of the spectrum.
H
= total peak height (amps) of the spectrum at mass number M .
M
h
= peak height contribution (amps) from gas g at mass M.
Mg
h
is related to the fragmentation pattern, the RGA’s sensitivity and the partial pressure of gas g by the
Mg
equation:
hMg = αMg Sg P
where:
α
= Fragmentation factor of gas g at mass M: Ratio of ion signal at mass M to the ion signal at the
Mg
principal mass peak for gas g.
g
(2)
SRS QMS Gas Analyzer
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1–18 Mass Spectrometry Basics
S
= RGA’s partial pressure sensitivity factor for gas g, in amp/Torr (see Partial Pressure Sensitivity
g
Factor below)
P
= Partial pressure of gas g in the system.
g
Equations (1) and (2) are combined to obtain the system of equations:
HM = Σg (Sg αMg) P
g
(3)
Since all gases have more than one peak in their fragmentation pattern, the number of peaks (M) in a
real spectrum is generally larger than the number of gases (g). As a result, the system of equations (3)
usually has more equations than unknowns. This situation is sometimes simplified eliminating some of the
extra equations; however, the best results are obtained using all the equations and a multiple linear
regression procedure to calculate the best possible fit to the data.
Obviously, accurate results can only be obtained if the constants αMg and S
are well known for the
g
RGA being used.
Note: The Analyze Utility of RGA Windows uses a multiple linear regression algorithm, as mentioned
above, to automatically calculate the composition of a “typical residual gas environment” at the end of
any 1-50 amu spectral scan. Please see the RGA On-Line Help files for details.
Standard fragmentation patterns (for example, the fragmentation patterns included in the RGA Library
of RGA Windows) can be used as a source of α
values in moderately quantitative determinations.
Mg
However, when very precise numbers are desired, one should obtain the appropriate fragment patterns
by introducing pure gas into the RGA being used. The fragment patterns must be obtained under the
same conditions that will be used during regular spectral analysis since they depend on many
instrumental parameters, including: electron energy, emission current, ionizer design, mass filter settings,
detector type, multiplier gain, etc. The principal mass peak of a fragmentation pattern is simply the most
intense peak of the spectrum, and the intensity of all the other peaks in the pattern are normalized to its
height for the calculation of fragmentation factors. Note that by our definition the αα
value for the
Mg
principal mass peak of any gas is equal to one. Principal mass peaks are used in the calculation of
the sensitivity of the RGA to different gases as shown below.
Partial Pressure Sensitivity Factors
The partial pressure sensitivity of the RGA to a gas g, Sg, is defined as the ratio of the
change (H-H
) in principal mass peak height to the corresponding change (P-P0) in
0
total pressure due to a change in partial pressure of the particular gas species. H0 and
P
are background values.
0
Sg = (H-H0) / (P-P0)
The units of Sg are of ion current per unit pressure (amp/Torr, for example).
SRS QMS Gas Analyzer
Page 29
Mass Spectrometry Basics 1–19
The sensitivity of the RGA varies with different gases, changes with time due to aging of the head, and is
a strong function of the operating conditions of the instrument. Careful quantitative analysis requires that
the sensitivity factor, Sg, be determined for every gas which may be a component gas in the system
being analyzed. The sensitivity factors must be obtained under the same operating conditions that will be
used during general partial pressure analysis since they depend on many instrumental parameters,
including: ionization energy, emission current, mass filter setting, type of detector, etc.
In order to separate the gain of the electron multiplier from the intrinsic sensitivity of the RGA head, the
sensitivity factors of the RGA are defined for Faraday Cup detection. A separate Electron Multiplier
Gain Factor, is used to correct the ion signals when the electron multiplier is turned on. See the
Sensitivity and Electron Multiplier Tuning sections of the RGA Tuning Chapter for details.
The basic procedure for determining the sensitivity of a particular gas in the RGA is the following:
• Introduce the pure gas into the vacuum system, at a known or calculable pressure (typically
around 10-6 Torr).
• Measure the output signal from the RGA for the principal mass peak of that gas using the
Faraday cup detector.
• The ratio of this output signal to the pressure of the gas is the sensitivity factor, S
.
g
During these measurements it is very important to insure that the partial pressures of all other gases in
the system are small enough so that they may be neglected. The sensitivity factors calculated can only be
applied to situations where the RGA is used with the same operating parameters. See the Sensitivity
Tuning section in the RGA Tuning chapter of this manual for more details on this calibration procedure.
A total pressure sensitivity factor is also needed by the RGA to convert the ion currents obtained
during total pressure measurements into total pressures. Total pressure sensitivity factors vary with
different gases and share many of the properties of the partial pressure factors. They are determined by
a procedure identical to the one described above, but with the partial pressure measurements replaced
by total pressure measurements.
The underlying assumption when using sensitivity factors in quantitative calculations is that there is a
linear relation between the partial pressure and the corresponding RGA signals of the gases. Deviations
from linearity are to be expected above 10-5 Torr due to space charge effects in the ionizer and ionneutral scattering interactions in the filter. A more thorough check of the RGA’s sensitivity involves
measuring the RGA signals over several orders of magnitude of partial pressure to determine the range
over which a linear relationship exists. The sensitivity factor for the gas is calculated as the slope of the
“signal vs. partial pressure” response over the linear range.
RGA Windows uses two sensitivity factors stored in the non-volatile memory of the RGA Head. The
sensitivity factors, one for total pressure and one for partial pressure, are used as conversion factors
between the ion currents received form the head and the pressure units selected by the user. The
sensitivity factors are measured with the Faraday Cup detector and can be updated or changed very
easily by using the Sensitivity Tuning command in the Head menu. A separate Electron Multiplier Gain
Factor, stored in the non-volatile memory of the RGA Head, is used to correct the ion signals for the
gain of the electron multiplier. The gain of the electron multiplier is highly mass dependent and defined
relative to the corresponding FC signal. An automatic Electron Multiplier Gain Adjustment command,
SRS QMS Gas Analyzer
Page 30
1–20 Mass Spectrometry Basics
built into the program, can adjust the CDEM voltage for any gain between 10 and 106. Consult the
RGA On-Line Help Files for details on the automated tuning procedures built into the RGA Windows
program. Also see the Sensitivity and Electron Multiplier Tuning sections of the RGA Tuning Chapter
for more general information.
The Table mode of RGA Windows offers scaling factors for all of its channels eliminating the limitations
imposed by the single sensitivity factor on multiple partial pressure calculations. For example, the scaling
factors can be used to display correct partial pressure for all the species in a table if the ratios between
the partial pressure sensitivities of the different components are known and only principal mass peaks
are used to monitor them. The scaling factors can also be adjusted to correct against the mass
discrimination of the electron multiplier’s gain.
Important: Following current industry standards, the partial pressure sensitivity factor
stored at the factory corresponds to N2 measured at 28 amu with ∆m
=1 amu,
10%
default ionizer settings and Faraday Cup Detection.
Single gas measurement example:
Monitoring the concentrations of one or few components in a system is easy in the absence of severe
spectral interference.
Suppose a system where argon is measured at 40 amu (principal mass), in the absence of any other
gases that contribute a signal at that mass value. The sensitivity to argon was previously measured at
S
=10-4 amps/Torr, and the electron multiplier is biased and its gain at mass 40 was previously
Ar
measured at g
=1.02.103 relative to the FC signal.
CDEM
The partial pressure of argon, PAr, is easily calculated measuring the intensity (i.e. peak height) of the
ion current at mass 40, I40.
P
The peak value, I
= I40 / (g
Ar
, can be extracted from a spectral scan or measured directly using the single mass
40
. SAr), units of Torr (4)
CDEM
measurement mode of the RGA. For example, a 10-9 amp peak value corresponds to 9.8 × 10-9 Torr of
Ar. Note that equation (4) is a particular case of equation (3), and that the fragmentation factor for the
principal peak of Ar is one by definition.
SRS QMS Gas Analyzer
Page 31
Chapter 2. Windows Software
The QMS system contains a mass spectrometer that belongs to a class of spectrometers
commonly referred to as residual gas analyzers or RGA. RGAs are low resolution, low mass
range, quadrupole spectrometers. The SRS spectrometer is used both as a stand-alone RGA and
in the PPR and QMS systems. The same software package is used to operate all three
instruments. The description in this chapter refers to the spectrometer as RGA.
More information about the RGA program can be found in the On-Line Help files provided on
the program CD. The RGA help system includes current and detailed description of all the
features, procedures, and commands available in the program.
In This Chapter
Overview ........................................................................................................................................................ 2-3
Program Structure..................................................................................................................................... 2-3
RGA Files................................................................................................................................................. 2-3
RGA files (.rga) .................................................................................................................................. 2-3
RGA Scan log files (.ana .hst .tbl) ......................................................................................................2-3
RGA ASCII Data files (.txt) ............................................................................................................... 2-4
RGA Graph Metafiles (.wmf) ............................................................................................................. 2-4
System Requirements ..................................................................................................................................... 2-4
Getting Started................................................................................................................................................ 2-4
Starting the RGA Software....................................................................................................................... 2-5
Connecting to a Head ............................................................................................................................... 2-5
Shutting Down the RGA System.............................................................................................................. 2-5
Features and Operation................................................................................................................................... 2-6
The RGA Window.................................................................................................................................... 2-6
Multiple Head Operation .......................................................................................................................... 2-6
Display Modes.......................................................................................................................................... 2-6
Analog (Mode Menu) ......................................................................................................................... 2-7
Histogram (Mode Menu) .................................................................................................................... 2-7
Table (Mode Menu) ............................................................................................................................ 2-7
P vs. T (Mode Menu).......................................................................................................................... 2-8
Leak Test (Mode Menu) ..................................................................................................................... 2-9
Annunciator (Mode Menu) ................................................................................................................. 2-9
Library (Mode Menu) ....................................................................................................................... 2-10
Data Acquisition ..................................................................................................................................... 2-10
Scan Data Logging ................................................................................................................................. 2-11
Graph Management ................................................................................................................................2-11
Head Management.................................................................................................................................. 2-12
Sensitivity Factors .................................................................................................................................. 2-12
Spectrum Analysis.................................................................................................................................. 2-12
Background Data .................................................................................................................................... 2-13
RGA Head and Scan Parameters.................................................................................................................. 2-13
Changing Scanning Parameters .............................................................................................................. 2-13
Changing Head Parameters..................................................................................................................... 2-14
Changing Scan Trigger Rates ................................................................................................................. 2-14
Scanning With The Filament Off ........................................................................................................... 2-15
Display Modes.............................................................................................................................................. 2-15
Changing Display Modes ....................................................................................................................... 2-16
SRS QMS Gas Analyzer
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2-2 Overview
Running in Split Display Mode...............................................................................................................2-16
Manual Scaling of Graphs.......................................................................................................................2-16
Using Scan Data as Background.............................................................................................................2-16
General Utilities............................................................................................................................................2-17
Using the Data Cursors ...........................................................................................................................2-17
Scheduled Saving of Data .......................................................................................................................2-18
Logging Scans ...................................................................................................................................2-18
Viewing Scans...................................................................................................................................2-18
Browsing Through the Gas Library ........................................................................................................2-19
Analyzing the Mass Spectrum ................................................................................................................2-19
Spectrum Analysis description..........................................................................................................2-19
Analysis Procedure............................................................................................................................2-20
Pressure Reduction..................................................................................................................................2-20
Head Calibration and Security......................................................................................................................2-20
Tuning the RGA Sensitivity....................................................................................................................2-20
Adjusting the CEM Gain.........................................................................................................................2-21
Peak Tuning the RGA Head....................................................................................................................2-22
Securing the RGA Head..........................................................................................................................2-23
RGA On-line Help........................................................................................................................................2-23
Help Search.............................................................................................................................................2-23
Help Index...............................................................................................................................................2-23
Commonly Asked Questions...................................................................................................................2-24
Sample Scans ..........................................................................................................................................2-24
SRS QMS Gas Analyzer
Page 33
Overview 2-3
Overview
Program Structure
The RGA program is a fully interactive Windows program capable of managing several RGA
Heads simultaneously. Fully interactive means that you can double-click on any graph object and
the program responds by executing a specific command such as editing the color of a data line.
RGA was designed to handle data acquisition from multiple heads simultaneously by assigning
one head for each window and by making all the windows independent from each other. Each
window can be thought of as a separate RGA Head control & display file. This file, called an
RGA file, stores all the information regarding the graph parameters, scan parameters, some head
parameters, what port the head was connected to, etc.
It is not necessary for an RGA file window to be connected to a specific head at all times. You
could, for example, open a file just to browse through a few dozen scans that were done
overnight. You can also copy the graph to the clipboard so you can paste it into a word
processor.
Once an RGA window is connected to a Head, you can start scanning (acquiring data) in any of
the display modes available. When you are done scanning, you can either close the file which
will automatically disconnect you from the head, or you can issue a disconnect command and
keep the file open for further data analysis. You can even connect the same window to a different
head on another RS232 port.
RGA Files
The RGA program is capable of saving files in a variety of formats. This section describes all the
file formats and their different uses.
RGA files (.rga)
RGA files are the main files used in the RGA program. They are binary files that contain scan
data with all the information on the graph parameters, scan parameters, scan schedule
parameters, etc. When the RGA file is saved, it saves the last scan data along with all the setup
parameters of that window. When the file is open again, it creates a window that has the same
parameters as when it was saved including the data from the last scan.
RGA Scan log files (.ana .hst .tbl)
RGA scan log files are binary files that contain mostly raw data from multiple scans acquired
over a period of time. These files do not have any of the RGA file setup parameters in them in
order to save disk space. They contain the time and date for each scan and the minimum scan
setup information to reconstruct the scan data graph. These log files are available for the Analog,
Histogram, and Table modes.
SRS QMS Gas Analyzer
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2-4 System Requirements
RGA ASCII Data files (.txt)
The RGA program can save the last scan data in an ASCII format that is easily read by
spreadsheet programs for data analysis. The file header contains the scan setup information
followed by the scan data. The RGA program does not read these ASCII file, it only writes them.
RGA Graph Metafiles (.wmf)
RGA can also save the active graph as a Windows Metafile. Windows Metafiles are easily read
by many word processing, page layout, and graphic programs. The graphs can be easily saved by
RGA and then recalled by any of these programs for documentation and presentations.
System Requirements
The RGA program system requirements vary depending on the performance required of the
RGA system and on how many heads are connected and run simultaneously. The RGA Head can
scan data at different scan speeds that effect noise floor and averaging. If you choose to scan at
the fastest speed and want the RGA program to "keep up" with the head, then a faster computer,
i/o card, and graphics card might be required.
Following, is the minimum recommended system to run a SINGLE head system:
• IBM PC compatible machine with 64 Mbytes of RAM
• Mouse or equivalent pointing device
• Serial port and DB-9 to DB-9 RS232 cable, or serial to USB adapter if your
computer does not have a serial port
• 100 Mbytes of free hard disk spaces (for RGA installation and runtime use)
and CD drive (for the installation disk)
• Super VGA graphics card running in 800x600, 1024x768, 1280x1024,
1600x1280 with minimum 256 color mode
• A sound card if the audio features are needed
• Microsoft Windows 98 or later
Getting Started
The following sections describe how to launch the RGA program and start acquiring data from
the RGA Head.
Note: For detailed information and command description of the RGA program please refer
to the RGA On-Line Help files provided on the program CD. The RGA help system
includes current and detailed description of all the features, procedures and commands
available in the program.
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Getting Started 2-5
Starting the RGA Software
To start the RGA software simply double-click on the RGA icon in the "SRS RGA" program
group created by the RGA installation program. You may also type the full path name of the
RGA program in the Run command from the Program manager.
The program also accepts a file on the command line. If a filename is specified, the program will
open that file at startup. You can open the program by double clicking on any file with the .rga
extension. You can open any .rga file from windows explorer by dragging it to the running
program.
You can only run one copy of RGA at the same time. If you attempt to start another copy of
RGA while it is already running, the existing copy is activated and brought to the front of the
screen.
Connecting to a Head
Connecting to an RGA Head requires an available RS232 port on your PC and a full DB9-DB9
RS232 cable. To connect to an RGA Head do the following:
1. Connect the RS232 cable to the DB9 connector on the Head labeled
"RS232/DCE/28.8k.Baud".
2. Connect the other end of the cable to the RS232 port on the PC side.
3. Turn On the RGA Head.
4. Start MS Windows and the RGA Program.
5. Select Connector List Setup from the Head menu.
6. Select the port to which the RGA head is connected.
7. Press the Connect button.
The connection is made when the connector icon turns green and the status of the instrument
shows string "connected". After you close the Connector List Dialog, the toolbar GO button
turns green, and the Scan and Head menu commands get enabled.
Shutting Down the RGA System
If the user simply exits the program, the RGA will be left in its current state. Disconnecting from
the RGA will also leave the RGA setup unchanged. This means that the filament and CEM can
remain on when the program is quit. This can be useful in some situations.
When the RGA is not in use, the filament and CEM should be turned off. This will extend their
life. To accomplish this remember to explicitly turn off the filament and CEM before leaving the
software. The entire ECU can be left on when the RGA is not in use. At idle the electronics draw
minimal power.
