PROBLEMS OF ANALYZING C-18 CIS-FATTY AND TRANS-FATTY-ACIDS OF MARGARINE ON THE SP-2340 CAPILLARY COLUMN

The problems of analyzing isomeric fatty acids in partially hydrogenated oils by gas liquid chromatography (GLC) on an SP-2340 fused-capillary column are evaluated using methyl esters from margarine fatty acids and 44 synthetic C18unsaturated fatty acids. Satisfactory separations of the geometrical isomers are obtained for a given double bond system, but overlaps occur between different double bond systems and between some mono- and diethylenic, as well as between di- and triethylenic fatty acids. In margarine the major cis-octadecenoic isomers (18:1Δ6c-Δ10c) coeluted with four trans-isomers (18:1 Δ12t-Δ15t). This overlap is greater in the print type of margarine than in the tub type. Another overlap, quantitatively less important, occurs between 18:1Δ14c-Δ15c with some trans, trans-octadecadienoic acids. Because of these overlaps, the isomers cannot be determined by GLC alone on the SP-2340 liquid phase or any other cyanosilicone phase. Fractionation of fatty acids according to the degree of unsaturation via methoxybromomercuric adduct and subsequent geometrical isomer separation by silver nitrate thin-layer chromatography (TLC) prior to GLC is recommended. © 1990 Oxford University Press.