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.
CCheck that adequate air (about 482 kPa or 70 psi) is supplied.
CCheck if the valve is rotating at all.
CIf the valve rotates, check for proper alignment of the actuator or mechanical binding or slippage of connecting parts.
CCheck 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 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.
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