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2-6 Features and Operation
The recommended shut-down procedure is:
1. Stop the scan if there is one in progress using the Stop Now command in the
Scan menu.
2. Turn off the filament and CEM.
3. Save the RGA file you have been working on, using the Save or Save As
command in the File menu.
4. Terminate the RGA program using the Exit command in the File menu.
5. Turn off the RGA Head.
Features and Operation
The RGA Window
The RGA window represents one SRS RGA Head operating in a specific display and scan mode.
The RGA window does not need to be connected to a head at all times. It can be used to print
graphs or to review some old scan logs. You may size, tile, minimize, or cascade several RGA
windows together even while scanning.
When an RGA window is sized, all the graph objects in it are also sized down or up. For
optimum viewing of a graph, maximize the RGA program window, and maximize the RGA file
you wish to view.
Multiple Head Operation
The RGA program can monitor several SRS RGA Heads simultaneously. Each RGA window
represents a separate RGA Head that is controlled independently from any other window/head
combination.
To connect an RGA Head to an RGA window, use the RS232 Setup command to assign a port
(that you know is attached to an RGA Head) to that window. The operation of the RGA program
is unchanged whether you have one or several heads connected and scanning. The only
difference you might see is a slowdown in display update, and that is dependent on the computer
hardware you use.
Display Modes
There are seven distinct display modes in RGA along with split modes that consist of
combinations of these modes.
Following are the seven distinct modes:
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Features and Operation 2-7
Analog (Mode Menu)
Analog mode is the spectrum analysis mode common to all Residual Gas Analyzers. The XAxis represents the mass range chosen in the Mass Spec Parameters menu. The Y-Axis
represents the ion current amplitudes of every mass increment measured.
Select the Schedule menu to set the scan trigger timing. Once the scan is in progress, the
AutoScale menu may be used to scale the data.
The Analog Trace parameters such as color and width may be changed by either double clicking
on a data point directly (if data is present) or by using the Traces command from the Graph
menu.
In Analog mode the RGA Head scans from the start to the stop mass using the Points Per AMU
variable specified in the Scan Parameters dialog box (Scan drop-down menu or click the Mass
Spec Scan Parameters menu button on the toolbar). This Points Per AMU variable is used to
determine the mass increments between data points. The time to acquire each partial pressure
depends on the scan speed selected (the plotting speed depends on the user's computer).
The control view bar may be used to zoom, scroll and view all data, even if a scan in progress.
Histogram (Mode Menu)
Histogram mode displays the individual mass amplitudes for the selected scan range. In this
mode the RGA head performs a peak-lock for each mass and calculates one amplitude per mass.
This peak is then plotted as a bar at the appropriate mass. The X-Axis represents the mass range
chosen in the Scan Parameters menu. The Y-Axis represents the ion current amplitudes of every
mass measured. A mass index indicator is located at the left of the X-Axis. The index displays
the mass that is currently being updated (assuming a live scan is in progress).
Use the Schedule command from the Scan menu to set the scan trigger timing. Once the scan is
in progress, the AutoScale command (Graph menu) menu may be used to scale the data.
The Histogram Traces (bars) colors may be changed by either double clicking on the bars or
selecting the Traces command from the Graph menu.
In Histogram mode the RGA Head scans from the start to the stop mass running 7 measurements
per AMU. After performing a peak lock on each 7 points it then sends the maximum value to the
RGA program. This results in one point per mass being plotted as a bar graph. The time to
acquire each partial pressure depends on the scan speed selected (the plotting speed depends on
the user's computer).
The control view bar may be used to zoom, scroll and view all data, even if a scan in progress.
Table (Mode Menu)
Table mode presents a tabular form readout of preselected gases along with alarm level
warnings. The gas masses, names, and parameters can be set using the Scan Parameters
command (Scan menu). The scan parameter dialog also can be evoked by clicking on the Mass
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2-8 Features and Operation
Spec Scan Parameters menu button on the toolbar
Table entries can be configured independently from each other. Some entries can use the
Channel Electron Multiplier (CEM), while others can have different scan speeds with the CEM
off.
The alarm control and level settings can be edited by either double- clicking on the Alarm text
of the desired table entry, or by clicking on the 'Alarm X' (where X is the channel number)
button for the appropriate table entry in the Table Parameters dialog box.
Select the Schedule command (Scan menu) to set the scan trigger timing and frequency. The
Table Traces may be edited by either double-clicking on the text directly (except for the Alarm
text), or by selecting the Traces command (Graph menu).
Important: Table mode, P vs. T mode, and Annunciator mode share the same table scan
parameters, alarm parameters, and graph trace colors.
The data acquisition method for the table scan will vary depending on the display mode selected:
In Table mode or Table mode split with P vs. T mode, each table entry value (partial
pressure) is acquired directly from the RGA head by individually querying the partial
pressure for the appropriate mass. This is done for all the selected masses using the
present scan schedule as a trigger. The time to acquire each partial pressure depends on
the scan speed selected for that mass (the plotting speed depends on the user's
computer).
In Table mode split with Analog or Histogram mode, the Table entry values are
extracted from the Analog or Histogram spectrum. No individual mass query is
performed. If a Table mass lies outside the Analog or Histogram mass range, its partial
pressure will show a zero value with the alarm indicating the appropriate zero value
warning. A table entry can be easily disabled using the Table Parameters dialog box.
P vs. T (Mode Menu)
Pressure versus Time mode is a scroll graph of up to ten gas masses in the same plot. The graph
scrolls to the left as the data fills the screen and the old data is saved in a history buffer for
review at any time. The gas masses, names, and parameters can be set using the Scan Parameters
command from the Scan menu.
P vs. T entries can be configured independently from each other. Some entries can use the
Channel Electron Multiplier (CEM) while others can have different scan speeds with the CEM
off. When this mode is split with Table mode, the result is a wealth of information on the
selected gases behavior. The Schedule command (Scan menu) may be used to set the scan trigger
timing and frequency. The P vs. T Traces may be edited by either double clicking on them
directly, or by selecting the Traces menu.
The control view bar may be used to zoom, scroll and view all data, even if a scan in progress.
Note that the Table mode, P vs. T mode, and Annunciator modes share the same table scan
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Features and Operation 2-9
parameters, alarm parameters, and graph trace colors.
The data acquisition method for the P vs. T scan will vary depending on the display mode
selected:
In P vs. T mode or Table mode split with P vs. T mode, each table entry value (partial
pressure) is acquired directly from the RGA head by individually querying the partial
pressure for the appropriate mass. This is done for all the selected masses using the
present scan schedule as a trigger.
In P vs. T mode split with Analog or Histogram mode, the P vs. T entry values are
extracted from the Analog or Histogram spectrum. No individual mass query is
performed. If a P vs. T mass lies outside the Analog or Histogram mass range, its partial
pressure will show a zero value. A table entry can be easily disabled using the Table
Parameters dialog box.
Leak Test (Mode Menu)
Leak Test mode provides the most effective way to study the behavior of a single gas. This mode
provides a scroll graph that monitors the gas trend over a period of time, an instantaneous partial
pressure readout, a bar meter, and an Alarm message. An Audible beep with a pitch proportional
to the partial pressure is also available if a sound card is installed. If the beep is disabled, an
Audio message can be enabled to reflect the status of the Alarm levels.
The gas mass, name, and scan parameters may be configured from the Scan Parameters
command (Scan menu). The Leak Trace, bar meter color, and text readout color can be edited by
double clicking on them directly (if data is present) or by selecting the Traces command form the
Graph menu. Only one color can be selected for all data objects. The Schedule command (Scan
menu) may be used to set the scan trigger frequency. The scan parameter dialog also can be
evoked by clicking on the tool button in the toolbar.
In Leak Test mode, the partial pressure for the specified gas is acquired by querying the RGA
head for that specific mass only. The frequency of data acquisition is set in the Schedule setup.
The time to acquire each partial pressure depends on the scan speed selected (the plotting speed
depends on the user's computer). The Leak Test display mode cannot be split with other modes.
The control view bar may be used to zoom, scroll and view all data, even if a scan in progress.
Annunciator (Mode Menu)
Annunciator mode provides an effective way to visually monitor gas warning levels from a
distance. The graph is composed of large green panels each representing a gas and alarm level.
The partial pressure value is clearly visible and so is the gas name. When an Alarm level is
reached, the panel color changes to bright red, and the appropriate alarm message is shown. The
gas masses, names, and parameters can be set using the Scan Parameters command from the
Scan menu.
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2-10 Features and Operation
The Annunciator channels can be independently configured. Some channels can use the Channel
Electron Multiplier (CEM) while others can have different scan speed with the CEM off.
The alarm control and level settings can be edited by either double clicking on the Alarm text of
the desired Annunciator entry, or by clicking on the 'Alarm X' (where X is the channel number)
button for the Annunciator channel in the Table Parameters dialog box.
The Schedule command (Scan menu) may be used to set the scan trigger frequency.
The Annunciator display mode cannot be split with other modes.
Important:
Table mode, P vs. T mode, and Annunciator mode share the same table scan parameters, alarm
parameters, and graph trace colors.
In Annunciator mode each channel value (partial pressure) is acquired directly from the RGA
head by individually querying the partial pressure for the appropriate mass. This is done for all
the selected masses using the present scan schedule as a trigger.
Library (Mode Menu)
Library mode displays the selected library gas fragment patterns in a histogram graph. When
split with a histogram or analog graph, the mass range of the library will automatically match
that graph's range for easy comparison of gases. All library information is read from the
"gaslib.dat ASCII file located in the same directory as "rga.exe". This file may be edited (with a
text editor) to add, delete, or modify gases in the library. The exact library file format must be
preserved for proper operation of this mode.
The Library Traces (bars) colors my be changed by either double clicking on the bars directly or
by using the Traces command (Graph menu).
The library scan parameters (displayed mass range) may be changed using the Library
Parameters command in the Scan menu. The cursor tool (Utility menu) can be used for the
analog, histogram, PVST, leak and library modes.
Data Acquisition
The terms Data Acquisition and Scanning are used interchangeably in the SRS RGA
documentation. A scan implies that the SRS RGA Head is performing ion current readings and
sending the data over the RS232 cable to the PC running the RGA program at a fixed rate of
28.8 kBaud. This data is graphed using the currently active display mode chosen by the user. The
speed of graphing depends on the hardware used and can vary with types of CPUs, video cards,
Serial ports etc.
The main procedure for establishing a live scan is outlined below:
1. Turn on the SRS RGA Head.
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Features and Operation 2-11
2. Establish a connection between the RGA program and the Head (Connector
List Setup in the Head menu, or Connector List button on the toolbar).
3. Select the desired display mode (Mode Menu).
4. Turn On the Filament (Head menu or Filament button).
5. Select the desired scan parameters (Scan menu).
6. Select the desired trigger rate (Scan menu).
7. Select the Start scan command (Scan menu or Start button).
Scan Data Logging
Scan Data logging (or scan logging ) is used for saving raw scan data to disk without the
overhead that comes with saving an RGA Head file.
Scan logging continuously saves scan data along with the time and date, with only the essential
information about each scan. The user can easily browse through the scans after the scan logging
is complete using the Next Item and Previous Item commands from the View menu.
Scan logging is implemented for all scanning modes. To save the data for the modes with time
axis, simply select the Save command from the File menu. Scan logging is enabled by default to
insure no data is every lost. Every scan mode has logging options. They can be changed buy
selecting Scan Logging in the File menu. Spectrum Analysis can also be logged.
Graph Management
RGA is a fully interactive Windows program that allows the user to directly manipulate
graphical objects on the screen by double-clicking on them with a point and click device such as
a mouse or track ball.
Almost every graph object in every display mode may be edited by double clicking on it. The
extent of the editing depends on the actual object. For example you may change the text color,
font, and size of any graph title but you may only change the color of the bar graphs in
Histogram mode. Some graph objects can be customized. Those objects can be relocated on
the screen with a mouse by left clicking and dragging a cursor. Some of them also can be
resized.
Every graph object that can be edited by double-clicking on it may also be edited by using an
equivalent command from the Graph menu.
All graph parameters relating to graph objects are saved when the Save or Save As commands
are used form the File menu.
Graph object parameters are local variables to the presently active display mode only. For
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2-12 Features and Operation
example, if the background mode in Analog mode is changed to yellow, the background mode of
all the other modes remains unchanged. Also, any new file using the Analog mode still uses the
default background color.
After an RGA file has been edited to have a desired look, it may be used as a template for new
files by clearing its data using the Clear Graph Data command, saving it using the Save As
command, and opening it again using the Open command.
Head Management
The SRS RGA Head has nonvolatile memory that stores most head variables such as sensitivity
factors, Channel Electron Multiplier gain, high voltage, calibration factors, etc. When the RGA
program connects to a head, all these variables are read from the head and stored temporarily in
the program for use in the active file. When a head parameter is edited using the command in the
Head menu, that parameter is stored back in the head immediately.
Saving RGA files does not save any of the head parameters to that file since each RGA Head has
unique parameters due to minor variations in electronic and physical properties. Head parameters
cannot be changed unless an SRS RGA Head is connected and turned on.
Protecting Your RGA
A Maintenance Schedule menu item lets you protect your RGA by automatically turning the
filament or CEM off if the RGA is left to idle for a selected time.
Sensitivity Factors
The RGA Head uses two sensitivity factors stored in its non-volatile memory. The sensitivity
factors, one for total pressure and one for partial pressure, are used as conversion factors
between the ion currents received form the head and the pressure units selected by the user.
The sensitivity factors can be changed using the Sensitivity Tuning command in the Head menu.
In order to set these factors, you need a reference ion gauge installed in the same vacuum system.
The reading from the gauge is used to set the new sensitivity factor for the RGA Head.
Note: The pressure limits you may enter will change if you have the Pressure Reduction option
enabled. If this option is enabled RGA lets you enter much higher pressure values.
Spectrum Analysis
Mass spectrum analysis takes you one step beyond visual analysis and Library lookup tables.
RGA looks at the spectrum from 1 to 50 amu and uses a matrix inversion technique to analyze
the composition of the residual gas and approximate its composition. Note that Analysis mode
requires a minimum span of 1 to 50 amu. The span can be higher than 50 amu. Analysis data can
also be logged by clicking the Enable Analyze Logging box in the Analyze Settings selection in
the Utilities menu.
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RGA Head and Scan Parameters 2-13
Background Data
This mode is helpful in providing the user with a clean baseline after the background data gets
subtracted from newly acquired scans. This utility is available in Analog mode, Histogram mode,
Table mode, and P vs. T mode. In Analog mode and Histogram mode the scan must be allowed
to finish at the Stop mass before the data can be used as background. Use the Stop at End
command from the Scan menu (if in continuous scan mode) to guarantee this condition.
In Analog and Histogram modes the full spectrum is subtracted from the newly acquired
spectrum. In Table and P vs. T modes, only the last acquired partial pressure of each mass
get used as a background data. When a Table or P vs. T mode is split with Analog or
Histogram mode AND background is enabled, the data in the Table or P vs. T graph is
extracted from the Analog or Histogram graph and NOT subtracted form its own previous
data.
RGA Head and Scan Parameters
The following topics describe how a user might change any of the RGA Head or scan
parameters. For specific command information please use the On-Line Help for the RGA
Program.
Note: For detailed information and command description of the RGA program please refer
to the RGA On-Line Help files provided on the program CD. The RGA help system
includes a detailed description of all the features, procedures and commands available in
the program.
Changing Scanning Parameters
Every display mode has scan parameters that are used to setup the display and to acquire data
form the RGA head. Some modes such as the P vs. T, Table, and Annunciator modes share the
same scan parameters. For example, if a scan parameter is changed in table mode, this change
will be reflected in the P vs. T and annunciator mode when that mode is activated.
It is not necessary to be connected to an RGA Head to modify the scan parameters. The head will
be updated when a connection is established.
To change the scan parameters do the following:
1. Select the desired display mode from the Mode menu.
2. Select the scan parameters command for the active display from the Scan
menu. The scan parameter dialog also can be evoked by clicking on the tool
button in the toolbar.
If the user accepts the new parameters, the display is recreated using the new parameters and, if
an RGA Head is connected, its parameters get updated also.
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2-14 RGA Head and Scan Parameters
Note: Changing scan parameters will result in loss of all displayed data on
the screen. Use the File menu to save the data in one of the formats available
before changing any scan parameters.
Changing Head Parameters
The head parameters menu items are available only when there is an SRS RGA Head connected
and turned on. The head parameters are variables that depend on the actual RGA Head and
reside in non-volatile memory in the RGA Head circuitry. These parameters are read from the
Head when a connection is established and vary from one Head to another due to physical
properties, electronic properties, and aging of the unit.
To change or view any of the Head parameters, do the following:
1. Connect to a head using the Connector List setup from the Head menu, or by
clicking on the Connector List button on the toolbar.
2. Select an item from the Head menu.
If the operation is not canceled, the new parameters are stored in the Head and are used in
subsequent operations.
Changing Scan Trigger Rates
The scan trigger rate (schedule) determines the frequency with which a scan is repeated. Not all
display modes have the same schedule options. When changing trigger rates keep in mind the
following:
• When a graph with a time axis is active, only timer triggered schedule is
allowed.
• When a split mode is active, the graph with the more restrictive schedule is
used for triggering scans.
