Characterization of epoxides of polyunsaturated fatty acids by mass spectrometry via 3-pyridinylmethyl esters.

The isomeric epoxides of linoleic, arachidonic and docosahexaenoic acids were prepared by reaction with m-chloroperoxybenzoic acid and, after separation by normal-phase high-performance liquid chromatography, were esterified with 3-pyridylcarbinol via the unstable imidazolide generated by the reaction with 1,1'-carbonyldiimidazole. The electron impact mass spectra of these derivatives showed a molecular ion and a sequence of peaks with two characteristic abundant ions that resulted from formal cleavage of the carbon-carbon bonds at the oxirane ring. Both these ions retained the ester group. This fragmentation pattern allowed the unequivocal identification of the separate epoxide isomers.