flame ionization detector
/ FLAYM eye-on-ih-ZAY-shun dih-TEK-ter /
Hold a candle near a sensitive radio and you might hear it crackle: a flame burning carbon-rich fuel briefly fills the air with charged fragments. A flame ionization detector turns that quirk into a measurement — it burns whatever leaves the column in a small hydrogen flame and counts the tiny burst of electrical charge each carbon-containing compound produces.
Formally, a flame ionization detector (FID) is a gas-chromatography detector in which the column effluent is fed into a hydrogen-air flame. When an organic compound burns, it momentarily creates ions and electrons; a voltage across the flame collects this charge as a small current, and the size of the current is proportional to the amount of carbon burning at that instant, so each eluting compound registers as a peak.
It matters because the FID is rugged, extremely sensitive over a very wide range, and responds to almost all organic (carbon-containing) compounds in roughly proportion to their carbon, making it the everyday workhorse detector for gas chromatography. Its honest limits are that it is blind to many compounds with little or no burnable carbon — water, carbon dioxide, the permanent gases — and that it destroys the sample as it burns it, so nothing downstream can re-examine what it consumed.
Checking a batch of solvent for purity, an analyst runs it on a gas chromatograph with an FID; the main solvent burns to give one large peak, and a faint extra peak betrays a trace organic impurity the flame readily senses.
Burn the eluate in a flame and count the charge: carbon compounds light it up.
Because its response tracks burnable carbon, the FID gives little or no signal for highly oxidised or carbon-poor species, so analysts sometimes pick a different detector when those are the target.