Flame ionization detector response for the carbonyl carbon atom in the carboxyl group of fatty acids and esters

Abstract

The response of the carboxyl group in free fatty acids, although markedly less than the equivalent response for a methylene carbon atom in the lower fatty acids, approaches this value in acids with six or more carbon atoms. The relative responses for these higher fatty acids are then proportional to the relative weight percent carbon content on a weight basis. In esters a complex molar response pattern is evident, apparently due to scission of the ester linkage with the lower acids to give an alcohol with a response of half a methylene carbon atom, and an acid or carboxyl group fragment with little or no response. In the methyl esters of saturated fatty acids with nine or more carbon atoms the net loss in response falls to the equivalent of one methylene carbon atom and the relative response is the proportional to the relative weight percent carbon content based on the number of carbon atoms in the fatty acid chain. Formate esters apparently give slightly higher responses than other esters of the same net number of carbon atoms. This suggests that the difference observed in the various esters may be due in the cases of those esters with higher responses to formation of a hydrocarbon in lieu of an alcohol during the initial scission of the ester group linkages. Alternatively in the esters other than formates the carboxyl group may give an increasing response as the fatty acid chain length increases.