Mechanism of the flame ionization detector. II. 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 hydrogen. The exception is methane for which an inverse deuterium isotope effect of 3.5% is observed for tetradeuteromethane. [¹³C]Methane shows an inverse isotope effect of 2%. The reason for the small or non-existent isotope effects is that H/2H exchange takes place in the pre-combustion hydrogenolysis in the flame. This was shown by taking samples from the lower part of the flame by means of a fused-silica capillary probe. By the same technique the hydrogenolytic reactions in the hydrogen flame of compounds added to the hydrogen gas in low concentrations were followed. Alcohols, ethers, ketones, and esters all produced methane and carbon monoxide, while amines produced methane and hydrogen cyanide, halogen compounds methane and hydrogen halide, etc. The FID response of an organic compound is the sum of the responses from the products formed in the hydrogenolysis.