Flame Photometric GC Detector
Introduction
The reason to use more than one kind of detector for gas chromatography
is to achieve selective and/or highly sensitive detection of specific compounds
encountered in particular chromatographic analyses. The determination of
sulfur or phosphorus containing compounds is the job of the flame photometric
detector (FPD). This device uses the chemiluminescent
reactions of these compounds in a hydrogen/air flame as a source of
analytical information that is relatively specific for substances containing
these two kinds of atoms. The emitting species for sulfur compounds is
excited S2. The lambda max for emission of excited S2 is approximately
394 nm. The emitter for phosphorus compounds in the flame is excited HPO
(lambda max = doublet 510-526 nm). In order to selectively detect one or
the other family of compounds as it elutes from the GC column, an interference
filter is used between the flame and the photomultiplier
tube (PMT) to isolate the appropriate emission band. The drawback here
being that the filter must be exchanged between chromatographic runs if
the other family of compounds is to be detected.
Instrumentation
In addition to the instrumental requirements for 1) a combustion chamber
to house the flame, 2) gas lines for hydrogen (fuel) and air (oxidant),
and 3) an exhaust chimney to remove combustion products, the final component
necessary for this instrument is a thermal (bandpass) filter to isolate
only the visible
and UV radiation emitted by the flame. Without this the large amounts
of infrared
radiation emitted by the flame's combustion reaction would heat up
the PMT and increase its background signal. The PMT is also physically
insulated from the combustion chamber by using poorly (thermally) conducting
metals to attach the PMT housing, filters, etc.
The physical arrangement of these components is as follows: flame
(combustion) chamber with exhaust, permanent thermal filter (two IR filters
in some commercial designs), a removable phosphorus or sulfur selective
filter, and finally the PMT.
Schematic of a gas chromatographic flame photometric detector
These notes were
written by Dr. Thomas G. Chasteen; Department
of Chemistry, Sam Houston State University, Huntsville, Texas 77341.
© 1995, 2007, 2009.
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