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Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
350 Flame Photometric Detector
(FPD)
Theory of operation
In the Flame Photometric Detector (FPD), the sample burns in a hydrogen-
rich flame, where some species are reduced and excited. The gas flow moves
the excited species to a cooler emission zone above the flame where they
decay and emit light. A narrow bandpass filter selects light unique to one
species, while a shield prevents intense carbon emission from reaching the
photomultiplier tube (PMT).
The light strikes a photosensitive surface in the PMT where a light photon
knocks loose an electron. The electron is amplified inside the PMT for an
overall gain of up to a million.
The current from the PMT is amplified and digitized by the FPD electronics
board. The signal is available either as a digital signal on the communications
output or as a voltage signal on the analog output.
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