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5
With all outlet ports plugged, pressurize the system to 20 - 30 psi.
This can be accomplished by adjustment of manual pneumatic
controls or Electronic Flow Control. For information on setting
carrier gas pressure see the Basic Operation section of this
manual.
6
Shut off the carrier supply at the source and monitor the displayed
pressure at the GC for 15 minutes. The pressure should not drop
more than 0.5 psi in 15 minutes.
Leak checking ensures that there are no leaks in the GC system up to and
including the injector assembly. If the indicated pressure on the GC drops by
more than 0.5 psi during the 15 minute test period, this indicates that there is
a positive leak in the system. Finding such a leak, particularly if it is a small
leak, can be very difficult. In general the best approach is to systematically go
through the pneumatic system from the source and tighten each fitting until
the leak is eliminated. It is important to note that leaks are often found in the
carrier gas supply to the GC.
Locate leaks with an electronic leak detector or leak detection fluid. An
electronic helium leak detector is available from Varian (P/N 01-900153-01).
This device can detect helium concentration in the air as low as 2 ppm and is
very effective at identifying the precise location of a helium leak from a GC
system.
Gas Purifier
Replacement
The most common gas purifiers used in gas chromatography are moisture
traps for removing water vapor, hydrocarbon traps for removing low levels of
hydrocarbon contaminants and oxygen traps for removing low levels of
oxygen. In general the choice of filter depends on the application and
detector used. Carrier gas should contain less than 1 ppm oxygen, moisture
and trace hydrocarbon contaminants. A combined moisture / hydrocarbon
filter is recommended for general applications to maintain detector
background levels at their lowest possible level. If the detector used is an
ECD or MS an oxygen filter should be employed as well.
Carrier gas filters are typically located on the rear panel of the 3800. Varian
supplies several types of carrier gas purifiers. These include a combined
moisture / hydrocarbon trap, indicating moisture trap, and indicating or non-
indicating oxygen traps. While the choice of purifier depends on the individual
configuration and performance expectations, the following configuration is
recommended for maximum carrier gas purity and protection.