• If a selected trigger period is smaller than the actual period of the scan, RGA
will change the trigger period to the minimum value.
To change the scan schedule for a mode do the following:
1. Connect to an RGA Head.
2. Select the desired display mode.
3. Select the Schedule command from the Scan menu.
4. Select the trigger mode or rate you want.
IMPORTANT:
Since Windows can be running one of many applications at once, it is possible for the RGA
program to miss a scan trigger while other applications are busy performing other tasks such as
printing or file copying. These facts should be taken into account if optimum performance is
desired when using Timer Triggered modes and display modes that use a time axis.
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Display Modes 2-15
Scanning With The Filament Off
The software can run experiments with the filament off. This is useful only to researchers who
perform experiments that generate their own source of ions.
The program will warn you if you start a scan without the filament turned on. This is done to
prevent casual use and accidents. However advanced users who are going to use this mode
frequently can disable the warning in the ionizer settings for a particular Rga Head, or just check
the box "never ask me again" in the warning dialog.
Note: Using this mode presents some risk to the RGA hardware
With the filament off, the RGA has no method to detect total pressure. If the CEM is on, the
RGA will not be able to detect a high pressure condition and will not automatically turn off the
CEM. Without this protection, it is possible for the pressure to reach a level where a sustained
electrical discharge will occur from the CEM to the surrounding wall.
Warning: The CEM will be destroyed by such a discharge.
The most likely situation during which this can occur is when shutting down a vacuum system.
As the chamber is slowly vented, the pressure will reach the discharge region. Do not leave the
RGA with the CEM on when a system is being shut down. Better yet, do not leave the RGA on
at all when the vacuum chamber is being shut down.
This can also occur when power failures trip out the turbo pump of the vacuum system. As the
pump coasts to a stop, the pressure rises to the discharge region. If you are going to use the RGA
unattended with the CEM on, consider using an interlock to shut off power to the RGA if the
turbo pump is not running. Many pump controllers have an outlet that is only powered when the
pump is running. Plugging the RGA into this outlet would protect against this failure mode.
Note, with the filament off, the RGA will appear dead to a user who is expecting to run a normal
experiment. The only indication that the filament is off, will be a lack of signal.
Because of these concerns, the "Filament On" mode is always initiated by default. Only a small
number of users need this ability to operate without the filament on, and they must be fully
aware that it is being enabled.
Display Modes
Note: For detailed information and command description of the RGA program please refer
to the RGA On-Line Help files provided on the program CD. The RGA help system
includes detailed description of all the features, procedures and commands available in the
program.
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2-16 Display Modes
Changing Display Modes
A display mode presents the user with a specific way to analyze the RGA data acquired. The
RGA program has several display modes including a combination of those modes (split modes).
To change the present display mode to any other mode do the following:
1. Stop the scan if one is in progress.
2. Select the desired display from the Mode menu or click on one of the mode
buttons in the Toolbar.
Running in Split Display Mode
Running in split display mode allows you to view scan data in two different formats or to scan
two different sections of the mass spectrum with different scan parameter settings.
The following is a list of all the split display modes available:
• Analog split with: Analog, Table, P vs. T, and Library
• Histogram split with: Histogram, Table, P vs. T, and Library
• Table split with : Analog, Histogram, P vs. T, and Library
• P vs. T split with : Analog, Histogram, Table, and Library
You cannot split with Leak or Annunciator modes
To split a graph do the following:
1. Make sure you are running in any of the Analog, Histogram, Table, or P vs. T
single graph modes.
2. Select any of the enabled graph modes from the Split Submenu of the Mode
menu
Manual Scaling of Graphs
Manual scaling applies to graphs with an X-Axis, Y-Axis or Time-Axis. To manually scale a
graph do the following:
1. Select the X-Axis, Y-Axis or Time-Axis by double-clicking on it or selecting
X-Axis, Y-Axis or Time-Axis form the Graph menu.
2. Enter the new desired limits for the axis in the From and To parameters.
3. Select OK.
Using Scan Data as Background
This mode is helpful in providing the user with a clean baseline after the background data gets
subtracted from newly acquired scans. This utility is available in Analog mode, Histogram
mode, Table mode, and P vs. T mode.
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General Utilities 2-17
To Enable the Background mode make sure the graph has valid data and the RGA head is
connected. In Analog and Histogram mode the scan must be allowed to finish at the Stop mass
before the data can be used as background. Use the Stop at End command (if in continuous scan
mode) to guarantee this condition.
To Enable the Background:
1. If a scan is in progress, issue the Stop at End command from the Scan menu.
2. Select the Background Setup command from the Utilities menu.
3. Click on the Enable Background Mode check box to enable this mode.
4. Select the desired data subtraction mode.
5. Press OK.
The data gets cleared and the next scan will be the result of subtraction selected.
To Disable the Background:
1. Select the Background command from the Utilities menu.
2. Click on the Enable Background Mode check box to disable this utility.
3. Press OK.
When the background mode is disabled, the old background data reappears on the screen.
General Utilities
Note: For detailed information and command description of the RGA program please refer
to the RGA On-Line Help files provided on the program CD. The RGA help system
includes a detailed description of all the features, procedures and commands available in
the program.
Using the Data Cursors
A real-time graphical cursor is available in all scanning modes. The cursor is on by default in all
modes. When you select the cursor command (Utilities menu) in all modes except the Table or
Annunciator mode a cursor appears on the graph. The vertical cursor can be moved to any point
in the graph to identify the desired data value. In Analog or Histogram modes the cursor values
are displayed in the text box on the right top portion of the graph. The cursor text box is an
active graph object. Thus you can customize the position of the cursor box by selecting it and
dragging it, or you can double-click on it to change the font or the size. The X and Y values of
the cursor are updated every time the cursor is moved, or a new data value is acquired at the
current cursor position.
In the Leak test mode the X value of the cursor text is displayed in time format.
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2-18 General Utilities
In PvsT mode, the cursor values are displayed in the legend box. If the legend view is toggled off
you cannot see the cursor values. This off-option can be useful if the legend box obscures some
data points. You may move the cursor by either clicking on or near a data point of interest, or by
clicking and dragging with the mouse with the left mouse button held down. You also can move
the cursor incrementally by using the LEFT or RIGHT arrow keys.
In split mode, the cursor appears in both displays. You can move the cursor incrementally in the
top display by using the LEFT or RIGHT arrow keys and in the bottom display by holding the
SHIFT key down while using the LEFT or RIGHT arrow keys.
Scheduled Saving of Data
Logging Scans
Scan logging options are available for all scan modes. Scan logging is enabled by default. The
logging settings are saved when a user exits the program.
To log scans to disk do the following:
1. Establish a connection between the RGA program and a Head.
2. Select a scan mode.
3. Customize the logging by selecting the “Scan Logging” command from the
File menu or by clicking the “logging Folder” button in the toolbar. The
logging directory must be selected for every scan mode or you can use default
values assigned by the installation program. The log file name is created
automatically. This saves valuable time for a user who always uses scan
logging. If you don’t want to keep unnecessary data on your hard drive, you
can periodically clean up the log dirs. You can select two different formats for
the log file name. The first format file name is created by the concatenation of
the title of the current document and the file timestamp. The second format
file name is created by the concatenation of a user defined name and the file
timestamp. The settings are stored, even if a user quits the program. More
details can be found in the help menu.
4. Start a scan.
To disable scan logging, use the scan logging dialog box again.
Viewing Scans
To view a scan log, the active RGA display mode must be the same type of display mode that
was used to log that scan.
To view the stored scans on disk do the following:
1. Select the Open Scan Logs from the File menu.
2. Select the desired scan log file ( it must be the same type of scan as the
currently active one).
SRS QMS Gas Analyzer
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General Utilities 2-19
3. Use the Next Item or Previous Item from the View menu to view the
sequential logs (The time and date of the scan appears on each log).
Browsing Through the Gas Library
Library Browsing Description
There are several ways to browse through the gas library depending on the display mode and
whether the Library Search utility is active. The Library display mode and search utility are
linked together to provide an intuitive interface to locate and view any gas in the library file.
Library Display mode only:
1. Activate the Library mode by selecting the Library command from the Mode
menu.
2. Use the Next Item and Previous Item in the View menu to plot the next and
previous gases in the list (you may also use the left and right arrow keys).
Library Search utility only:
You may start this utility from any display mode
1. Start the search utility by selecting the Library Search command from the
Utilities menu.
2. Press the Show All button to list all available gases.
3. Use the scroll bar, and/or the keyboard to search the list or use the Search
Masses section to look for specific gases.
Library Display mode AND Search Utility:
The combination of these two modes provides a very quick way to look for gases.
1. Make sure both the Library mode and the Library Search utility are active (see
procedure above).
2. If you want to plot a gas, double click on its name in the Search utility dialog
box or click on it once (to highlight it) and press the plot button. The gas
fragment pattern will then appear as a histogram in the Library display.
Analyzing the Mass Spectrum
Spectrum Analysis description
This utility provides the user with immediate analysis of residual gas based on a complete
histogram or analog scan. Using a matrix inversion technique, the composition of the residual
gas is analyzed and the best approximation to its composition is given in either pressure units or
percentages. The user can enable up to 12 common gases for analysis.
Note: Analysis mode is only available in Analog mode or Histogram mode. The mass range must
be at least from 1 to 50 amu.
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2-20 Head Calibration and Security
Analysis Procedure
Make sure the RGA window is in either Analog mode or Histogram mode and connected to an
RGA head. Set the scan range to be from 1 amu to at least 50 amu.
1. Select the Analyze command from the Utilities menu to bring up the Spectrum
Analysis dialog box. You may place the dialog box anywhere on the screen.
2. Press the Setup button if you need to change the gas selection or the analysis
units.
3. Select the Start command form the Scan menu (if not already scanning) to
begin a scan.
4. When the scan is complete (the stop mass is reached) the analysis results will
be displayed in the Spectrum Analysis dialog box. The analysis will be
repeated every time the scan reaches the stop mass.
Note: You may enable or disable the analysis procedure even while the RGA Head is performing
a scan.
The “Analysis Settings“ option (Utilities menu) allows customization of the Analyze mode. The
list of 12 gases used in the Analyze mode can be changed interactively by the user directly from
the program. A user also can enable/disable the logging of analysis data to the disk.
Pressure Reduction
If you purchased a pressure reduction system from SRS this feature allows you to display true
pressure values using the pressure reduction factor that corresponds to your system. If pressure
reduction is enabled, all pressure readings are multiplied by the pressure reduction factor before
they are displayed.
IMPORTANT: Entering the wrong pressure reduction factor will result in erroneous
measurements being displayed.
Head Calibration and Security
Note: For detailed information and command description of the RGA program please refer
to the RGA On-Line Help files provided on the program CD. The RGA help system
includes current and detailed description of all the features, procedures and commands
available in the program.
Tuning the RGA Sensitivity
WARNING! The sensitivity tuning procedure should be performed by qualified personnel
only. A mistuned RGA Head could give erroneous readings until it is retuned properly.
SRS QMS Gas Analyzer
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Head Calibration and Security 2-21
Please refer to the RGA Tuning Chapter for more information about tuning and
calibration.
While performing the tuning procedure the total pressure in the vacuum chamber should
be around 10E-6 Torr.
In order to set the sensitivity factors of the RGA Head you must have a reference pressure gauge
installed on your vacuum system. There are two sensitivity factors, one for total pressure and one
for partial pressure. Both factors are stored in non-volatile memory in the RGA Head.
To tune the sensitivity factors do the following:
Make sure you are connected to the head.
1. Select the Sensitivity Tuning command from the Head menu.
2. Select which sensitivity factor you would like to adjust from the Measurement
Mode section (Partial or Total).
3. If you selected Partial Pressure then enter the mass whose pressure you will
be measuring in the Mass Selection edit box.
4. Enter the pressure indicated by your pressure gauge in the Reference Pressure
Reading.
5. Press the Measure button.
6. Press Accept to start using the new factor(s) or Undo to revert back to the old
one(s).
Adjusting the CEM Gain
The CEM gain may be set to any desired value between 10 and 1,000,000. The RGA program
will automatically search the required high voltage for such a gain if the Adjust button is
pressed.
All data acquired while the CEM is ON gets divided by the gain automatically before it is
displayed.
To change the CEM Gain do the following:
1. Connect to an RGA Head with a CEM option (Utilities menu).
2. Select the Channel Electron Multiplier command form the Head menu.
3. Select the Pressure Measurement Mode and the Partial Pressure Mass (if
partial pressure is selected).
4. Enter the desired gain in the Gain edit box.
5. Press the Adjust button and wait for the procedure to finish.
6. Press OK.
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2-22 Head Calibration and Security
Peak Tuning the RGA Head
WARNING!
The peak tuning procedure should be performed by qualified personnel only. A mistuned
RGA Head could give erroneous readings.
Please refer to the RGA Tuning chapter of this manual for more information about tuning
and calibration.
All the variables displayed are initially read from the SRS RGA Head when the dialog box is
activated. When you are done with this procedure, the new parameters are stored in non-volatile
memory in the RGA Head.
Activating this dialog box automatically puts the display mode in Dual Analog mode. The span
is set to 10 amu with the two peak tuning gas masses set in the middle of each span. If you press
Scan and you have sufficient gas amounts of each peak tuning gas, you should see the two peaks
on the graphs. Use these peak for width and position adjustments.
Every time you make a change to any of the parameters, press the Scan button to see the effect of
the change on the graph. Use the graph scaling command AutoScale to scale the data.
The peak tuning procedure must be performed in the order described below. You may repeat the
entire procedure as needed until you are satisfied with the results.
To tune the RGA Head peaks do the following:
1. Select the two gases you are going to use for peak tuning. Use the Gas
Selection command (Head Menu)to enter the gas names and masses.
2. Press Scan to see where the peaks are positioned. Rescale the graph as needed
so you can see the gas peaks.
3. Enter a Position Shift value for the Low Mass Peak if the peak does not fall
exactly at its required mass. Press Scan. Repeat this step as needed until the
peak is at the correct mass.
4. Repeat step 3 for the High Mass Peak Position
5. Iterate between steps 3 and 4 as needed
6. Enter a Width Adjust value for the Low Mass Peak (if needed ) and press scan
(The full width of the mass peak at 10% of its maximum should be less than
or equal to 1 amu).
7. Repeat step 6 for the High Mass Peak Width
8. Iterate between steps 7 and 8 as needed
When you are done you may:
• Press Done if you are satisfied with the results, or
SRS QMS Gas Analyzer
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RGA On-line Help 2-23
• Press Undo ALL to revert to the initial settings, or
• Press Factory Settings to recall factory set values.
Securing the RGA Head
Use this feature in an environment where you would like to restrict access to the head
parameters. When the RGA Head is locked, you cannot perform certain parameter editing
procedures such as Peak Tuning, Sensitivity Tuning, and Ionizer parameter editing. The
encrypted password is stored in the rga.ini file in the Windows directory.
To lock the head parameters do the following:
1. Select the Lock Head Parameters command form the Head menu or click on
the Lock button in the main Toolbar.
2. Enter an alphanumeric password. The password is case sensitive.
3. Confirm the password exactly as entered above.
4. Press Lock.
To Unlock the head parameters do the following:
1. Select the Lock Head Parameters command form the Head menu or click on
the Lock button in the main Toolbar.
2. Enter you password.
3. Press Unlock.
RGA On-line Help
The RGA Help system contains detailed information on the operation of the program that is not
contained in this manual. Use the On-Line Help to get up to date detailed information on the
RGA Windows program. The following sections describe the different ways to use the RGA
Help system.
Help Search
You may search the RGA Help system for any topic or keyword. Once you have the RGA Help
system window open, select the search command and enter any keyword to display that topic.
Help Index
Displays a comprehensive index to information contained in the RGA Help system.
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2-24 RGA On-line Help
To see the entries for a topic, click the first letter of the word you want to look up, or press TAB
to select the letter and then press ENTER. Click on any entry highlighted in green and the topic
for that entry is displayed automatically.
Commonly Asked Questions
A Q&A help files has been included with the RGA program. This file includes the most
commonly asked questions about the SRS RGA system. Double click on the RGA Q&A Help
Icon to view this file.
Sample Scans
The RGA Windows program includes several samples scan files that demonstrate some of the
SRS RGA capabilities. The scan files are located in Scans subdirectory of the main RGA
program directory. Double click on the Sample Scans Help Icon to view the description of the
each scan file.
SRS QMS Gas Analyzer
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Chapter 3.
Measurement Techniques
This chapter discusses procedures to help the user make accurate measurements with the QMS.
Several sections are devoted calibration and tuning procedures. The last sections discuss specific
measurement techniques.
