On the formation of conjugated linoleic acid diagnostic ions with acetonitrile chemical ionization tandem mass spectrometry

Acetonitrile chemical ionization tandem mass spectrometry has recently been shown to be a rapid method for the identification of double‐bond position and geometry in methyl esters of conjugated linoleic acids (CLAs); however, the structures of intermediate and diagnostic ions and their mechanisms of formation are not known. A mechanism is proposed here in which the m/z 54 ion, (1‐methyleneimino)‐1‐ethenylium (MIE), undergoes nucleophilic attack preferentially by the cis double bond in CLAs with mixed geometry (cis/trans, trans/cis), favoring the observed CC cleavage vinylic to the trans double bond. The [M+54]+ addition product intermediate is consistent with a heterocyclic six‐membered ring resulting from the two‐step addition of MIE to the CLA. Experiments with isotopically labeled CLAs and acetonitrile, and from MS/MS/MS experiments, yield data consistent with this proposal. The proposed mechanism is also consistent with known ion—molecule chemistry in smaller compounds, and explains most phenomena associated with MIE‐CLA ion chemistry. Copyright © 2005 John Wiley & Sons, Ltd.