Troubleshooting
6850
6850
Released: April 2004 6850 Series Gas Chromatograph Service Procedures 316
Flame photometric detector
Quenching effects
Hydrocarbon quenching occurs when a high
concentration of carbon dioxide from a hydrocar-
bon peak is in the flame at the same time as the
sulfur species. Part of the light emitted by the
sulfur species is absorbed by some CO
2
species.
Self-quenching occurs at high concentrations of
the heteroatom species. Some other ground state
(inactivated) species reabsorbs the emitted
photon, preventing it from reaching the PMT.
These effects are reduced by good chromato-
graphic practices. The column should provide
good separation of the compounds, those that
contain sulfur or phosphorus as well as those
that do not but may absorb light. A careful,
multilevel calibration is well worth the invest-
ment!
Detector and gas cleanliness must be maintained
to have consistent responses. Since most sulfur
and phosphorus compounds contain chemically
active sites, the injection and column systems
must be kept very clean.
PMT saturation
The photomultiplier tube may saturate if light
intensity is too high. When this happens, increas-
ing concentration produces little or no increase in
signal and peak tops are rounded or flattened.
The sample must be diluted to prevent satura-
tion.
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