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
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.
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.
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).
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
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.
O2 doping flow:
- Stop the "rungc" cycle -- click here for more help on shutting down
- Type "labdoit" in an open shell -- click here for more help
on opening a shell.
- 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".
- Before proceeding -- print a copy of the channel
4 flow measurement worksheet.
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
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
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.