HP 5890 Series II Plus Reference Manual page 215

Chromatographic Troubleshooting
Troubleshooting valve systems
Loss of peaks in specific areas of the chromatogram
Entire sections of chromatographic data can be lost due to a valve that
does not rotate or one that rotates improperly. Other than obvious
component failures (i.e., solenoid, actuator, etc.), generally improper
adjustments and misalignments cause most problems.
Baseline upsets
Frequently baseline upsets may be seen on chromatograms when valves
are switched. These upsets are normally 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. When possible,
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. Needle valves (Nupro) can be used as
adjustable restrictors; however, they are used typically where a matched
restriction is desired and 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.
Check that adequate air (about 482 kPa or 70 psi) is supplied.
Check if the valve is rotating at all.
If the valve rotates, check for proper alignment of the actuator or
mechanical binding or slippage of connecting parts.
Check for blocked flow paths with valve in both positions.
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