Silver ion chromatography and gas chromatography-mass spectrometry in the structural analysis of cyclic monoenoic acids formed in frying oils

Abstract

The nature of the cyclic monoenoic fatty acids formed from linoleic acid in sunflower oil heated to 275°C has been determined by gas chromatography-mass spectrometry of the picolinyl ester derivatives, before and after hydrogenation and deuteration, and following simplification by silver ion high-performance liquid chromatography. In addition, they were examined by gas chromatography-Fourier transform infrared spectroscopy. Cyclopentene fatty acids (50% of the total monoenes) were formed from C-8 to C-12 and C-10 to C-14 of the original chain in equal amounts with unique stereochemistry. In some isomers the double bond appeared to remain in its original position, and in others it migrated to a substituted ring carbon. Isomers (25% of the total monoenes) were formed with a cis double bond in position 9 or 12 in equal amounts, and either a cyclopentane ring involving C-5 to C-9 or C-13 to C-17 or a cyclohexane ring involving C-5 to C-10 or C-12 to C-17 of the original fatty acid chain. With these compounds, most if not all of the possible configurational isomers were produced. Related compounds (25% of the total monoenes) with a trans double bond in position 9 or 12 were found with only six-membered rings with a restricted stereochemistry. Some bicyclic fatty acids were also present in the mixture.