Charge-remote fragmentation characteristics of monounsaturated fatty acids in resonance electron capture: differentiation between cis and trans isomers

Abstract

In the present study we have examined cis and trans isomers of monounsaturated fatty acids (i.e. oleic, cis-vaccenic and erucic acids, and their trans isomers) using resonance electron capture (REC) at 7.2 eV. Differences between cis and trans isomers could be observed, which became more pronounced by lowering the electron energy within the limits of the resonance. The ion of diagnostic value does not belong to the regular charge-remote fragment ions corresponding to the combined loss of two hydrogen radicals and an alk(en)yl radical and is rationalized by a vinylic cleavage and loss of an alkyl radical directly from the excited molecular radical anion. In addition, collision-induced dissociation experiments have been performed on deprotonated cis- and trans-13-docosenoic acids generated by REC at 7.2 eV. These experiments demonstrated that the [M − H] − ions were the precursors of all the regular charge-remote fragment ions observed in the REC spectra. The latter ions are rationalized by an initial loss of a hydrogen radical from different positions of the fatty acid chain yielding [M − H] − ions (containing a radical site in the fatty acid chain) which fragment further by the combined loss of an alk(en)yl radical from the chain and a hydrogen radical from the carboxyl group.