Preparative separation of cis- and trans-isomers of unsaturated fatty acid methyl esters contained in edible oils by reversed-phase high-performance liquid chromatography.

In order to measure exactly the trans-fatty acids content in food materials, a preparative group separation of cis- and trans-isomers of unsaturated fatty acid methyl esters (FAMEs) was achieved by an isocratic reversed-phase HPLC (RP-HPLC) method. The trans-isomers of 16:1, 18:1, 18:2, 18:3, 20:1 and 22:1 FAMEs were readily separated from the corresponding cis-isomers by a COSMOSIL Cholester C18 column (4.6 mm I.D. x 250 mm, Nacalai Tesque) or a TSKgel ODS-100Z column (4.6 mm I.D. x 250 mm, TOSOH), using acetonitrile as the mobile phase. This method was applied for determining the trans-18:1 fatty acid content in partially hydrogenated rapeseed oil. The methyl esters of cis- and trans-18:1 isomers of the oil were collected as two separate fractions by the developed RP-HPLC method. Each fraction was analyzed by gas chromatography (GC) for both qualitative and quantitative information on its positional isomers. By a combination of RP-HPLC and GC methods, a nearly complete separation of cis- and trans-18:1 positional isomers was achieved and the trans-18:1 fatty acid content was able to be evaluated more precisely than is possible by the direct GC method. The reproducibility of cis- and trans-18:1 isomers fractionated by the RP-HPLC method was better than 98%. These results suggested that the preparative RP-HPLC method developed in this study could be a powerful tool for trans-fatty acid analysis in edible oils and food products as an alternative to silver-ion chromatography.