Flame-ionization detector response to methyl, ethyl, propyl, and butyl esters of fatty acids

The concept of theoretical response factors is not directly applicable to methyl esters of short-chain fatty acids (FA), since their carbon deficiency is larger than expected from theory. Substituting the methyl group by an ethyl, propyl, or butyl group improved the flame-ionization efficiency of fatty acid esters gradually, up to the point where the empirical response factors of the butyl esters were identical within experimental error to the theoretical values. Butyl esters of FA have a uniform flame-ionization detection (FID) response irrespective of the number of carbon atoms contained in the FA. They exhibit a carbon deficiency of 1.0, i.e. the carbonyl carbon atom does not respond, as expected from theory. Compared to methyl esters, which have a carbon deficiency of 1.4-1.5 for short-chain FA, use of butyl esters has the advantage that a precalculation of the FID response enables the analyst to judge whether the analytical system employed works properly and the data produced are accurate and reliable. Both acid (BF3 or H2SO4)-and alkali (butoxide)-catalyzed butyl ester preparation were equally effective, giving the analyst a choice of methods so that different analytical needs can be addressed efficiently. Computing response factors and comparing the theoretically expected values with those obtained experimentally gives the experimenter an indication whether the analytical system employed for FA profiling (transesterification plus the subsequent gas-liquid chromatographic separation and quantitation by FID) works properly. This setup is particularly useful for an accurate analysis of the FA profile of milk fat.