MECHANISM OF THE FLAME IONIZATION DETECTOR .2. ISOTOPE EFFECTS AND HETEROATOM EFFECTS

The relative molar flame ionization detection (FID) response (RMR) for a hydrocarbon does not change when deuterium is substituted for hydro gen. The exception is methane for which an inverse deuterium isotope e ffect of 3.5% is observed for tetradeuteromethane. [C-13]Methane shows an inverse isotope effect of 2%. The reason for the small or nonexist ent isotope effects is that H/H-2 exchange takes place in the pre-comb ustion hydrogenolysis in the flame. This was shown by taking samples f rom the lower part of the flame by means of a fused-silica capillary p robe. By the same technique the hydrogenolytic reactions in the hydrog en flame of compounds added to the hydrogen gas in low concentrations were followed. Alcohols, ethers, ketones, and esters all produced meth ane and carbon monoxide, while amines produced methane and hydrogen cy anide, halogen compounds methane and hydrogen halide, etc. The FID res ponse of an organic compound is the sum of the responses from the prod ucts formed in the hydrogenolysis. (C) 1997 Elsevier Science B.V.