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.