UNITY 2TM
Air Server Series 2 Operators
Manual
Version 1.0
February 2008
1. Introduction to the Air Server Accessory for UNITY 2............................................................ |
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1.1. |
Summary of Operation............................................................................................... |
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2. Developing a UNITY 2-Air Server method for whole air / gas analysis ..................................... |
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2.1. UNITY 2-Air Server method ........................................................................................ |
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2.2. Setting the carrier gas type ........................................................................................ |
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2.3. Parameters for on-line mode methods ......................................................................... |
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2.4. |
On-line methods ....................................................................................................... |
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3. Automatic sequencing of whole-air / gas analyses................................................................ |
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3.1. |
Performing analyses .................................................................................................. |
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3.2. Using the sequence builder and creating sequences of runs ............................................ |
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3.3. Setting up your first sequence .................................................................................... |
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3.3.1. Editing individual lines in a sequence .................................................................... |
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3.3.2. Recycling sequences ......................................................................................... |
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3.4. |
Running the sequence.............................................................................................. |
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3.5. Starting at a predefined date and time....................................................................... |
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3.6. |
Stopping a sequence................................................................................................ |
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1. Introduction to the Air Server Accessory for UNITY 2
The UNITY 2 Thermal Desorber normally requires sample introduction by means of a standard sample tube. This could be a sorbent tube (used for diffusive or pumped sampling) or an empty tube into which solid or liquid samples are weighed for direct desorption. The Air Server accessory connects to UNITY 2 and extends the compatible sample range to include whole air / gas samples e.g. continuous monitoring of air / gas streams or samples collected in whole-air containers such as canisters or Tedlar bags. Tubes can still be desorbed using a UNITY 2 configured with the Air Server accessory.
The Air Server contains appropriate inert switching valves and link tubing to interface the accessory to UNITY 2. The Air Server is controlled via the UNITY 2 software. It features selection between three (or eight) inlet ports as standard, using inert, PTFE-body solenoids, and is compatible with gas-phase samples ranging in pressure from sub-atmospheric to 50 psig. Three ports are commonly used for sample, calibrant and zero gas streams, but could just as easily be used for three (eight) different samples.
1.1. Summary of Operation
The gas stream selected is directed out of the Air Server accessory (via the dryer - if one is fitted) and into the UNITY 2 cold trap via the Link tube. During this ‘sampling phase’, UNITY 2’s cold trap is electrically cooled and volatiles from acetylene to n-C14 can be quantitatively sampled and retained. The sample / standard gas, once stripped of its target VOCs, exits through the desorb flow vent and is directed to the ‘flow in’ port of the electronic mass flow controller on UNITY 2. It is not possible to split during sampling. Set and actual sampling flows are displayed via the UNITY 2 software.
If the sample / standard gas is pressurised (>10 psi), the flow through the entire UNITY 2-Air Server system is driven by this pressure. However if the sample is at low (<10 psi), atmospheric or just below atmospheric pressure, a pump (U-ASPM1 / U-ASPM2) is required to ‘pull’ the gas-phase sample through the system.
Note: the flow controller and the optional pump are located downstream of the cold trap and thus do not come into contact with the sample prior to trapping.
At the end of sampling and after a purge of carrier gas to eliminate air from the trap, the UNITY 2 focusing trap heats in the normal way, transferring compounds of interest to the analytical system and triggering the measurement cycle. The outlet split at this point may be entered in the software and is controlled by the mass flow controller. Collection of the next sample can begin, if required, as soon as the cold trap has re-equilibrated at its trapping temperature.
The UNITY 2-Air Server sequence of operation is as follows:
1.Standby
2.Leak test (optional)
3.Pressure release
4.Pre-purge
5.Sampling
6.Line purge
7.Trap purge
8.Trap fire
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Key Air Server applications include: -
Continuous on-line measurement of C2 to C10 hydrocarbons in ambient air using UNITY 2-Air Server with GC-FID
Process monitoring of trace level volatiles in CO2 using UNITY 2-Air Server with process MS
Process monitoring of aroma during food and beverage production using UNITY 2 Air Server combined with electronic nose technology
2. Developing a UNITY 2-Air Server method for whole air / gas analysis
2.1. UNITY 2-Air Server method
Once the software has been downloaded to the Air Server, the only operating mode available will be On Line Air and an on-line sampling method is displayed. The system is now ready for use with whole-air samples or for continuous on-line monitoring of air or gas streams.
2.2. Setting the carrier gas type
Under the ‘View’ menu select ‘Options’ and then ‘Gas’.
Access the carrier gas selection by clicking on the box with the small arrow to the right of the gas type.
The three carrier gas options - He, H2 and N2 - will then be displayed.
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Click on the desired gas and then click on the ‘OK’ button to select and set the carrier gas type.
2.3. Parameters for on-line mode methods
All on-line method parameters (except Sample Gas and Sample Channel) are accessed and changed in the same way as tube desorption method parameters in other method modes - see the UNITY 2 operators manual.
This section of the manual only covers those parameters which differ from those used in standard tube desorption methods. For common parameters, such as the flow path temperature, reference is made to the relevant sections of the main UNITY 2 operators manual.
Split on in standby: This is the same parameter used in tube desorption methods (see the UNITY 2 operators manual) except that, in on-line mode the flow through the split in standby is controlled by the electronic mass flow controller. The flow range is from 5-100 ml/min and may be set as required.
Leak test: Given that UNITY 2-Air Server systems are commonly used in a continuous or semicontinuous sequence by recycling or linking methods and given that there is no need to break into the sample flow path by using the lever mechanism to change a tube between samples, the leak test is optional in on-line mode.
However, if a sequence of air analyses is to be run in on-line mode and with the leak test deselected, it is recommended that an initial leak test be carried out to check system integrity before initiating the series of runs.
To do this, first set up the run with ‘leak test’ selected - i.e. check the appropriate box. Ensure that the UNITY 2 to Air Server Link tube is properly located in the desorption oven with the lever mechanism fully down.
Initiate the run using the ‘Start run’ command. Watch as the instrument performs the complete leak test. If the leak test is successful, stop the run immediately as described in the main UNITY 2 operators manual.
The on-line mode method can then be modified to de-select the leak test before running the sequence of samples. Save the method at this stage.
If the leak test is not successful, re-check the seal of the UASL link tube and all the connections between UNITY 2 and the Air Server.
Pressure release: The Air Server can be used to sample using a pump or directly from a pressurised container. In standby, the UNITY 2 trap and internal flow path is at the pressure of the column. In the case of a pressurised source, if the carrier gas pressure is greater then the pressure of the sample, then carrier gas will flow into the container when the sequence enters pre -purge. In order to avoid dilution of the sample with carrier gas the pressure of the UNITY 2 flow path is first reduced by venting carrier gas through the split line prior to opening any sampling valves. The duration of this vent is set in the View >Options>Sequence section of the UNITY 2 software
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