Direct Measurement of Total trans- and cis-Octadecenoic Fatty Acids Based on a Gas--Liquid Chromatographic Class Separation of trans-18:1 and cis-18:1 Fatty Acid Methyl Esters

Abstract

A gas-liquid chromatographic (GLC) class separation is described whereby the contents of total trans- and cis-octadecenoic fatty acids can be obtained for relatively simple samples such as margarines and for more complicated food mixtures and biological specimens. The method is an adjunct to the standard process commonly used for quantitating the general fatty acid profile of a sample (extraction of lipids, preparation of methyl ester derivatives of fatty acids, GLC separation on Carbowax-20M capillary columns). There is no incremental work beyond the addition of one chromatographic run on a cyanosilicone capillary column at 242°C that lasts only 20-25 min. The chromatograms provide a composite trans peak and a composite cis peak with sufficient resolution between them to provide direct estimation of total trans- and cis-octadecenoic fatty acids. A concurrent analysis for the general profile of fatty acids in an unknown (using Carbowax20M columns) is included with every analysis for trans content to check for internal consistency and for general quality control.