Flame ionization detector
The purpose of a FID is to detect compounds as they exit the gas chromatography column. The compounds that are exiting the column are in a gaseous state. Detection is done by first turning the gas compounds into ions and electrons and then collecting the charged molecules. The FID turns compounds into ions and electrons by putting them into a flame. The fuel used to make the flame in FIDs is usually made of a mixture of hydrogen and oxygen.  As these charged materials are collected, they make a current which is measured. The larger the current the more charged compounds are being collected. Currents for FIDs are usually about 10-12 amps and are measured by a high-impedance picoammeter.