is sensitive to temperature changes. The carrier gas must be exceptionally pure as oxygen, air and water
at levels exceeding about 10 ppm affect performance, as does column bleed. Halogenated solvents
should be avoided in sample preparation as residual traces can deactivate the detector.
Flame Photometric Detector (FPD)
This is a selective detector for phosphorus and sulphur-containing compounds which makes use of the
characteristic emission of radiation by and HPO• species in a suitable flame. Earlier versions
incorporated special burners to produce a flame with hydrogen enveloping the air/carrier gas mixture
(inverted or inside-out flame). More recently, modified FID burners have proved satisfactory (Figure
4.22). The emission at 394 nm and the HPO• emission at 526 nm are selected by means of
appropriate narrow bandpass filters and the lower half of the flame is shielded to reduce background
emission which is due largely to and CH•. The emission intensity can be monitored with a fibre
optics light pipe connected to a remote photomultiplier tube, as the response of the tube is temperature-
sensitive. An alternative design utilizes heat filters, a long tube and a metal block to act as a heat sink.
Sensitivity is high (Table 4.11) but the response to phosphorous is linear over a range of only 10^4 , while
for sulphur it is proportional to the square of the sulphur concentration, varies with the type of sulphur
compound and is linear over an even smaller range.
Figure 4.22
Schematic cross-section of a flame photometric detector (FPD).