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Often confused with baseline upsets, an offset is
a shift in the baseline that does not return
quickly to the original level. Baseline offsets may
be caused by air leaks but more commonly are
due to a change in gas purity or flow rate in the
detector. Poor carrier gas or improperly condi-
tioned filters and traps should be suspected
whenever offsets occur.
Variation in peak area and retention
time
The amount of sample contained in the loop and,
therefore, the amount injected onto the column is
affected by loop pressure and temperature. Vari-
ations in pressure and temperature lead to vari-
ability in peak areas. Flow restrictors or back-
pressure regulators help to maintain constant
loop pressure, and valve boxes help maintain
temperature.
Released: April 2004
Leaks can occur in the valve itself or at any of
the connecting points with transfer lines. Leaks
usually cause area irreproducibility, retention
times changes, and increases in the area of air
peaks (with thermal conductivity detectors).
Leaks in rotors can sometimes be fixed by tight-
ening the nuts holding the rotor in the valve body.
Leaks in connections are usually found with an
electronic leak detector or with a liquid leak
detection fluid (for example, Snoop).
Pressure check
Leak checking the plumbing involved in a valve
system must be done carefully and methodically.
The pressure check method below will indicate,
but sometimes not isolate, a leak in the flow
path. Since this method does not necessarily
isolate the leak, other leak check methods may
be needed to locate the leak specifically.
6850 Series Gas Chromatograph Service Procedures
337
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