In This Chapter
Calibration..........................................................................................................................................................................3-2
Effect of Total Pressure .............................................................................................................................................3-3
Operating Off the Design Pressure....................................................................................................................3-3
Total Pressure and Composition........................................................................................................................3-4
Calibration of Partial Pressure...................................................................................................................................3-5
Initial Calibration..................................................................................................................................................3-6
Basic Recalibration...............................................................................................................................................3-7
Calibration for Multiple Operating Conditions................................................................................................3-8
Calibration with Fixed Reservoir........................................................................................................................3-8
Peak Tuning Procedure..............................................................................................................................................3-8
General Procedure ..............................................................................................................................................3-10
Peak Position Tuning Algorithms....................................................................................................................3-11
Peak Width Tuning Algorithms .......................................................................................................................3-12
Temperature Effects on the Mass Scale Calibration.....................................................................................3-14
Electron Multiplier Tuning Procedure...................................................................................................................3-15
Techniques .......................................................................................................................................................................3-16
Correcting for the Chamber Background..............................................................................................................3-16
Correcting for Multiple Species..............................................................................................................................3-17
Operation with Condensable Gases.......................................................................................................................3-17
SRS QMS Gas Analyzer User’s Manual
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3-2 Calibration
Calibration
The QMS has been calibrated at the factory to measure the partial pressure of nitrogen correctly. For
many purposes this will be suitable. Overtime the calibration can change or operating conditions may
change. There are many factors involved in calibrating the QMS and interpreting the mass spectra. To
make accurate measurements, the following conditions need to be met:
• The total pressure needs to be known.
• The main sensitivity factor needs to be calibrated. Sensitivity factors change as a factor of time
due to aging and periodic recalibration is necessary.
• For careful quantitative analysis, it is important that the sensitivity of the RGA be determined for
every gas which may be a component of the system. Each gas component in the sample will
fragment differently and will have slightly different sensitivities.
• Correct calibration of the mass scale is essential during qualitative analysis for the correct
assignment of mass numbers to the different peaks. The mass scale will effect the peak height if
it is more than 0.3 amu out of calibration.
• The mass resolution of the quadrupole mass filter, ∆m
avoid overlap between adjacent peaks. Changes in ∆m
aging, severe contamination and large temperature changes) will cause variations in the
sensitivity of the instrument and the shapes of the fragmentation patterns of the molecules,
affecting all quantitative measurements.
• The gain of the electron multiplier is mass dependent and needs to be determined prior to
performing measurements with the device. The gain characteristics of the multiplier change with
time and periodic recalibrations are very important.
The following sections of this chapter describe several procedures designed to assure that all the
calibration conditions described above are satisfied prior to a set of partial pressure measurements. All
tuning procedures can be executed from RGA Windows. Users writing their own programs can
implement the procedures themselves, using the RGA Command Set and the instructions of this chapter.
, must be kept at or under 1 amu to
10%
during the measurements (caused by
10%
All the tuning procedures require the ability to introduce pure gases (or a mixture of gases of known
composition) into the system and a reference pressure gauge. All pressures are absolute; it is not
possible to make measurements in gauge pressure.
Important: Tuning should only be attempted after the unit has been warmed up (with the filament on
and under typical operating conditions) for at least one hour.
SRS QMS Gas Analyzer
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Calibration 3-3
(
)
Q
in out
Effect of Total Pressure
Increasing the total pressure at the inlet of the capillary will increase the flow through the capillary. The
higher flowrate in turn will increase the pressure at the RGA. This effect is not linear and in applications
where the inlet pressure varies, the user needs to understand the flow at the inlet. The flowrate or
throughput of the capillary is characterized by
C P P
= − ,
where Q is the throughput (a mass flowrate), C is the conductivity of the capillary, and Pin & P
out
are
the pressures at the inlet and outlet of the capillary. The inlet pressure is much larger than the outlet
pressure, which allows P
to be approximated as zero. The conductivity is a function of the capillary
out
dimensions, the viscosity of the gas, and the pressure drop across the capillary. This results in the
throughput, Q, being proportional to the square of the inlet pressure. The diaphragm pump dictates the
pressure at the outlet of the capillary according to
= ,
out
Q
S
DP
P
where SDP is the speed of the pump. The speed of the diaphragm pump varies with throughput
according to its characteristic curve, referred to as a speed curve. The speed curve is not linear.
Because the pump has an ultimate vacuum it can achieve, the intercept of the curve is not even zero.
The aperture and turbo pump respond linearly to P
. Although all these factors can be modeled, the
out
overall response of the QMS to total pressure is best characterized experimentally. A short experiment
with the specific gas of interest, equipment and operating conditions will yield a curve describing how
the pressure at the RGA varies with Pin.
Operating Off the Design Pressure
Each system is specified for one inlet pressure, the design point, which is atmospheric pressure for the
standard capillary. The capillary accomplishes the first stage of the pressure reduction from the chosen
design point to about 1 mbar. The aperture in the QMS is fixed, and designed to reduce the pressure
from 1 mbar to about 5×10-6 mbar at the RGA. Each capillary is designed for the specific inlet
pressure; mainly by choosing length and bore diameter. The inlet pressure to the capillary must always
be such that the outlet pressure is less than 5 mbar. Operating the inlet at high pressures which would
cause the outlet pressure of the capillary to exceed 5 mbar would cause two unacceptable effects: First,
the turbo pump exhaust pressure would be excessive and slow the pump. The high pressure would
increase the work load and cause excessive heating of the pump bearings. In the QMS, these fault
conditions are prevented. The turbo pump contains a thermocouple which monitors the bearing
temperature and shuts down the pump before it overheats. Also, the system microcontroller will shutoff
the turbo pump and not restart it until the pressure is acceptable. The second effect is excessive
pressure at the RGA, which can degrade the filament if it occurs for long periods. This fault condition is
also prevented by two means. The RGA will shut off the filament when it senses the pressure is high
and the system microcontroller will close the sample valve. These operating limits restrict the dynamic
User’s Manual
Page 58
3-4 Calibration
x
i
∑
range of the QMS with respect to increasing the inlet pressure above the design point. The instrument
has little “head-room” and the capillary should be designed for the maximum expected pressure.
Below the design point, the QMS can tolerate large decreases in the inlet pressure. The ultimate
vacuum of the diaphragm pump limits the lowest pressure at the outlet of the capillary, typically to 0.5
mbar. This pressure is the only operating limit; below it gas would flow out of the QMS. With respect
to measurements, operating the outlet of the capillary near the ultimate vacuum of the diaphragm pump is
inadvisable. At the ultimate vacuum, the flow through the pump is effectively zero. Operating the QMS
with zero flow obliterates the fast response time of the instrument. While there are no physically harmful
effects to operating below the design point, the ability to make measurements is lost at very low inlet
pressures. Best performance of the system requires that the outlet of the capillary is between 1 to 3
mbar. The Technical Reference Manual discusses in greater detail the design of capillaries.
Total Pressure and Composition
The RGA measures partial pressure of the components in a gas stream. For ideal gases, the partial
pressure is related to composition by
P
i i
T
P
= ,
where Pi is the partial pressure of the i-th component, xi is the mole fraction of the i-th component and
PT is the total pressure. It is evident from this equation that a measurement of Pi cannot determine both
xi and PT. To determine the composition, xi, a value for PT must be known. In many applications total
pressure is constant and therefore partial pressure is proportional to mole fraction. When total pressure
is not constant, a method of determining its value must be employed to allow composition to be
determined.
In theory, the sum of the partial pressures determines the total pressure, i.e.
T
P P
=
.
i
In practice, this summation requires care. For example, when using the P vs. t mode to acquire data,
make sure to record all the major components of the gas being analyzed. Beware of overlapping peaks;
these complicate the analysis. For example, consider a 50/50 mixture of nitrogen and carbon dioxide.
The parent peaks for these gasses are at 44 and 28. Referring to the library in the software shows that
nitrogen produces a peak at masses 28 and 14 that are 93% and 6% of the partial pressure and that
carbon dioxide produces a peaks at masses 44 & 28 that are 78% & 9% of the partial pressure. For a
1000 mbar total pressure, the spectrum would show peaks at 28 & 44 with heights of 510 and 390
mbar. An error can be demonstrated by using these peak heights and reversing the calculation, while
ignoring the interference. The carbon dioxide partial pressure would be correctly calculated as 500
mbar (=390/0.78), but the nitrogen would be erroneously calculated as 548 mbar (=510/0.93) mixture.
The mixture appears to be present at a total pressure of 1048 mbar and a composition of 52/48. The
correct calculation would first subtract the component of the peak at mass 28 which was caused by
carbon dioxide, before calculating the nitrogen partial pressure. More discussion of the quantitative
analysis of complex mixtures can be found in the texts listed in the Reference section.
SRS QMS Gas Analyzer
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Calibration 3-5
sensitivity factor (A Torr )
Calibration of Partial Pressure
All quantitative calculations performed with the RGA rely on the assumption that there is a linear relation
between the partial pressure and the corresponding RGA signals of the gases. Each gas ionizes
differently, and its ions make it through the mass filter with different efficiencies. As a result the
proportionality constant relating the ion current of a gas to its partial pressure is dependent on the
specific gas.
The partial pressure sensitivity of the RGA to a gas g, Sg, is defined as the ratio of the change (H-
H0) in principal mass peak height to the corresponding change (P-P0) in total pressure due to a change
in partial pressure of the particular gas species. H0 and P0 are background values.
Sg = (H-H0) / (P-P0)
The units of Sg are of ion current per unit pressure (amp/Torr, for example).
The sensitivity of the RGA changes with time due to aging of the probe, and is a strong function of the
operating conditions of the instrument. Careful quantitative analysis requires that the sensitivity factor,
S
, be determined for every gas which may be a component gas in the system being analyzed. The
g
sensitivity factors must be obtained under the same operating conditions that will be used during general
partial pressure analysis since they depend on many instrumental parameters, including: ionization
energy, emission current, mass filter setting, type of detector, etc.
Important: In order to separate the gain of the electron multiplier from the intrinsic sensitivity of the
RGA head, the sensitivity factors of the RGA are defined for Faraday Cup detection. A separate
Electron Multiplier Gain Factor, is used to correct the ion signals when the electron multiplier is used.
See the Electron Multiplier Tuning section for details.
Two calibration factors are used in the QMS system: the RGA sensitivity and the pressure reduction
factor. The RGA sensitivity is the factor which converts the ion current that is measured by the
electrometer to partial pressure at the ionizer. The pressure reduction factor accounts for the large
pressure reduction performed by the two stage inlet (capillary and aperture). Determination of these
factors requires comparing the system with a known-accurate pressure gauge and calculating the factor
that makes the QMS agree with the standard. Calibration is not necessary on a frequent interval, but is
required whenever operating conditions change.
The RGA intrinsically measures an ion current, which is proportional to the partial pressure at its ionizer.
While the software can be set to report ion currents, most users will need to measure partial pressure at
the inlet of the capillary. To convert between the two, the partial pressure reported by the software is
calculated by the formula:
P = ion current (A)
i
× ×
pressure reduction factor
= ion current (A) overall factor (Torr A )
-1
-1
.
The pressure reduction factor is a function of the capillary dimensions, the performance curve of the
pumps, and the dimensions of the aperture. The sensitivity factor is a function of the precise dimensions
of quadruple and ion optics, the state of the detector, the ionizer filament, and the four parameters which
User’s Manual
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3-6 Calibration
control the filter (electron energy, focus voltage, ionizer current, and ion energy). In the equation above,
the two factors are unknown. During calibration only the standard partial pressure and measured ion
current are known. Therefore, both factors cannot be determined; only the overall factor can be
determined. Both factors can be determined if a second reference pressure gauge is introduced into the
RGA chamber. While this approach would yield another reference pressure and allow both factors to
be accurately determined, it has no practical benefit.
Because only the ratio of the two factors is relevant, strictly speaking, one of the factors could be
chosen to be any number. An obvious choice is to make one of the factors equal to one and use only
the other. Because the sensitivity factor is stored in the RGA, this choice causes practical problems.
The RGA firmware limits the sensitivity factor to reasonable values. The sensitivity factor must be on
the order of 10-4 to 10-5 A Torr-1, which are typical values. The pressure reduction factor is stored by
the software in each .RGA file. With the two values stored in different locations, there are benefits for
using each to account for various components of the overall calibration factor.
Both factors cannot be determined; therefore, each time the instrument is calibrated, one of the factors
will be assumed to be correct, and the other will be adjusted to make the measured and reference
values agree. The strategies for using each value are discussed in the following sections, starting with the
basic technique that was performed at the factory.
Initial Calibration
Initially, a default value is stored in the RGA for its sensitivity factor. This factor is displayed by
selecting the “Head|Get Head Info...” menu item in the software. This value was determined at the
factory using a reference ion pressure gauge. Users can resort to this default value when they wish to
completely recalibrate the instrument. The pressure reduction factor is calibrated using the partial
pressure of nitrogen present in air using the following steps:
1. Determine the barometric pressure, which is typically reported in “in Hg”. Example: on a
typical clear day the pressure is 29.95 in Hg or 761 Torr (1 in Hg = 25.4 Torr).
2. Nitrogen is naturally present at 78.1% of total pressure. Multiply by this factor to yield the
partial pressure. Example: on that day nitrogen is present at 594 Torr.
3. Gases break into molecular fragments in the ionizer. For common gases, fragmentation factors
exist that indicate what fraction of the molecules remain intact. For nitrogen 92.6% of the
molecules will remain intact and will be measured at mass 28. Multiply the partial pressure by
this fraction to determine the reference value. Example: the reference value would be 550 Torr.
4. With the pressure reduction factor disabled (or set to 1), measure the peak at mass 28.
Example: the system indicates 1.3 × 10-6 Torr at mass 28.
5. The pressure reduction factor is the reference value divided by the measured value. Example:
the factor is calculated to be 4.2 × 108 for this instrument.
6. Enter this number in the pressure reduction factor dialog box and check the enable box.
SRS QMS Gas Analyzer
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Calibration 3-7
This completes the calibration. All modes of the software will now report partial pressure at the inlet to
the capillary. Be sure to record these values as they can be used to diagnose system performance. The
pressure reduction factor is saved in the .RGA file; make sure to select File|Save to record the new
pressure reduction factor.
Basic Recalibration
Some situations will require recalibration of the instrument. For example:
• aging of the diaphragm pump and ionizer filament
• small changes in the total pressure at the capillary inlet
• small dimensional changes to capillary and aperture
For users of one capillary and one input stream, an easy method of making small changes to the
calibration values is available with the Sensitivity Tuning feature of the software. Under this method, we
assume that the pressure reduction factor is correct and change the RGA sensitivity factor.
Nitrogen is the most common recalibration gas, and in this example, we assume that the partial pressure
of nitrogen in air is used as a reference. While air is convenient, the recalibration can be performed with
any test gas as a reference. Follow these steps to recalibrate the QMS:
1. Make sure that the pressure reduction factor is enabled and correct in the dialog box that
appears under the “Utilities|Pressure Reduction...” menu item.
2. Setup the QMS to sample the reference gas.
3. Choose the “Head|Sensitivity Tuning...” menu item to make the Sensitivity Tuning dialog box
appear. Make sure the Measurement Mode is set to Partial Pressure and that the Mass
Selection is set to the parent peak of the reference gas.
4. Enter a value in the Reference Pressure Reading edit box that is the expected value for the
pressure at the capillary inlet. This value should be adjusted for the fragmentation factor as
done above (92.6% for peak 28 from nitrogen). As in the example in the previous section, the
reference reading is 550 Torr for nitrogen in air at a barometric pressure of 29.95 in Hg.
5. Observe the value in the Sensitivity Factor text box and then press the Measure button. A new
value will be displayed in the text box, which should be close to the old value.
6. Press the Accept button to store the newly calculated value into the RGA or the Undo button to
restore the previous button.
The instrument is now recalibrated. Note that the new sensitivity factor is only correct when used with
.RGA files that contain the matching pressure reduction factor. This procedure can be repeated
frequently to make minor adjustments to the overall sensitivity factor. Because the range of the RGA
sensitivity factor is limited by firmware, this procedure cannot be used to account for large changes in
the overall sensitivity factor.
User’s Manual
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3-8 Calibration
Calibration for Multiple Operating Conditions
The QMS capable of being used over a variety of operating conditions, which in turn require different
overall sensitivity factors. Examples are:
• one QMS system used with multiple capillaries
• measurements of gas streams at different total pressure, temperature, or composition
• measurements at multiple ionizer conditions
The RGA sensitivity factor is not meant to be directly adjusted by the user. The software only allows
this value to be changed via the Sensitivity Tuning dialog box using a reference gauge reading. Users
cannot type a new value into the Sensitivity Factor text box. Instead, the Pressure Reduction Factor
feature is provided to account for widely varying operating conditions.
The Pressure Reduction Factor is stored in the .RGA files. When using the QMS with various
operating conditions, one .RGA file can be made for each set of conditions. Each of these files will
contain a different pressure reduction factor. The procedure to determine the pressure reduction factor
is the same as used in the Initial Calibration section above. Briefly: disable the pressure reduction factor,
compare the measured value with a reference, and calculate a new pressure reduction factor. When
determining the pressure reduction factor for each set of conditions, make sure that the RGA sensitivity
factor has not been changed.
To use the QMS at one of the multiple conditions, simply open the appropriate .RGA file and connect
the window to the ECU (if the ECU is already connected to another window, disconnect from that
window first). The software will now be ready to make measurements.
It is worth restating that the pressure reduction factor is only accurate when used with the matching
RGA sensitivity factor. The value in the RGA electronics can be changed by other users, so the RGA
sensitivity factor should be recorded or locked using the security feature of the software.
