Analysis of autoxidized fats by gas chromatography-mass spectrometry: I. Methyl oleate

A structural investigation of autoxidation products of methyl oleate was carried out by gas chromatography-mass spectrometry (GC-MS) of trimethylsilyl (TMS) ether derivatives. GC-MS using computer plots of selected masses afforded structural assignments of GC peaks due to incompletely resolved mixtures. This method provided evidence of epoxy and keto esters which are not completely separated from the main components consisting of the TMS derivatives of the allylic hydroxy esters. Use of an MS-computer system also showed that the hydroxyoctadecanoate TMS ethers were partially separated by GC. The use of synthetic hydroxyoctadecanoates for the first time enabled us to demonstrate the quantitative reliability of a GC-MS computer summation approach to analyze the isomeric composition of oleate hydroperoxides (as the saturated TMS ether derivatives). Consistently higher concentrations were found of the 8- and 11-hydroperoxides than of the 9- and 10-hydroperoxides. Minor products of autoxidation identified by GC-MS include allylic enones, isomeric epoxyoctadecanoates, dihydroxyoctadecenoates, and dihydroxyoctadecanoates.