Channel 4 -- HCFC-22, CH₃Cl, CH₃Br

Channel 4 is the most complicated channel on the Chromatograph for Atmospheric Trace Species (CATS).  It utilizes two gas sample valves (GSV) to manipulate the flow through a trap and two capillary columns (a pre and main column).  The trap is enclosed in a cryo-cooled chiller block which can be cooled to -75 C.  An 80 cc injection of sample (either air or a calibrated gas mixture) is flushed onto the cooled trap.  Once the air is captured the trap is rapidly heated to quickly drive the concentrated sample off to the columns for separation.

Skip the theory -- How do I check the flows?
 

Plumbing

The first figure is a state diagram of the two GSV and the trap heater.  The following figures show the plumbing in the three separate states: GSV 4 in load and GSV 5 in inject, GSV 4 in inject GSV 5 in load, and GSV 4 in load and GSV 5 in load.  In the second state, the trap heater is flashed on for a few seconds to quickly drive the sample off of the trap.  The trap heater is subsequently turned during the third state to clean the trap.

 

 

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The next three figures illustrate the flow paths of the Helium carrier gas and sample flow during an injection.  At time zero the sample loop is flushed with Helium carrier gas onto the cold trap (blue tubing).  If the trap is not cold enough the sample would be vented.  Also during this same time, the pre-column is backflushed (flow is in the opposite direction) to remove any species remaining on the column (green tubing).  The chromatogram (the signal from the ECD) is dependent on the main flow through the main column (pink tubing), the nitrogen (N₂) "makeup", and the oxygen (O₂) doping flow.

State 1

After about 300 seconds of "loading" the sample onto the trap both gas sample valves switch and the trap is flash heated (see the figure 1 for state diagram).  At this point the trapped sample is flushed though the pre-column and onto the main column (blue tubing).

State 2

About 500 seconds into the injection gas sample valve 4 switches back to load.  While in load, the trap is heated again to clean any remaining sample from the trap (blue tubing).  The flow through the pre-column is reversed thus flushing any remaining specie from the column.  The species of interest (HCFC-22, CH₃Cl, and CH₃Br) and few others that passed through the pre-column are now on the main column.  During the next several minutes the species are pushed and separated through the main column on their way to the electron capture detector (ECD).  Also during this state, the next sample is flushed through the sample loop in preparation for the next injection.

State 3

 Measuring the flows:

There are several important flows to measure on the fourth channel.  To make these measurements you will have to stop the instrument from its normal sequence.  Below is are step-by-step instructions for measuring the appropriate flows.

1. Stop the "rungc" cycle -- click here for more help on shutting down the cycle
2. Type "labdoit" in an open shell  -- click here for more help on opening a shell.
3. Once labdoit starts up you can manually move the gas sample valves from either the instrument itself or by sending a command from the "chroms" screen.  An example of a "chroms" screen command is "GSV 4 load".
4. Before proceeding -- print a copy of the channel 4 flow measurement worksheet.

O₂ doping flow:

This is a very slow flow of about 1 cc/min that is fed into a tee just prior to entering the detector.  Unplumb the doping line, cap or plug the tee, and measure the flow.  If using a digital flowmeter make sure to set the gas to air -- we use zero air (20% O₂ and 80% N₂) as the doping gas.

N₂ makeup flow:

The nitrogen is fed directly into the detector via the makeup port on the lid of the detector.  To measure the flow, unplumb the makeup line (make sure you use two wrenches -- one for the tube fitting and another for the bulkhead fitting on the can).  Plug the bulkhead.  Measure the flow.

Sample loop flush flow:

Set the GC to state 1 (GSV 4 to load and GSV 5 to inject).  Measure the flow out of the vent tubing on GSV 5 port 7.  As with most of the flows, it takes about a minute for the flow to stabilize after the GSV is switched.

Helium flow through main column:

With the GC still in state 1 unplumb the main column out from the O₂/ECD tee and plug or cap the tee.  Measure the flow from the column.  You can leave this unplumbed temporarily for a later measurement.

Helium backflush flow through pre-column:

Still in state 1 measure the flow from GSV 4 port 10

Main Helium flow through trap and both columns:

Switch to state 2 (GSV 4 to inject and GSV 5 to load).  Wait at least a minute for the flows to stabilize.  Measure the flow out of the main column leading into the tee.  (Same place as the "Helium flow through main column" measurement).  Plumb the column to the tee.