Calibration with Fixed Reservoir
Air is a convenient calibrant gas for the QMS, but only provides nitrogen as a useful reference. The
other major components, e.g. oxygen, water, and carbon dioxide, are not present at reliable
concentrations. For more precise calibrations a reservoir and pressure gauge can be used as a
calibrant. When using this method, be aware that the QMS continuously draws 1-5 milliliter per minute
of gas, depending on the capillary. The reservoir should be large or the total pressure will change
quickly.
Peak Tuning Procedure
When analyzing a sample, you expect the peaks of the different gases to be displayed at their correct
mass-to-charge ratio values and the peak widths to be less or equal than 1 amu at 10% of peak height.
The correct location of the peaks is essential for accurate qualitative analysis, and unity resolution
(∆m
and the widths of the peaks can vary with time due to aging of the head. Changes in resolution are
particularly serious since they affect the sensitivity of the RGA and introduce errors in the partial
pressure measurements.
=1 amu) minimizes the overlap between adjacent peaks. Both the calibration of the mass scale
10%
SRS QMS Gas Analyzer
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Calibration 3-9
The Peak Tuning procedures described in this section allow the user to calibrate the mass scale and the
resolution, ∆m
, of the mass spectrometer. The RGA has a very solid design and this type of tuning
10%
procedures should rarely be needed.
WARNINGS
The peak tuning procedures should be performed by qualified personnel only. A mistuned
RGA Head will give Erroneous Readings until it is retuned properly.
Peak Tuning should only be attempted after the unit has been warmed up (with the filament
on and under typical operating conditions) for at least a one hour.
Peak Tuning requires a mixture of gases whose mass spectra is well known. In general, a two gas
mixture, one with low mass peaks and one with high mass peaks, is sufficient. The sample is introduced
into the vacuum, and the quadrupole mass filter parameters, referred to as Peak Tuning parameters, are
adjusted based on the sample analog spectra. The mass scale is adjusted so that all peaks are displayed
at their correct mass-to-charge values, and the peak widths, ∆m
, are adjusted to unity (or smaller)
10%
values. The two tuning procedures are referred to as Peak Position and Peak Width Tuning,
respectively.
Note: A mixture of He, Ar, Kr and Xe inert gases is used at SRS to Peak Tune the RGA’s. The
resolution is adjusted to 1 amu and peak tuning parameters are saved into the RGA ECU before
shipping. The inert gases cover a broad spectral range, and being inert they do not interact with the
RGA probe and do not contribute to its aging.
Peak tuning can be easily performed with RGA Windows using the Peak Tuning command of the Head
Menu. The program provides password protection for locking out the Peak Tuning Parameters so that
casual users cannot alter the mass scale calibration or the spectrometer’s resolution.
An extra copy of the tuning parameters determined at the factory for the mass filter is saved in the RGA
Head before shipping, and those values can be retrieved at any time in case they are necessary. Consult
the RGA Windows chapter of this manual or the RGA On-line Help Files included with the program for
details.
Note to Supervisors: A calibration disable jumper (JP100) can be configured to block any attempt to
change the value of the mass filter settings in the RGA Head. The jumper is located on the top
electronics board of the ECU box, next to the microprocessor chip (i.e. biggest component on the
board), and its two settings are clearly indicated as CAL DIS and CAL EN. Supervisors may use this
feature to prevent accidental changes in the calibration parameters by inexperienced operators. Peak
tuning is completely disabled when the jumper is configured to the CAL DIS setting.
Tip: Virtually every vacuum system will have detectable amounts of hydrogen (2 amu), water (18 amu),
carbon monoxide (28 amu) and carbon dioxide (44 amu). Become familiar with these species and their
fragmentation patterns, and use their peaks to quickly verify the correct performance of the instrument
(i.e. mass scale calibration and mass resolution) while operating the RGA.
User’s Manual
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3-10 Calibration
General Procedure
Peak tuning is a simple procedure that requires the introduction of two known gases into the vacuum
system. A low mass gas (1-20 amu recommended) is used to adjust the low end of the mass axis, a high
mass gas, with a mass-to-charge ratio close to the upper limit of the instrument’s mass range, is used to
adjust the high end of the mass scale.
Several analog scans are performed on the sample and the peak positions and widths are checked and
adjusted as necessary. Changes in resolution affect the sensitivity of the RGA, and a Sensitivity tuning
procedure should always be performed at the end of the peak tuning process.
The entire procedure can be carried out with the help of the Peak Tuning command (Head Menu) of
RGA Windows. The program guides you through the calibration procedure and automatically updates
the Peak Tuning Parameters in the RGA Head based on the results of the calibration.
The overall adjustment procedure is very simple and must follow the order described below:
1. Low mass peak position adjustment
2. High mass peak position adjustment
3. Repeat 1 & 2 in that order one or two more times until no more changes in peak positions are
observed.
4. Low mass resolution adjustment
5. High mass resolution adjustment
6. Repeat 4 &5 in that order until no more changes in peak width are observed.
7. Repeat 1 & 2 in that order one or two more times in the case steps 4 & 5 caused changes in
peak positions.
Important: Collect a fresh analog scan for each step. The peak positions are adjusted such that all
peaks throughout the scanning range of the RGA fall within +/- 0.25 amu of their known mass-tocharge ratio (This is needed to make sure the Peak-locking algorithm used for single mass
measurements always finds the mass peak within its search window). The peak width, ∆m
, must be a
10%
constant, and less than 1 amu throughout the whole scan range.
Example: The following figure shows the result of peak tuning the RGA based on the 18H2O+ (low
mass= 18 amu) and 86Kr+ (high mass=86 amu) calibration peaks. All peaks are at their correct mass
settings and show absolute resolution values of about 0.9 amu. Also note a 20 amu peak in the low
mass spectrum corresponding to 40Ar++ at 20 (i.e. 40/2) amu.
SRS QMS Gas Analyzer
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Low Mass High Mass
Ar
Calibration 3-11
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
15 16 17 18 19 20 21 22 23 24 25
H20
+
++
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
80 81 82 83 84 85 86 87 88 89 90
86Kr+
Users writing their own computer code can write Peak Tuning Commands for their own programs using
the Tuning Commands of the RGA Command Set and the instructions of the following two sections.
Peak Position Tuning Algorithms
The magnitude of the RF determines the mass-to-charge ratio of the ions that can pass through a
quadrupole mass filter without striking the rods (i.e. with stable oscillations). A linear relationship
between mass and RF amplitude is one of the most attractive features of these type of filters. The
regulated output of the RF source that powers the RGA’s quadruple rods is controlled by, and linearly
related to, the voltage output of an RF Driver circuit. The RF Driver uses an 18 bit digital-to-analog
converter and some additional electronics to program its output voltages.
The purpose of the Peak Position Tuning Procedure is to determine the voltages that the RF Driver must
output at 0 and 128 amu so that all the peaks in an analog spectrum appear in the right place in the mass
axis.
The calibrated voltage settings, in mV, are saved in the non-volatile memory of the RGA Head (RI and
RS commands respectively) and used by the firmware to generate the internal scan parameters used to
step the RF during scans and single mass measurements. Please consult the Tuning Commands List in
the RGA Programming chapter of this manual for details on the RS and RI commands.
As described above, the peak position tuning procedure requires the introduction of two known gases
into the vacuum system. A low mass gas (1-20 amu recommended) is used to adjust the low mass end
of the mass axis, a high mass gas, with a mass-to-charge ratio close to the upper limit of the scanning
range of the RGA, is used to adjust the high mass end of the mass scale. Several analog scans are
performed, and the values of RI (RF Driver output @ 0 amu) and RS (RF Driver output @ 128 amu)
are adjusted until all mass peaks appear at the correct position in the mass
User’s Manual
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3-12 Calibration
scale. An increase in RI causes the low end of the analog spectrum to displace towards lower masses
(A small effect is seen at the high masses). An increase in RS results in the spacing between peaks in a
scan to decrease (with the largest effect seen at the high mass end).
The formulae used to correct the calibration parameters during peak position adjustment are presented
next:
Low Mass Peak Position Adjustment: To displace a low mass peak by a distance ∆m amu in the
mass axis the value of RI must be modified from its original value RI0 according to:
RI = RI
- ∆m
0
.
(RS / 128)
Notes:
• The new RI value must fall within the acceptable parameter range of the RI command.
• This change mostly affects the position of the peaks at the low end of the spectrum.
• A decrease in RI shifts the low mass peaks to the right (peaks show up at higher masses) , an
increase in RI shifts the same peaks to the left (peaks show up at lower masses).
High Mass Peak Position Adjustment: To displace a high mass peak, m
, by a distance ∆m amu in
0
the mass axis modify the value of RS from its original value RS0 to:
RS = RS0 . [ m0/(m0+∆m)]
Notes:
• The new RS value must fall within the acceptable parameter range of the RS command.
• Modifying RS affects the spacing between peaks in the spectrum. An increase in RS results in
the peaks getting closer together, and a decrease in RS results in the peaks getting further apart
from each other.
• The effect is more significant at the higher masses and should have very little effect on the
position of the low mass peaks (that is why you do this adjustment second).
• The lower the mass-to-charge ratio of the low mass gas the less this adjustment will effect the
position of its peak.
• See that a decrease in RS results in the high mass peaks moving to higher masses (right shift),
and vice versa.
Iterations: In most cases it will be necessary to repeat the two position adjustments one or two more
times until both low and high mass peaks show up at their known positions.
Peak Width Tuning Algorithms
Constant absolute resolution (∆m
) in a quadrupole mass filter requires DC voltages linearly related to
10%
the mass, with a slight negative offset at low masses (i.e. negative intercept).
The RGA Head adjusts the DC levels of the quadrupole filter during measurements so that constant
mass resolution is automatically available throughout the entire mass range of the spectrometer. The bulk
of the DC voltage is supplied by a DC power supply whose output is linearly related to the RF
amplitude. The rest of the DC voltage (DC_Tweek) is provided by the output of an 8 bit digital-toanalog converter (DAC). The firmware uses two Peak Tuning Parameters : DI (Intercept) and DS
SRS QMS Gas Analyzer
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Calibration 3-13
(Slope), stored in the non-volatile memory of the RGA, to calculate the 8 bit settings of the DAC
according to the linear equation:
DAC8 (m) = DS . m + DI (DC_Tweek (m) = (DAC8(m) - 128) . 19.6 mV)
where m is the mass in amu, and DAC8(m) is the 8 bit setting at that mass.
The purpose of the Peak Width Tuning Procedure is to determine the values of DI and DS so that all
the peaks in an analog spectrum have the desired peak width (typically Dm
=1 amu). The calibrated
10%
parameters are saved in the non-volatile memory of the RGA Head (DI and DS commands,
respectively) and used by the firmware to generate the internal scan parameters used to adjust
DC_Tweek during scans and single mass measurements. Please consult the Tuning Commands List in
the RGA Programming chapter of this manual for details on the DS and DI commands.
Note: The sensitivity of the peak widths to the DC_TWEEK voltage can accurately be approximated
to: -1 amu per 550mV (28 bits). In other words, a 550 mV increase in DC_Tweek voltage causes a 1
amu decrease in the width of any peak. The DC_Tweek voltages span from -2.5 to +2.5 V with
increments of 19.6 mV per bit (255 total bits). This corresponds to a peak width adjustment range of
+/- 4.5 amu and minimum increments of 0.036 amu per bit.
As described above, the peak width tuning procedure requires the introduction of two known gases into
the vacuum system. A low mass gas (1-20 amu recommended) is used to adjust peak widths at the low
end of the mass scale, a high mass gas, with a mass-to-charge ratio close to the upper limit of the
scanning range of the RGA, is used to adjust the widths at the high end of the mass scale. Several
analog scans are performed on the sample and the values of DI ( nominally 128) and DS (nominally 0)
are adjusted until all peaks appear at their correct setting. The formulae used to correct the calibration
parameters during peak position adjustment are described next:
Low Mass Peak Width Adjustment: To increase the low mass peak width by an amount ∆m
AMU’s modify the value of DI from its original value, DI0 , to:
DI = DI
- ∆m * 28
0
Notes:
• The new DI value must fall within the acceptable parameter range of the DI command.
• A change in DI affects the width of all the peaks in the spectrum.
• A decrease in DI results in broader peaks at a rate of 0.036 amu per bit removed.
High Mass Peak Width Adjustment: To modify the high mass peak width by an amount ∆m
AMU’s, modify the value of DS from its original value DS0 to:
DS = DS
- 28 * (∆m) / m
0
0
Notes:
• The new DS value must fall within the acceptable parameter range of the DS command.
• Modifying DS affects the width of peaks at the higher end of the spectrum. An increase in DS
results in the peaks getting sharper, and a decrease in DS results in the peaks getting broader
User’s Manual
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3-14 Calibration
• The effect is more significant at the higher masses and that is why we do this adjustment second
after the width has already been modified by the change in DI.
• If the mass-to-charge ratio of the low mass gas is real low this adjustment will have a small
effect on the width of its peak.
Iterations: In most cases it will be necessary to repeat the two width adjustments one or two more
times until both low and high mass peaks show the desired widths.
Note: Changes in the peak positions will also be observed, at a rate of -0.40 amu per amu of increase
in peak width, and a Peak Position Tuning procedure will be needed to put the peak positions back
where they belong.
Important: The Peak Width Tuning Procedure can be used to adjust the resolution of the RGA to the
ultimate theoretical limit of the mass filter. See the Quadrupole Mass Filter section of the RGA Probe
chapter of this manual for details on that limit.
Temperature Effects on the Mass Scale Calibration
As the temperature of the RGA Head changes two different effects affect the calibration of the mass
scale:
• For small temperature changes: Drift in the voltage output of the RF Driver that controls the
RF power supply can cause the mass peaks to shift their position in the mass spectrum. In
order to correct against this effect, the RF driver output is checked at the beginning of each
analog and histogram scan at 0 and 128 amu. The internal calibration parameters, used by the
firmware to step the RF during scans, are updated so that the control levels specified by the RI
(0 amu) and RS (128 amu) parameter values are correctly set at the present temperature.
• For large temperature changes: The sensitivity of the RF power supply to its controlling
voltages might be affected or, more fundamentally, the relationship between mass and RF levels
in the filter might change. In this case a Peak Tuning procedure will be necessary to reestablish
the mass axis scale.
SRS QMS Gas Analyzer
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Calibration 3-15
Electron Multiplier Tuning Procedure
Accurate quantitative measurements with the electron multiplier detector require the determination of the
CDEM gain for all the ion peaks being measured. Frequent recalibrations are recommended to correct
against aging of the device.
The gain of the electron multiplier (CDEM) in the RGA is defined relative to the Faraday Cup output
(which is assumed to be mass independent). It can be programmed anywhere from 1 to 107 adjusting
the high voltage applied across the device, is highly mass dependent, and changes with time due to
aging.
The electron multiplier gain is easily calibrated in the RGA since it is possible to measure the same ion
current with and without the CDEM. The common method of calibrating the electron multiplier gain for
a given mass peak is to measure the peak intensity with the Faraday cup, and then repeat the same
measurement with the electron multiplier without changing anything else. The gain of the multiplier is the
ratio of the multiplier output current to the Faraday cup output current. (Note that there is no need to
change the sign of the electron multiplier signal prior to the division since the firmware automatically
reverses its sign before transmitting the value.)
The RGA Head can store a single set of [High Voltage, Gain] values for the electron multiplier in its
non-volatile memory. RGA Windows uses the voltage value to bias the CDEM and the gain value to
divide the ion currents when the CDEM is turned on. See the HV, MV and MG commands in the RGA
Command set for details on the command-level implementation of this procedure.
RGA Windows can automatically program the gain of the electron multiplier for any mass using the
automatic Electron Multiplier Gain Adjustment function of the Electron Multiplier command (Head
Menu). Select a gain value between 10 and 1,000,000, choose a mass value for partial pressure
measurements , and the RGA program automatically calculates the required high voltage setting when
the Adjust button of the Electron Multiplier Window is pressed. The HV and gain settings are saved
into the RGA Head and used every time the CDEM is turned on. All data acquired while the CDEM is
on gets divided by the gain automatically before it is displayed.
User’s Manual
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3-16 Techniques
Techniques
Correcting for the Chamber Background
Even with the sample flow and capillary flow valves closed, their will be a noticeable background in the
mass spectrum. This background in the analyzer chamber is caused by outgassing from the chamber
surfaces, backstreaming through the turbomolecular pump, and gas production from the ionizer of the
RGA. These three processes account for the ever present background of hydrogen, water, nitrogen,
oxygen, and carbon dioxide seen in high vacuum. The outgassing of water can be minimized by
extensive pumping with both valves closed; typically the system can achieve water partial pressures
around 1 × 10
be reduced. The ultimate vacuum of the turbo pump causes nitrogen to be present at no lower than
2 × 10
-9
from 10-9 to 10-7 mbar. The software contains a background subtraction feature that allows the
chamber background to be removed from the mass spectrum.
