Agilent Technologies 6890 Series Service Manual page 673

1150 Troubleshooting
Chromatographic symptoms
Baseline upsets
Frequently, baseline upsets may be seen on chromatograms when valves are
switched. These upsets are caused by pressure changes within the system,
injections of large volume samples, or by changing the amount of restriction
in the flow path. These upsets will become more of a problem when high
sensitivity is required. Addition of a fixed restriction downstream from the
valve may help minimize the upset. Changes in column length may also help
reduce the upsets.
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).
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Valves
Agilent 6890 Gas Chromatograph Service Manual
Jun 2001