Troubleshooting And Maintenance; Chromatographic Symptoms; Loss Of Sensitivity Or Excessive Drift; Loss Of Peaks In Specific Areas Of The Chromatogram - HP 5890 SERIES II Service Manual

TROUBLESHOOTING AND MAINTENANCE

Chromatographic Symptoms

Troubleshooting valves and their related plumbing is primarily a matter of systematic checking and verifica-
tion of unimpaired mechanical operation of any moving part. This requires an understanding of how the
valve functions internally and how the plumbing is configured. A plumbing diagram is essential for effective
troubleshooting. The following symptom cause list gives the most commonly encountered problems and
solutions found with valves.

LOSS OF SENSITIVITY OR EXCESSIVE DRIFT

Several possible causes exist for overall deterioration of the chromatogram. Contamination in the valve
requires a thorough cleaning. Internal leakage necessitates a complete disassembly and inspection of the
mating surfaces.
Poor temperature control may require a full check of electronic and thermal components.
Lack of proper conditioning techniques, columns, etc.
Failure or deterioration of other components (i.e., columns, detectors, etc.).

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 ad-
justments and misalignments cause most problems. 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 connect-
ing parts.
Check for blocked flow paths with valve in both positions.

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 chang-
ing the amount of restriction in the flow path. These upsets will become more of a problem when high sensi-
tivity 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 im-
mediately 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; howev-
er, 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
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