-8
mbar. The other two process (backstreaming and ionizer) are fundamental and cannot
mbar and oxygen at 1/4 of that level. Carbon dioxide from the ionizer will be present at levels
The background spectrum is correctly measured with the sample valve closed and the capillary flow
valve open. When the capillary flow valve is open, the pressure at the exhaust of the turbo pump is the
same as it will be when the sample is measured. Thereby, the backstreaming component of the mass
spectrum will be the same as in the sample measurement. The software can only subtract two spectra
when they cover the same mass range. Set the software to acquire the spectrum (either histogram of
analog) at the speed, range, and schedule you require. To obtain a background subtracted spectrum,
follow these steps:
1. Measure one complete analog or histogram mode spectrum with the sample valve closed. If the
software was set to scan on a continuous schedule, you can select “Stop at End” from the
“Scan” menu to stop when the current scan in progress is complete. The data displayed must
be a complete scan, and be measured with the same parameters as the scans to follow.
2. Under the Utilities menu, select “Background” and select “Scan Data - Background” from the
dialog box. Check the box next to “Enable” and select “OK” to close the dialog box. This
makes the current spectrum the background and all spectra displayed subsequently will have
this spectrum subtracted from it.
3. Open the sample flow valve and start the scan with the “GO” button. The newly acquired
spectra are the background corrected result.
The ability to subtract background is limited by signal proportional noise, which is typically present at
between 1-10% of the signal magnitude. Because this noise originates in the ionizer of the RGA,
subtraction cannot remove much more than 90% of the background. This limits the ability to see small
changes of less than 1% at the same mass as peaks present in the background.
SRS QMS Gas Analyzer
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Techniques 3-17
O
2
O
4
CO
Correcting for Multiple Species
As discussed above, the QMS is calibrated at one mass number. Because every gas behaves
differently, analog scans can only show peak heights that are correct at the one mass number. It is not
possible to correct the analog and histograms at every mass number. The RGA would have to know
what species was causing the ion current at each mass. As an example: is ion current at a m/z of 16
caused by a fragment of H2O (
+1
), a fragment of O2 (O
in a mass spectrum have multiple sources. To demonstrate this to yourself, use the library search on
almost any single mass number. Except for a few values, the search will retrieve multiple species for
almost all the low mass peaks. Without knowledge of what species is causing what peak, the correction
cannot be made automatically. Practically, this is not a problem; the histogram and analog modes are
intended to show qualitative composition.
The table modes (table, P vs. t, and annunciator) contain a calibration factor for each species. These
factors are provided to resolve the problem with the mass spectrum just discussed. The user can use
these factors to tell the software how to correct for each peak. To determine the correct calibration
factor to use, you must know (or assume) what species is causing each peak and choose masses that
are not complicated by other species. Choosing the correct peaks is complicated and requires
understanding of the mass spectrum. For instance in a 50/50 mixture of nitrogen and carbon dioxide,
you cannot use mass 28 to measure the nitrogen. With this mixture, about 10% of the peak at 28 would
be caused by a
+
fragment. A better choice would be to use the peak at 14 to measure nitrogen.
Once you have chosen a mass that is representative of each species of interest, you then refer to
published fragmentation factors (see references) to determine how to correct back to the parent peak.
Even if the instrument was calibrated for nitrogen, it was likely calibrated on the peak at 28. The peak
at 14 will not be correct; it will be about 7% of the correct value. Entering 14.2 (1/0.07) as a
calibration value in the table parameters allows the software to correct the partial pressure reading.
Similar calibration factors are entered for each species being measured. A calibration gas of known
composition makes process of determining factors easy. First make a measurement of the standard
with all the factors set to 1. From this measurement, a correction for each species can be calculated
and entered into the tables. The references listed at the end of this manual contain discussions about
how to interpret mass spectra from RGA’s.
+2
, or
+1
), or CH
+1
. Many of the peaks
Operation with Condensable Gases
The QMS is designed to sample gases that are nominally at room temperature. Under these conditions,
any species that is a gas in at inlet conditions can be expected to travel through the instrument without
condensing. Without a heat input, a gas will cool as it expands through a capillary and pressure
reduction aperture (according to its Joule-Thompson coefficient). In the QMS system, the absolute
pressure difference across the aperture is small and the flow rate is small; under these circumstances the
interior metal surfaces can provide sufficient heat to the expanding gas to keep it from condensing. If
problems due to condensation are suspected, the capillary can be wrapped with heating tape to test for
condensation. The goal of heating the capillary is to increase the heat transfer rate to the sample gas.
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3-18 Techniques
When the gas being measured is significantly hotter than the QMS system, condensation is likely and
presents a problem. If the species at the inlet are gases only at temperatures above room temperature,
they can condense when they reach the QMS. The condensed material will continually build up in the
QMS and cover the valve seats and aperture. Two approaches can prevent this problem: control the
location of condensation or prevent condensation. The first approach can be very simple: place
screens or metal plates before the inlet to provide sacrificial surfaces for condensation. The sacrificial
surfaces should have good thermal connections to the outer walls so that they stay at room temperature.
These surface will act like a trap and prevent the unwanted materials from passing into the QMS. The
second approach involves operating the entire QMS inlet above the condensation temperature of the
condensable material. This may be feasible if the operating temperature is below 100 °C. All the tubing
components in the inlet can be heated to 100 °C or more. The electronics and pumps cannot operate
at elevated temperatures. This approach (keeping the tubing hot and electronics cool) is a difficult task
and therefore is not recommended.
SRS QMS Gas Analyzer
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Glossary
The following is a listing of some of the most important terms used throughout the SRS RGA Operations
Manual. For a more complete listing of terms relevant to partial pressure analyzers in general, refer to:
“A Dictionary of Vacuum Terms used in Vacuum Science and Technology, Surface Science, Thin
Film Technology and Vacuum Metalurgy”, edited by M. S. Kaminsky and J. M. Lafferty, published
by the American Vacuum Society, 1979.
A. Basford et. al., J. Vac. Sci. Technol. A 11(3) (1993) A22-40: “Recommended Practice for the
Calibration of Mass Spectrometers for Partial Pressure Analysis. Update to AVS Standard 2.3”.
Anode Grid (Ionizer Component): Positively biased grid cage of the ionizer within which the ionization
of the gas molecules takes place. Note: In the SRS RGA, the voltage bias of the anode grid, in
Volts, sets the ion energy (in eV) for the spectrometer.
Atomic mass unit (abbreviation: amu). A unit of mass equal to one twelfth the mass of a neutral
carbon atom having six protons and six neutrons (12C); equivalent to 1.660566.10
-27
kg.
Background signal, H0. Output signal, measured with respect to baseline, which is obtained before the
introduction of any gas in the chamber.
Base pressure (also Background pressure). Total pressure before introduction of any gases into a
vacuum system. Usually, the base pressure is the lowest pressure that is typically achieved in the
vacuum chamber.
Baseline. The output signal from the RGA when no ions are arriving at the detector.
CDEM. Abbreviation for the type of electron multipliers known as: Continuous Dynode Electron
Multiplier.
CF Nipple. A short section of vacuum pipe terminated with a standard Conflat flange connector at each
end.
CF. Abbreviation for Conflat type flange.
Charge, Q. The electron charge of an ion. Ion charge occurs in multiples of the electron charge, e-.
Cleanup time - is a measure of how quickly the response returns to zero when a gas is removed from
the inlet (not necessarily equal to response time).
Delay time - is a measure of the time between when a step change is presented at the inlet and the
response is first seen at the QMS (can be very long without degrading the response time).
Drift. A change in time in the average output signal at constant partial pressure.
ECU. Abbreviation for electronics control unit.
Electron emission current. The electron current from the filament to the grid in milliAmperes.
Electron emission. The release of electrons from the heated filament in the ionizer. The electrons are
accelerated into the anode grid where the ionization of the gas molecules occurs.
SRS QMS Gas Analyzer
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Glossary
Electron Energy. The kinetic energy of the electrons (in eV) used for electron bombardment in the
ionizer. Note: In the SRS RGA the Electron Energy is equal to the voltage difference (in Volts)
between the filament and the anode grid.
Electronics Control Unit (abbreviation: ECU). Electronics box that attaches directly to the probes
feedthrough flange and contains all the necessary components to operate the quadrupole mass
spectrometer and communicate with a host computer.
Faraday Cup. A charged particle detector, consisting of a metal electrode, for the direct collection and
detection of charged particles. Note: Typical designs are cup shaped to minimize secondary
electron losses.
Filament (Ionizer Component). The source of the ionizing electrons. Thin, thoria coated, Iridium wire
that operates at a negative potential relative to ground and is resistively heated to incandescence
with an electrical current from the emission regulator. The thermionically emitted electrons are
accelerated towards the anode grid which is positively charged with reference to the filament and
ground.
Focus Plate (Ionizer component). Ionizer’s electrode plate that serves the double purpose of drawing
the ions away from the anode grid, and containing the ionizing electrons inside the source.
Fragment ion. An ion of mass smaller than that of the original parent molecule.
Fragmentation (or cracking) pattern. The fragment distribution of ionic species which results from
dissociation and ionization of multi-atomic molecules of given species in the ionizer.
Fragmentation factor. The fragmentation factor of gas g at mass M is defined as the ratio of ion signal
at mass M to the ion signal at the principal mass peak of gas g.
Fragmentation of molecules: The breaking of multi-atomic molecules into units of fewer atoms, some
of which are usually electrically charged.
Head (also RGA Head). The combination of the Probe and the Electronics Control Unit.
ID - inside diameter
Inert Gas. A gas that does not normally react chemically with other substances. Example: He, Ar, Kr
and Xe. Typically used for calibration purposes in RGA’s.
Ion current signals scale linearly with the electron emission current. Note: The available emission current
range in the SRS RGA is 0 to 3.5 mA.
Ion current. The rate of ion flow into the detector. Usually expressed in units of amps.
Ion Energy. The kinetic energy of the ions as they move down the quadrupole mass filter, and
expressed in eV. Note: In the SRS RGA the Ion Energy is equal to the voltage biasing of the
anode grid in Volts and has two possible settings (i.e. Low and High).
Ion. An atom or molecule which has gained or lost one or more electrons and therefore has a negative
or positive charge. Note: Most RGA’s use electron bombardment to ionize molecules and detect
only positive ions.
SRS QMS Gas Analyzer
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Glossary
Ionization efficiency. The ionization probability normalized to the probability of ionization of a
reference gas.
Ionization Potential. The minimum energy per unit charge (often in eV) required to remove an electron
from an atom (or molecule) to infinite distance. Note: In the SRS RGA the Electron energy must
be set above the ionization potential of the molecules for ionization to occur.
Ionization probability. The number s of ion pairs (equal amounts of positive and negative charges
appear as ions) produced by an electron traveling a unit distance (typically 1 cm) through a gas at
unit pressure (typically 1 mTorr). It depends on the ionization potential of the electrons used for
bombardment.
Ionization. The process that results in the formation of ions from neutral atoms or molecules. During
ionization, electrons are added or removed from the molecules to form negative or positive ions
respectively. Note: In the SRS RGA the ionization is caused by electron bombardment; outer
electrons are removed by the impact of energetic electrons on the molecules.
Ionizer (Probe Component). The portion of the mass spectrometer probe that generates the ions and
accelerates them as a beam. Note: The components of the SRS RGA ionizer are: anode grid,
repeller, focus plate, and filament.
Linear response range. The partial pressure range over which linearity in the signal response is
observed. See linearity.
Linearity. The extent to which the change in output signal of the RGA is proportional to the
corresponding change in partial pressure.
Mass analyzer (Probe component). The portion of the mass spectrometer probe that separates the
ion beam into its various mass-to-charge ratio components. Also referred to as Quadrupole mass
filter.
Mass Number, M. The mass number, M, is the sum of the number of protons and neutrons in an atom
or molecule.
Mass peak. The ion current pattern in the vicinity of maximum amplitude by scanning through a small
portion of the mass range containing ions of a single mass-to-charge ratio. Very often, the term
“mass peak” refers to the signal developed from singly charged ions. For example, nitrogen is said
to have a “mass 28 peak”.
Mass Range. The range of mass numbers defined by the mass number of the lightest and the heaviest
singly charged ions which can be detected by the RGA.
Mass spectrometer. An instrument which produces a beam of molecular ions from a sample,
separates the resulting mixture of ions according to their mass-to-charge ratios, and provides
output signals which are measures of the relative abundances of the ionic species present.
Mass spectrum. A graph of ion abundance vs. mass-to-charge ratio.
Mass-to-charge ratio: The mass-to-charge ratio, M/Q, is defined as the ratio of the mass number M
of the ion to its charge Q, measured in units of the electron charge e-. For example: doubly charged
ions of argon isotope 36 (36Ar2+) and singly charged ions of water, 1H
16O1+
, have M/Q = 18, and
2
SRS QMS Gas Analyzer
Page 76
Glossary
cannot be differentiated from each other with most mass spectrometers. Note: Mass
spectrometers do not actually measure the molecular mass directly, but rather the mass-to-charge
ratio of the ions. For singly charged ions, the mass to charge ratio is numerically equal to the mass
of the ion in atomic mass units (amu).
Minimum Detectable Partial Pressure change (MDPP). The partial pressure change
corresponding to the smallest signal change which can be distinguished from noise. A common
prescription is, ∆p
= σ/Sg, where σ is the noise and Sg is the partial pressure sensitivity for the
min
gas being measured.
Noise. Random fluctuation in the output signal unrelated to a change in the partial pressure of the gas
from which the signal is derived. The appropriate measure of noise is the standard deviation σN of
N independent determinations of the average output signal obtained at constant partial pressure.
Open Ion Source (Also Nude Ion Source). An ionizer with electrodes of high transparency to gases.
Outgassing. The evolution of gas molecules from the internal wall surfaces of the vacuum system.
Parent ion. An ion of the same mass as that of the original parent molecule.
Partial Pressure Analyzer (abbreviation: PPA). A compact mass spectrometer used to analyze the
residual gas composition in a vacuum system.
Partial Pressure. The pressure of a designated component of a gaseous mixture within a system as it is
exerted on the chamber walls. The sum of the partial pressures of all the kinds of gases gives the
total pressure.
Peak height (also peak intensity). The maximum ion signal developed in the RGA for a given mass-
to-charge ratio peak.
Peak width (∆∆Mv%). The difference between the mass-to-charge ratio values on either side of a mass
peak at which the signal has dropped to v% of the peak height, H. See also Resolution.
Principal mass peak. The most intense peak in the fragmentation pattern of any molecule.
Probe (also RGA Probe). Quadrupole mass spectrometer sensor consisting of an ionizer, a mass
analyzer and a detector (Faraday cup or optional electron multiplier).
QMS - quadrupole mass spectrometer
Repeller (Ionizer Component). The repeller grid cage completely encloses the ionizer, is biased
negative relative to the filament, and prevents the loss of electrons from the ion source.
Residual Gas Analyzer (abbreviation: RGA). A compact mass spectrometer used to analyze the
residual gas composition in a vacuum system (same as Partial Pressure Analyzer).
Resolution, or Absolute Resolution, ∆∆M
. The width ∆∆M of the pass band of the filter, defined
10%
as the full width at which the ion current falls down to 10% of the maximum value. In mass-tocharge ratio units. See also Peak width.
Resolving Power: R= M/∆∆M
resolution, ∆∆M
SRS QMS Gas Analyzer
, at that mass. Dimensionless ratio.
10%
: Ratio between a particular mass-to-charge ratio M and the
10%
Page 77
Glossary
Response time - is a measure of the steepness of the response when a step change is presented at the
inlet.
RGA - residual gas analyzer (a class of QMS)
RGA Cover Nipple. CF Nipple that covers the RGA Probe.
Scan Speed (mass spectrometer). The speed at which the RGA scans through a range of successive
mass numbers.
Scanning. The procedure of continuously changing the mass tuning of the quadrupole mass
spectrometer to bring successive mass numbers into tune.
Sensitivity calibration. The act of establishing a correspondence between the change in ion current
and the corresponding change in partial pressure of the gas from which the ion is produced. The
correspondence might be represented graphically or as a table of values. See also Sensitivity.
Sensitivity.
Partial Pressure: The partial pressure sensitivity of the RGA to a gas g, Sg, is defined as the ratio
of the change (H-H
) in principal mass peak height to the corresponding change (P-P0) in total
0
pressure due to a change in partial pressure of the particular gas species. H0 and P0 are
background values: S
= (H-H0) / (P-P0). The units of S
g
are of ion current per unit pressure
g
(amp/Torr, for example).
Total Pressure: The total pressure sensitivity of the RGA to a gas g, is defined similarly as the ratio
of the change (H-H
) in total ion current to the corresponding change (P-P0) in total pressure due
0
to a change in partial pressure of the particular gas species. H0 and P0 are background values.
Space charge. The electrical charge carried by a cloud of free electrons or ions. Space charge can
causes serious changes in the potential distributions of the ionizer, and ultimately limits the pressure
operating range of all RGA’s.
STP. Standard temperature, 273.15 K, and pressure, 1.01325 bar, for gasses. Volume alone is
insufficient to describe the flow of gasses. When temperature and pressure are also defined, the
state is completely defined.
Total Pressure. The average normal force per unit area exerted by all the gas molecules impacting on
the internal surfaces of the vacuum chamber. Units are typically Torr, mbar or Pascal. Note: The
SRS RGA is capable of collecting total ion currents that can be turned into total pressure
measurements with the help of gas dependent total pressure sensitivity factors.
UHV. Abbreviation for Ultra High Vacuum. Pressure < 10-9 Torr.
