Isomer Subfractionation Effects in the Study of Longer Chain Monoethylene Fatty Acids in Hydrogenated Oils

Methyl esters of fatty acids from a commercial margarine based primarily on a partially hydrogenated marine oil showed four distinct bands of monoethylenic unsaturation when the methyl esters were examined by TLC on silica gel impregnated with silver nitrate. The compositions of the four bands were compared by GLC on open-tubular columns coated with Apiezon-L. It was shown that each band included part of each of the four major chain lengths of marine oils (C16, C18, C20, C22), with the longer chain lengths more concentrated in the most mobile band. Fractionation within each band was based on the trans isomers being more mobile than the cis isomers and to a further subfractionation of both cis and trans materials because of the greater mobility of isomers with unsaturation closest to the terminal methyl group of the chain. The application of both TLC and GLC to such complex mixtures of isomers requires extreme care to minimize overlaps of cis and trans materials.