Analysis of Photosensitized Oxidation Products of Unsaturated Triglycerides and Vegetable Oils by Gas Chromatography Mass Spectrometry

Photosensitized oxidation of trioleoylglycerol (TO), trilinoleoylglycerol (TL), trilinolenoylglycerol (TLn) and vegetable oil triacylglycerols (triglycerides, TG) was carried out in isopropanol using methylene blue as a photosensitizer. Isomeric compositions of hydroperoxy fatty acid components of the oxidized TG were determined by hydrogenation, methanolysis and mass chromatographic analysis of the resulting methyl hydroxy octadecanoate. TO gave 9- and 10-isomers; TL, 9-, 10-, 12- and 13-isomers; and TLn, 9-, 10-, 12-, 13-, 15- and 16-isomers. It was concluded that each unsaturated fatty acid component of vegetable oil TG yields isomeric hydroperoxides during photosensitized oxidation in a manner similar to the corresponding unsaturated fatty acid methyl ester. TL monohydroperoxides were isolated from the photooxidized TL and hydrolyzed by pancreatic lipase. The hydrolysis products consisted of dilinoleoylglycerol, monolinoleoylglycerol, linoleic acid and their respective hydroperoxides. Formation of a hydroperoxy fatty acid component was observed during photoirradiation of vegetable oils in the bulk phase without methylene blue. The isomeric compositions of the resulting methyl hydroxy octadecanoate support the idea that singlet oxygen is responsible for the formation of hydroperoxides in the initial stage of photooxidation.