Separation of geometrical isomers of unsaturated fatty acids by gas-liquidchromatography on CP-Sil 88 and DB-23 columns

Gas-liquid chromatography with two capillary columns of different polarity (highly polar CP-Sil 88 and medium polar DB-23) was applied for the separat ion of geometrical isomers of unsaturated fatty acids. A mixture of methyl esters of all geometrical isomers of unsaturated fatty acids, obtained from a rapeseed oil, has been analysed chromatographically at seven different t emperatures. The values of equivalent ch ain length (ECL) for all identifie d acids were calculated. The dependence of the ECL values on temperature wa s linear in all cases. All geometrical isomers of C18:2 acid were fully sep arated using a highly polar column, while with medium polar column two isom ers were unresolved at lower temperatures. Different effects of temperature on the ELC values in dependence of fatty acid polarity were found. In case of a CP-Sil 88 column, the separation and identification of all geometrica l isomers of C18:3 acid were hindered by the presence of C20 acids having s imilar ECL values. For both columns, it is difficult to find a temperature at which a separation is most favourable. The complete qualitative analysis of all geometrical isomers of C18 and C20 unsaturated acids is possible us ing bath columns at least at two essentially different temperatures.