Determination of diacylglycerol isomers in vegetable oils by solid-phase extraction followed by gas chromatography on a polar phase

Diacylglycerol (DG) isomers in vegetable oils were determined by several chromatographic techniques. The use of reversed-phase high-performance liquid chromatography on LC-18 under isocratic conditions was inappropriate, since no complete resolution of the 1,2- and 1,3-isomers appearing at low retention time was achieved. In addition, overlapping of some peaks, absence of peaks of the minor components and interferences by sterols were observed. On the other hand, the isolation of the polar fraction by solid-phase extraction (SPE) using a bonded diol phase and further analysis of the silyl derivatives by capillary GC on 65%-phenyl-methylsilicone, was carried out successfully. No interferences by other components were found and isomerisation by passing the DGs through the SPE column was negligible. The procedure is easy, fast and reproducible, allowing the quantitation and separation of DGs according to their carbon number, their isomeric structure (1,2 and 1,3) and the degree of unsaturation. The effect of unsaturation on the DG retention times depends on the number and arrangement of the double bonds in the molecule. Applications to some vegetable oils are shown.