Variation In Peak Area And Retention Time; Pressure Check - Agilent Technologies 7820A Service Manual

Agilent 7820 GC Service
255
Fixed restrictors are used immediately before flame detectors to
prevent flameout and are used in some instances to prevent
pressure surges from damaging TCD filaments. An adjustable
restrictor (needle valve) can also be used where a matched
restriction is desired but not for preventing pressure or flow
surges.
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 conditioned 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. Variations in pressure and temperature lead to
variability in peak areas. Flow restrictors or back-pressure
regulators help to maintain constant loop pressure, and valve
boxes help maintain temperature.
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 tightening 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 (e.g., 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.
Each valve in a system has two flow paths, ON and
OFF. A leak sometimes occurs in only one of these
two positions. Check both.