Studies of structure and mechanism in acetonitrile chemical ionization tandem mass spectrometry of polyunsaturated fatty acid methyl esters

Recently it has been shown that acetonitrile chemical ionization tandem mas s spectrometry (CI-MS/MS) is a rapid, on-line means to determine double bon d position in fatty acid methyl esters (FAME). The mechanism of this gas ph ase condensation reaction has been studied. Evidence of the (1-methyleneimi no)-1-ethenylium ion (m/z 54), formed upon the reaction of acetonitrile wit h itself, adding across the double bond in a [2 + 2] cycloaddition reaction is observed. When this nascent complex undergoes collision-induced dissoci ation, two diagnostic ions emerge. One of these ions results from loss of t he hydrocarbon end of the FAME, whereas the other ion results from loss of the methyl ester end, acid when considered together, the diagnostic ions lo calize the positions of the double bonds in the FAME. Several labeling and MS/MS/MS experiments on the two diagnostic ions were performed to determine a plausible fragmentation mechanism of the stable (1-methyleneimino)-1-eth enylium-FAME complex. The first generation product ions, or diagnostic ions , appear to be formed though a charge-driven mechanism, whereas the second generation product ions are formed via charge-remote fragmentations. Plausi ble mechanisms for the formation and subsequent dissociation of the diagnos tic ions are presented for the monounsaturated, diunsaturated, and polyunsa turated (3 or more double bonds) FAME. (C) 1999 American Society for Mass S pectrometry.