SRS QMS Gas Analyzer
Page 78
References
General RGA information
Dawson, “Quadrupole Mass Spectrometery and Its Applications”, AIP Press, NY, 1995.
Drinkwine and D. Lichtman, “Partial Pressure Analyzers and Analysis”, AVS Monograph Series
published by the Education Committee of the American Vacuum Society
Basford et. al., J. Vac. Sci. Technol., A 11(3) (1993) A22-40: “Recommended Practice for the
Calibration of Mass Spectrometers for Partial Pressure Analysis. Update to AVS Standard 2.3”.
Batey, Vacuum, 37 (1987) 659-668: “Quadrupole Gas Analyzers”
Fu Ming Mao et. al., Vacuum, 37 (1987) 669-675: “The quadrupole mass spectrometer in practical
operation”
Dawson, Mass Spectrometry Reviews, 5 (1986) 1-37: “Quadrupole mass analyzers: Performance,
design, and some recent applications”
Vacuum Diagnosis
Studt, R&D Magazine, October 1991, p. 104: “Design Away Those Tough Vacuum System Riddles”
Applications of RGAs to process control
O’Hanlon, J. Vac. Sci. Technol. A 12 (4), Jul/Aug 1994: “Ultrahigh vacuum in the semiconductor
industry”
Vic Comello, R&D Magazine, September 1993, p. 65: “Process Monitoring with “Smart” RGAs”
Waits, et. al., Semiconductor International, May 1994, p. 79: “ Controlling your Vacumm Process:
Effective Use of a QMA”
Rosenberg, Semiconductor International, October 1995, p. 149: “The Advantages of Continuous
On-line RGA Monitoring”.
Lakeman, Semiconductor International. October 1995, p. 127: “Increase overall Equipment
Effectiveness with In Situ Mass Spectrometery”.
Quantitative measurements
Bley, Vacuum, 38 (1988) 103-109: “Quantitative measurements with quadrupole mass spectrometers:
important specifications for reliable measurements”
Cowen, et. al., J. Vac. Sci. Technol. A 12(1), Jan/Feb 1994: “ Nonlinearities in sensitivity of
quadrupole partial pressure analyzers operating at higher pressures”
Multiple linear regression analysis algorithms
William H. Press, et. al., 1992, Numerical Recipes in C, The Art of Scientific Computing, Second
Edition, Cambridge Univ. Press, section 15.4, page 671.
Bevington, P.R., 1969, Data Reduction and Error Analysis for the Physical Sciences, New York,
McGraw-Hill, Chapters 8-9.
SRS QMS Gas Analyzer
Page 79
Technical Reference Manual
QMS 100 Series
Gas Analyzer
Version 3.2 (1/2012)
Page 80
Certification
Stanford Research Systems certifies that this product met its published specifications at the time
of shipment.
Warranty
This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.
Service
For warranty service or repair, this product must be returned to a Stanford Research Systems
authorized service facility. Some components may be serviceable directly from the supplier.
Contact Stanford Research Systems or an authorized representative before returning this product
for repair.
Trademarks
Ultra-Torr®, Swagelok® and VCR® are registered trademarks of Swagelok Co.
Tygon® is a registered trademark of Norton Co.
All other brand and product names mentioned herein are used for identification purposes only,
and are trademarks or registered trademarks of the respective holders.
Information in this document is subject to change without notice.
Copyright © Stanford Research Systems, Inc., 1996, 2006, 2012. All rights reserved.
Stanford Research Systems, Inc.
1290-D Reamwood Avenue
Sunnyvale, California 94089
408 744 9040
Printed in USA
Page 81
Table of Contents
Safety ........................................................................................................................................................................... iv
Specifications .................................................................................................................................................................v
Materials List .............................................................................................................................................................. vii
Command List ............................................................................................................................................................ viii
Chapter 1. Principles of Operation ............................................................................................................................ 1-1
Internal View .................................................................................................................................................. 1-2
Overall System ............................................................................................................................................... 1-3
Pressure Reducing Inlet .................................................................................................................................. 1-5
Capillary Design ........................................................................................................................................... 1-10
Chapter 2. Quadrupole Spectrometer ......................................................................................................................... 2-1
Introduction .................................................................................................................................................... 2-2
ECU ................................................................................................................................................................ 2-3
Electrometer ................................................................................................................................................... 2-4
Mass Filter Power Supply .............................................................................................................................. 2-6
Ionizer............................................................................................................................................................. 2-7
Quadrupole mass filter ................................................................................................................................. 2-11
Ion Detector .................................................................................................................................................. 2-16
Chapter 3. Programming the RGA ............................................................................................................................. 3-1
Introduction .................................................................................................................................................... 3-3
The RGA COM Utility ................................................................................................................................... 3-4
Command Syntax ........................................................................................................................................... 3-5
Communication Errors ................................................................................................................................... 3-7
Programming the RGA ECU .......................................................................................................................... 3-9
RGA Command Set ...................................................................................................................................... 3-25
Error Byte Definitions .................................................................................................................................. 3-64
Chapter 4. Trouble Shooting ...................................................................................................................................... 4-1
System Problems ............................................................................................................................................ 4-2
Internal Error Detection in the SRS RGA ...................................................................................................... 4-4
Windows Software Error Codes ..................................................................................................................... 4-8
Chapter 5. Service ...................................................................................................................................................... 5-1
Warnings! ....................................................................................................................................................... 5-2
Component Notes ........................................................................................................................................... 5-3
Accessing Internal Components ..................................................................................................................... 5-7
General Checks ............................................................................................................................................ 5-11
Cleaning ....................................................................................................................................................... 5-13
Component Replacement .............................................................................................................................. 5-20
Replacement Parts ........................................................................................................................................ 5-28
Factory Service ............................................................................................................................................. 5-29
Appendix A. System Electronics .............................................................................................................................. A-1
Description of Schematics ............................................................................................................................. A-2
Parts List ........................................................................................................................................................ A-4
Appendix B RGA Circuit Description
Overview of the RGA ................................................................................................................................... B-3
Circuit Description ........................................................................................................................................ B-4
Description of Schematics ............................................................................................................................. B-6
Parts Lists .................................................................................................................................................... B-18
Appendix C. Drawings & Schematics ....................................................................................................................... C-1
....................................................................................................................... B-1
SRS QMS Gas Analyzer Technical Reference
Page 82
iv
Safety
Line Voltage
The QMS system is specified for power line of either 110 V / 60 Hz or 220 V / 50 Hz. The
diaphragm pump will only operate on the specified voltage. Operating at other voltages will
damage the motor. For 110 V operation use one 3 A fuse. For 220 V operation, two 1.5 A fuses
must be used in the power entry module.
Exhaust
As shipped, the QMS system exhausts to the atmosphere. If the system is analyzing hazardous
gases, the user must make provisions to handle the exhaust from the system. A standard 1/4 inch
connection is provided for this purpose.
Ventilation
The QMS system requires forced air cooling to operate at a reasonable temperature. Do not
block the air inlet or exhaust on the back of the unit. Components will fail without this cooling.
Elastomer Seals
Silicone has been reported to react adversely and irreversibly with the glass contained in an
electron multiplier. In systems containing an RGA w/electron multiplier, do not use silicone
greases or oils on seals; use only hydrocarbon based materials.
Lifting
The QMS system is heavy. Two people are recommended for lifting the system. The handle is
provided for occasional use and short periods. Do not hoist or hang the system by the handle.
SRS QMS Gas Analyzer
Page 83
v
p
p
Specifications
Inlet
Type capillary: available in stainless steel, PEEK, and glass lined
lastic
Flowrate 1 to 10 milliliter per minute at atmospheric pressure
Response time <400 ms
Pressure selectable from 10 mbar to 1 bar
Mass Spectrometer
Operational:
Mass filter type Quadrupole
(Rod diameter: 0.25”, rod length: 4.5”)
Detector type Faraday cup (FC) - standard
Electron multiplier (CDEM) - optional
Resolution
(per AVS standard 2.3).
Greater than 0.5 amu @ 10% peak height
Adjustable to constant peak width throughout the entire
mass range.
Sensitivity (A/Torr)* 2.10-4 (FC)
<200 (CDEM). User adjustable throughout high voltage
range.
Minimum detectable partial
ressure (MDPP)*
5.10
5.10
* Measured with N
Torr (FC).
-14
Torr (CDEM).
@ 28 amu with 1 amu full peak width
2
-11
@ 10% height, 70 eV electron energy, 12 eV ion energy
and 1 mA electron emission current.
Operating pressure range 10-4 Torr to UHV (FC)
-6
Torr to UHV (CDEM)
10
Max. bakeout temperature
(without ECU)
Technical Reference
350 C (FC)
300 C (CDEM)
Page 84
vi
p
Recommended bakeout
100 - 250 C
temperature
Ionizer:
Design Open ion source. Cylindrical symmetry
Operation Electron impact ionization.
Material SS304 construction
Filament Thoriated Iridium (dual) with firmware protection. Field
replaceable.
Degas 1 to 10 W Degas ramp-up.
Electron energy 25 to 105 V, programmable.
Ion energy 8 or 12 V, programmable.
Focus voltage 0 to 150 V, programmable.
Electron emission current 0 to 3.5 mA, programmable.
System
Pumps hybrid turbomolecular/drag pump, 70 liter/s,
ultimate pressure 2 10-9 mbar
diaphragm pump with ultimate pressure less than 1 mbar
rotection class IP44
Materials
(see full materials list for details)
construction: SS304 and SS316
insulators: alumina, ceramic
seals: Viton, buna-N, and nitrile butyl rubber
misc: aluminum, Tygon
General
Startup time 2 minutes from full stop
Max. Ambient Operating
35 C
Temperature
Power requirement either 110 V / 60 Hz or 220 V / 50 Hz (not field
selectable) less than 600 W total
Dimensions
44 cm H 20 cm W 61 cm D ( 17 in H 8 in W 24 in
D )
Weight 34 kg (75 lb. )
SRS QMS Gas Analyzer
Page 85
vii
Materials List
SRS receives many requests for information about corrosion compatibility. It is our policy not to
state the compatibility of our system with various corrosive environments. We simply cannot
test the myriad combinations of environments that our customers use.
We do provide a list of all the materials exposed to the gas being introduced into the system.
Our expectation is that users who need to measure corrosive environments already have some
type of system that creates, handles and contains the corrosive gases. Given that they have
designed and operate said system, they are the best people to decide the compatibility of the
materials in our system with the specific corrosive environment.
The QMS system contains the following materials:
Body
304 stainless steel - high vacuum tube
316 stainless steel - quarter inch tube and fittings
molybdenum - electrical feedthrough
ceramic - electrical feedthrough
AgCuIn - braze material on feedthroughs
alumina - contained in the spectrometer
aluminum - body of diaphragm pump
Replaceable Components
glass - body of electron multiplier
chromium - surface of the electron multiplier
IrO
·ThO2 - filament of analyzer (can be substituted with tungsten)
2
Seals
copper - seals in the CF high vacuum flanges
316SS - major component of VCR
silver - a thin layer on the VCR
Viton - o-ring seal in the inlet fitting (can be substituted)
buna-N - seals in the valves
neoprene - diaphragms in diaphragm pump
nitrile butyl rubber(NBR) - diaphragm pump valves - backing line
Tygon
®
- exhaust from diaphragm pump (can be substituted)
®
seals
®
seals to prevent gauling
Technical Reference
Page 86
viii
Command List
Initialization
Name Description Parameters Echo
ID
IN
Ionizer Control
Name Description Parameters Echo
DG
EE
FL
IE
VF
Detection Control
Name Description Parameters Echo
Identification Query ? ID String
Initialization 0,1,2 STATUS Byte
Degas Ionizer 0-20, * STATUS Byte
Electron Energy 25-105, *, ? STATUS Byte or
query response
Electron Emission Current 0-3.50, *, ? STATUS Byte or
query response
Ion Energy 0, 1, *, ? STATUS Byte or
query response
Focus Plate Voltage 0-150, *, ? STATUS Byte or
query response
CA
CL
HV
MO
NF
Scan and Measurement Control
Name Description Parameters Echo
AP
HP
HS
MF
SRS QMS Gas Analyzer
Calibrate All none STATUS Byte
Calibrate Electrometer none STATUS Byte
CDEM’s High Voltage 0-2490, *,? STATUS Byte or
query response
Multiplier Option ? CDEM option
Noise Floor 0-7,? Query response
Analog Scan Points ? Query response
Histogram Scan Points ? Query Response
Histogram Scan Trigger 0-255, *, none Ion Currents
Final Mass 1-M_MAX, *, ? Query response
Page 87
ix
MI
MR
SA
SC
TP
Parameter Storage
Name Description Parameters Echo
MG
MV
SP
ST
Mass filter control
Name Description Parameters Echo
ML
Initial Mass 1-M_MAX, *, ? Query response
Single mass measurement 0, M_MAX Ion Current
Steps per amu 10-25,*,? Query response
Analog Scan Trigger 0-255,*, none Ion Currents
Total Pressure measurement 0, 1, ? Ion Current
CDEM gain storage 0.0000-2000.0000,? Query Response
CDEM Bias Voltage storage 0-2490, ? Query Response
Partial Pressure Sensitivity 0.0000-10.0000,? Query response
Total Pressure Sensitivity 0.0000-100.0000, ? Query response
Mass Lock 0.0000-M_MAX none
Tuning
Name Description Parameters Echo
CE
DI
DS
RI
RS
Error Reporting
Name Description Parameters Echo
ER
Calibration Enable Query ? Query response
DI parameter
0-255, *, ? Query response
(Peak Width Tuning)
DS parameter
-2.5500-2.5500, *, ? Query response
(Peak Width Tuning)
RF_Driver @0 amu
(Peak Position Tuning)
RF_Driver @ 128 amu
(Peak Position Tuning)
-86.0000-86.0000, *,
?,none
600.0000-1600.0000,
*, ?,none
Query response
Query response
STATUS Byte Query ? Query response
EP
Technical Reference
PS_ERR Byte Query ? Query response
Page 88
x
ED
EQ
EM
EF
EC
DET_ERR Byte Query ? Query response
QMF_ERR Byte Query ? Query response
CEM_ERR Byte Query ? Query response
FIL_ERR Byte Query ? Query response
RS232_ERR Byte Query ? Query response
Note: M_MAX= 100 for RGA100, 200 for RGA200 and 300 for RGA300.
SRS QMS Gas Analyzer
Page 89
Chapter 1.
Principles of Operation
The QMS consists of two main subsystems: the gas handling inlet and the mass spectrometer. These
two subsystems operate independently of each other. The gas handling subsystem delivers the sample
to the spectrometer chamber. It has no control of the spectrometer or communications with it. The
spectrometer only analyzes the sample. It has no knowledge of the pressure reduction or the gas
handling subsystem. This chapter describes the overall instrument and the components that are
responsible for delivering the sample gas to the mass spectrometer. The next chapter contains a
detailed discussion of the spectrometer.
In This Chapter
Internal View......................................................................................................................................................................1-2
Overall System...................................................................................................................................................................1-3
Power............................................................................................................................................................................1-3
Controller.....................................................................................................................................................................1-3
Structure.......................................................................................................................................................................1-4
Exhaust.........................................................................................................................................................................1-4
Pressure Reducing Inlet...................................................................................................................................................1-5
Flow Calculations.......................................................................................................................................................1-6
Diaphragm Pump.........................................................................................................................................................1-8
Turbo Pump .................................................................................................................................................................1-9
Capillary Design..............................................................................................................................................................1-10
Length & Bore...........................................................................................................................................................1-10
Materials and Fittings..............................................................................................................................................1-12
Extensions..................................................................................................................................................................1-13
SRS QMS Gas Analyzer
Page 90
1-2 Internal View
Internal View
Figure 1. Main components of the QMS are labeled:
1. microcontroller 2. aperture 3. sample valve 4. pressure gauge 5. bypass valve 6. quadrupole electronics
7. mass spectrometer 8. analyzer chamber 9. 24V power supply 10. turbo pump controller
11. diaphragm pump 12. turbo pump
SRS QMS Gas Analyzer
Page 91
Overall System 1-3
Overall System
The QMS was designed to be a self contained instrument. Only line power is required to operate the
instrument. One serial cable between the instrument and a computer is required to run the software.
No ancillary gas supplies are necessary.
Power
Internally the QMS uses line power (110 or 220) and 24 VDC. The line power is distributed to the 24
V power supply and a relay connected to the diaphragm pump. When the main power switch is turned
on, 24 V becomes available throughout the instrument. The turbo pump, spectrometer, control
electronics and fan use 24 V. The fan will operate whenever the main power switch is on.
Whenever main power is on, the mass spectrometer is active. As soon as the instrument is powered,
programs can communicate with the spectrometer. If the pumps have not been activated, then only a
limited number of the spectrometer’s commands will be operable. Most commands, e.g. turn on the
filament, will only work when the pumps are active. The identification command (ID?) can be used to
determine the presence of an active spectrometer. Because the spectrometer uses hardware
handshaking, programs can also check the serial port to determine the presence of an active
spectrometer.
The microcontroller is active as soon as the main power is on. It has control of the two pumps, two
valves, and the front panel.
Controller
The two valves and two pumps are directly controlled by the microcontroller. The microcontroller
considers the four switches on the front panel to be requests to turn on or off the associated component.
