The ECD uses a radioactive emitter (electrons) to ionize some of the carrier gas & produce a current.
The ECD is sensitive to compounds with high electron affinities (halogen-containing compounds).
Gas entering the detector is ionized by a high energy radioactive source that gives off electrons (often 63 Ni).
When organic molecules that contain electronegative functional groups, such as halogens, phosphorous, & nitro groups pass by the detector, they capture some of the electrons & reduce the current measured between the electrodes.
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.
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 & the photomultiplier tube (PMT) to isolate the appropriate emission beam.
The final component necessary for this instrument is a thermal filter to isolate only the visible & UV radiation emitted by the flame.