Gow-Mac 5900 Series, Operating Manual

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GOW-MAC Instrument Co. 30
B. Sample Injection Valve: The most common use of the valve is for sample injection. The
gas sample valve may be a 6 port single sample loop valve or a 10 port dual sample loop
valve. A sample loop, a length of tubing of known volume, makes a connection between
two ports on the valve. The following procedure is for gas injection on a valve outfitted
with a sample loop.
The valve starts in the counterclockwise position to purge the sample through the loop
as the carrier gas passes through the valve to the column. When the valve is rotated
clockwise, the carrier gas pushes the sample from the loop and carries it through the
column. The valve is then returned to the counterclockwise position to refill the sample
loop and prepare for another injection.
Care must be taken to allow sufficient time for the sample loop to be completely filled
with sample before injection. This time is easily calculated from the carrier gas flow and
sample loop volume. Similarly, allow sufficient time for carrier gas to sweep the loop
before switching the valve from INJECT to PURGE.
Compressibility of the sample can be a problem when using a gas sample valve. If the
sample and the standard are at different pressures, there can be more or less volume
of gas in the loop. To yield reproducible results, always inject the sample at atmospheric
pressure to compensate for line pressure variations.
C. Column Selection Valve: The 8 port column selection valve is used to place one column
in-line with the gas injected from the sample loop into the DID, while the second column
is purging with carrier gas. This allows two different applications/impurity analysis on the
sample gas by two separate injections.