The rules programmed into the controller determine whether the request will be performed or rejected.
The two valves are actuated with 24 V solenoids, which are normally closed. The controller sends
these valves power when they need to be opened. The valves can only be fully open or fully close. To
limit the sudden inrush of high pressure gas when the capillary flow is started, the controller opens the
valve in short pulses.
A three phase motor drives the turbo pump. A separate turbo pump control board synthesizes the
motor drive from the 24 V supply. The microcontroller sends a signal to the turbo pump control that
indicates on or off. The turbo pump control responds with a signal that indicates that the pump is at full
speed, which the microcontroller monitors. The turbo pump control also outputs a current monitor that
indicates how much power the pump is consuming. This signal is displayed on the front panel as a rough
indicator of the pump power. The display is not intended to be analytic.
The diaphragm pump operates on line power. The microcontroller actuates the relay to turn the pump
on or off.
Technical Reference
Page 92
1-4 Overall System
The pressure gauge operates on 15 VDC, which is provided by the microcontroller. The gauge has onboard electronics that provide a setpoint comparison. The setpoint check is used by the microcontroller
to determine if the system is operating acceptably. The gauge also outputs an analog signal related to
the pressure. This signal is displayed on the front panel. The output of the gauge is not linear, and
thereby the spacing of the numbers on the display is irregular. The front panel display is too coarse to
be used analytically.
Structure
The QMS is built with a thin aesthetic skin surrounding a structural steel chassis. Figure 1 shows the
instrument with the skin, bezel, and main side panel removed. (The Service chapter discusses disassembly.)
As seen in Figure 1, most of the components are attached to the left wall of the chassis (as viewed from
the front). Because of this, if the instrument is to be laid on its side, the left side is preferred; the weight
of the components is directly supported.
For mounting the instrument, the feet and handle can be removed. The threaded inserts provide
convenient mounting locations. The handle holes accept 1/4-20 bolts and the feet accept 6-32 screws.
Note that the inserts only support tension; they will push-out if large thrust loads are applied. Do not
permanently hang the instrument from the handle mounts, support the weight from the bottom. There
are components behind the threaded inserts. Do not use screws that extend more than 1/2 inch into the
chassis.
Exhaust
The exhaust connection on the back panel is provided for handling hazardous gases. The connection is
a 1/4 inch Swagelok tube stub. The slight indent is intended to provide a perfect seal to other Swagelok
tube fittings. Use with other brand tube fittings cannot be recommended.
The exhaust can also be used to operate the QMS in a closed loop configuration, i.e. the exhaust
stream is returned to the supply. The closed loop operation is feasible because the QMS does not
significantly change the sample gas. The amount of the sample that is ionized by the mass spectrometer
is microscopic, less than 1 nanoliter per minute (STP). The ionizer produces a microscopic amount of
CO2 , typically less than 50 nanoliters per minute (or 26 ml per year). No air leaks into the system and
no carrier or detector gases are added to the sample. Applications like glove box monitoring are well
suited to closed loop operation, because the instrument does not require a continuous bleed from the
box.
SRS QMS Gas Analyzer
Page 93
Pressure Reducing Inlet 1-5
[
]
exp
Pressure Reducing Inlet
The gas handling subsystem is designed to achieve several goals:
• reduce the pressure of the sample gas to the operating range of the mass spectrometer
(<10-5 mbar)
• provide a quick response time to changes of sample composition at the inlet
• allow for easy connection to system being measured
• use conventional materials
If only the first goal where important a single stage pressure reduction would be suitable.
For example, about 50 cm of 50 µm capillary would perform the required
pressure reduction. In a single stage design, all the gas that enters the capillary is
delivered to the spectrometer chamber. For a 70 l s-1 pump, the flowrate at the
capillary inlet is 70 nl s-1 ! The velocity of the gas near the capillary inlet is very
small. To demonstrate, the figure at the right shows the end of a 1/16 OD capillary
and a hemi-spherical boundary of the same diameter. The time it would take to
drain the volume inside the boundary is a measure of the response. For the small
volumetric flowrate in this example, it would take 30 seconds to drain the tiny
volume. This low speed implies that the capillary relies on diffusion or forced
convection to respond to concentration changes.
The effect of a low flowrate on leak detection is catastrophic. Imagine trying to
locate a leak by moving the capillary tip around a fitting on a gas line. With the capillary in the example
above, you could be 1/16 of an inch away from the leak source and not detect it for 30 seconds. To
locate the leak, you would either have to move incredibly slow, or be exactly at the leak.
Low flowrates also severely restrict the inlet. At the left is shown a capillary
with a syringe tip filter installed. The gap between the filter element and the
end of the capillary (about 1/16 inch in this example) creates a dead volume.
If this volume is approximated to be well mixed, the time constant is over 5
seconds. The time constant relates to the rate at which a system responds to
a step change and is characterized by the familiar
In this example, a sudden change outside the filter would not be completely
detected for over 15 seconds.
To achieve fast response, the QMS uses a bypass flow configuration, which
draws 200-1000 times larger flowrates through the capillary. The same
amount of sample is delivered to the spectrometer. As the name implies, most
of flow is bypassed around the spectrometer. Also the pressure drop across
the capillary is now only a factor of 1000 (as opposed to 1 billion). This
means the capillary can either have a larger bore or be shorter than in a single stage system. The lower
pressure drop makes feasible many capillary materials commonly used in gas chromatography. The
following sections discuss details of the flow of the sample gas within the instrument.
1− −
t tc response.
Technical Reference
Page 94
1-6 Pressure Reducing Inlet
(
)
Q
in out
(
)
Q
total cap a
cap a
(
)
Q
total
Flow Calculations
The pressure and flowrates of the sampled gas can be calculated with simple formulas. The calculations
here assume that gases behave ideally, which is a reasonable approximation at the temperatures and
pressures involved. Actual system performance compares well with these simple calculations.
The pressure drop across a length of tube is related to the flowrate and dimensions by:
C P P C P
= − = ∆ (1)
where Q is the throughput, C is the conductivity, and ∆P is the pressure drop. Throughput is a measure
of mass flowrate commonly used in vacuum systems. Typical units for Q are mbar liter s-1, which unless
stated otherwise, implies a standard temperature. At STP (273.15 K and 1013.25 mbar) 1 mbar liter
s-1 is equal to 1.013 cm3 s-1 (sometimes abbreviated sccs). The conductivity of various geometries is
calculated with the standard formulas available in texts discussing vacuum (see References).
The pressures and flow at every point in the system is determined by applying equation 1 to each
section of tube. A simplified flow schematic of the QMS is shown below.
P
C
a b
cap ap
C
P
P
c
RGA
aperture
capillary
turbo pump
diaphragm pump
P
d
Figure 2. Schematic of key components of system.
For this example there are four points at which the pressure is unknown, Pa, Pb, Pc, and Pd. Applying
equation 1 between each pair of points will yield a set of equations to solve. First all the sampled gas
flows through the capillary:
C P P C P
= − ≅ . (2)
b
Because Pa >> Pb , the approximation can be made. When the flow reaches the tee it is split into two
streams, bypass and sample. The sample flow is a small fraction of the total flow, so we can assume
that the bypass flow is equal the total and write the equation from the tee to the diaphragm pump as:
C P P
= − , (3)
bd b d
SRS QMS Gas Analyzer
Page 95
Pressure Reducing Inlet 1-7
(
)
Q
sample ap
c ap
Q
c sample
=
Q
total
=
where Cbd is the conductivity of the tube from the tee to the diaphragm pump. The flow through the
aperture is:
C P P C P
= − ≅ , (4)
b
b
where again the large pressure drop allows the approximation to be used. The turbo pump is an active
component that is characterized by
P
S
, (5)
where S is the speed of the pump and has the same units as conductivity (liter s-1 ). For the pump in the
QMS, the speed is a constant at 70 liter s-1 (except for He and H2). Lastly, the diaphragm pump
equation is
P
d
S
. 6
DP
The speed of the diaphragm pump is not a constant; it is a function of the inlet pressure. A typical speed
curve is shown in the graph below.
The curve shows an important feature: the speed goes
to zero at a finite pressure. This pressure is the
ultimate pressure of the pump. At this pressure, gas
travels backward through the pump at the same
speed at which it is being pumped forward.
Therefore, even though the pump is still operating, it
effectively has no speed.
Equations 2 to 6 completely describe the system.
speed (liter/minute)
Solution of the entire set would determine all the
unknowns, except that there are more unknowns than
equations. Typically, the inlet pressure is know and
the desired pressure at the spectrometer is known.
0
inlet pressure (mbar)
The tubing dimensions determine Cbd leaving 6
unknowns. To simplify the calculations, Pd is chosen
Figure 3. Speed curve for a diaphragm pump.
to be an acceptable value and the remaining values
are solved for.
These equations demonstrate some important characteristics of the QMS.
• The pressure in the spectrometer chamber is directly proportional to the pressure at the exit of
the capillary and largely controlled by the aperture. Choosing a different aperture has
insignificant effect on the remainder of the system.
• The mass flowrate through the capillary is insensitive to changes in pressure at the exit of the
capillary. This characteristic is very helpful to predicting the performance of different capillaries.
If this where not true, the exit pressure of the capillary would be complexly related to the speed
curve of the diaphragm pump. (This can happen when designing capillaries for low pressure).
• The pressure at the exit of the capillary is dominated by the speed curve of the diaphragm pump
and the mass flowrate through the capillary.
Technical Reference
Page 96
1-8 Pressure Reducing Inlet
1 1 2 2
=
These last two characteristics greatly simplify the selection of alternate capillaries and is discussed later
in this chapter.
Diaphragm Pump
A measured speed curve for the diaphragm pump is
shown in the figure at the right. The speed is the
volumetric flowrate at that pressure. Because
mechanical pumps have much lower flowrates than
turbo pumps, the speed is usually expressed in
volume per minute. A pressure of 1 mbar is a typical
operating point for the QMS, which means the pump
speed is 1.5 liter min-1 . The mass flowrate at the
diaphragm pump and capillary inlet are the same. At
the higher pressure of the capillary inlet, 1000 mbar,
the corresponding volumetric flowrate is 1.5 milliliter
min-1. The simple relation
P V P V
is a good approximation as long as temperature is
constant. The speed is 2.6 liter min-1 at 5 mbar,
which corresponds to 13 milliliter min-1 at 1000 mbar.
Figure 4. Measured pumping speed vs. the pressure at
the pump inlet for the pump in the QMS.
The turbo pump only operates with exhaust pressures
up to approximately 5 mbar, which limits the useful range to that shown in Figure 4. Thereby, capillaries
are always chosen to draw volumetric flowrates of 1-10 ml min-1. The complete speed curve would
keep increasing up to the pumps specification of 13 liter min-1 at atmospheric pressure.
3
2.5
2
1.5
1
speed (liters/min)
0.5
0
0 2 4 6
pressure (mbar)
The ultimate pressure of the diaphragm pump can age, mainly by degradation of the valve seats. This
aging will shift the zero intercept of the speed curve (Figure 4) to higher pressures. For many operating
pressures the effect is minimal, but pressures near the ultimate pressure will show drastic speed changes.
The implication is that operating the QMS near the ultimate pressure of the diaphragm pump requires
careful monitoring and should be avoided when feasible.
SRS QMS Gas Analyzer
Page 97
Turbo Pump
The pump attached to the
spectrometer chamber is hybrid
turbomolecular/drag pump. The
hybrid design of this pump allows it to
exhaust at high pressure (relative to
conventional turbomolecular pumps).
The pumping speed is constant at the
nominal value of 70 l s-1 over a large
range of exhaust pressures. As the
exhaust pressure approaches the
maximum value, the speed begins to
drop. Figure 5 shows a
representative turbo pump speed
curve, overlaid with the speed curve
of the diaphragm pump. The two
curves overlap for a small region of
pressures, which determine the
operating range of the system.
Pressure Reducing Inlet 1-9
turbo pump
diaphragm pump
speed
pressure
Figure 5. Representative speed curves for two pumps. The speed
values have been scaled to show both pumps on the same graph. The
pressure is the exhaust of turbo and the inlet of the diaphragm pump.
In the case the QMS, the overlapping region is 1-5 mbar. As the turbo pump exhaust pressure
increases approaches 5 mbar, the work it performs will increase. The current monitor display on the
front panel is a good indicator of the power consumed by the pump. Running the pump near its limit
mainly causes the bearing temperature to increase. The turbo pump controller will detect if the bearings
are overheating and avoid damage by shutting down the pump. Long term operation near the
temperature limit mainly will age the bearings more quickly. The only advantage to operating the turbo
pump at higher exhaust pressures is an increase in the flow rate through the capillary. The higher
flowrate can help response time, but given the cost of a turbo pump rebuild, response time is better
addressed through capillary design.
Technical Reference
Page 98
1-10 Capillary Design
L
2
Capillary Design
The inlet uses a bypass configuration that results in a fast response time. A large flow is drawn through
the capillary tube, which drops the pressure 3 decades. The typical capillary used at atmospheric
pressure has a bore diameter of 0.125 mm and a length of 0.7 m. Any number of combinations of
length and bore diameter can achieve the same flowrate and pressure drop. Capillaries are available in
several materials. The factors affecting the choice of capillary are:
• inlet pressure
• required response time
• distance to sample point
• material restrictions
• cost
The possibilities for capillary choice are numerous, and SRS offers only a few types. The standard
capillary shipped with the QMS is mainly provided to test the system and provide a reference. The
following sections contains some guidelines to designing a capillary suited to the users application.
Length & Bore
To choose the dimensions of the capillary, three parameters must be fixed: the inlet pressure, exit
pressure, and flowrate. These three values allow the conductivity to be determined (equation 1). A
typical design point is an exit pressure of 1-2 mbar and a corresponding flow rate of 1.5 to 3.5 milliliter
min-1 (see figure 4). The inlet pressure is determined by the users application. The capillary conductivity
is a function of the geometry, pressure, temperature, and gas properties. A common formula for air
flowing through a tube at 20°C is
4
P P
d
C
=+135
where C is the conductivity in l s-1 , d and L are in cm, and the pressures are in mbar. More general
formulas including temperature and viscosity are contained in the texts listed in the references. This
formula also assumes laminar flow.
There are many approximations used in equation 7 and users may be concerned about its accuracy.
The formula shows that the conductivity is a strong function of diameter. As a practical consequence,
this strong dependence means the standard manufacturing tolerances on bore diameters will cause more
uncertainty than the formula itself. A typical 0.005 inch bore capillary might have a ±10% tolerance.
While it is reasonable that the bore could vary from 0.0045 to 0.0055 inch, this uncertainty causes the
conductivity to vary by about ±40%. Use the standard formulas as a guide, but cut the capillary long to
begin with. Measure the actual performance and trim as necessary.
1 2
, (7)
SRS QMS Gas Analyzer
Page 99
Capillary Design 1-11
The chromatography industry uses a large variety of capillaries, from which we can select capillaries for
the QMS. The figure below shows the conductivity for several commonly available bore diameters.
1E-02
1E-03
7
1E-04
4
1E-05
6
5
C (liter/s)
1E-06
1E-07
1E-08
1 10 100 1000
Figure 6. Conductivity of five different capillaries as a function of length. The curves are labeled with the bore
diameter in thousandths of an inch. Both axis are logarithmic.
2
length (cm)
As an example, consider a capillary for atmospheric pressure and an capillary exit pressure of 1 mbar.
From the speed curve for the diaphragm pump, the throughput, Q, is 2.5 × 10-2 mbar liter s-1 (Q = P S)
and the required conductivity is 2.5 × 10-5 liter s-1. The horizontal dotted line in figure 6 shows that for
each capillary diameter there is an appropriate length. The application dictates which bore diameter is
appropriate. If the system being measured was far away from the QMS, 3 meters of the 7/1000
capillary would be best. On the other hand, for something close 0.7 meters of 5/1000 would be less
expensive. The gas velocity through the capillary quickly reaches the speed of sound. Thereby, until the
capillary becomes very long, it is not an important contributor to the response time of the QMS.
Technical Reference
Page 100
1-12 Capillary Design
Materials and Fittings
Users will find vendors of gas chromatography supplies a good source for capillaries and fittings.
Capillaries are available in many materials. No material is ideal for all applications. The following table
list features of several materials
material min. bore
diameter
stainless steel 0.005 in · rugged
PEEK 0.005 in · highly flexible
glass lined plastic 0.002 in · smallest bore · cost
advantages disadvantages
· difficult to cut without
· high temperature
· durable connections
· can be cut by user
clogging the bore
· marginal flexibility
· weaker connections
· not flexible
· low temperature
Three conventional methods are available for making connections to the capillaries: metal compression
fittings, graphite seals, and o-ring seals. Metal compression fittings are suitable for stainless steel tube.
The steel is capable of deforming to make the seal. The outside diameter of plastic capillaries is not
round enough to make a good seal to metal ferrule. Graphite ferrules in a metal fitting are a better
choice. The graphite will conform to any irregularities in the surface of the capillary. In addition, the
graphite ferrule does not permanently deform the capillary as a steel ferrule would. O-ring seals, e.g.
Ultra-Torr, make good seals. They only lack in the ability to operate at high temperature.
SRS QMS Gas Analyzer
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