SRS Labs QMS 100 Series User Manual
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)
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
iii
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
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.
SRS QMS Gas Analyzer
v
SRS QMS Gas Analyzer
vi
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
SRS QMS Gas Analyzer
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
SRS QMS Gas Analyzer
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. )
SRS QMS Gas Analyzer
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
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
SRS QMS Gas Analyzer
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
SRS QMS Gas Analyzer
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
SRS QMS Gas Analyzer
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.
SRS QMS Gas Analyzer
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
SRS QMS Gas Analyzer
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.
SRS QMS Gas Analyzer
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
SRS QMS Gas Analyzer
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.
SRS QMS Gas Analyzer
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.
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
SRS QMS Gas Analyzer
h igh
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
SRS QMS Gas Analyzer
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
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
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.
SRS QMS Gas Analyzer
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
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.
SRS QMS Gas Analyzer
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
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
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
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
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
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