Resonant electron capture mass spectrometry of free fatty acids: examination of ion structures using deuterium‐labeled fatty acids and collisional activation

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Abstract
The structures of [M–H]− ions generated from free fatty acids in resonant electron capture at energies of 1.2 and 7.2 eV were investigated using deuterium‐labeled isotopomers and collision‐induced dissociation. The [M–H]−· ions occur in both a carboxylate anion and a carbanion form. While the formation of the carboxylate anion at 1.2 eV involves the loss of a carboxylic hydrogen, that at 7.2 eV involves the loss of a hydrogen from different positions in the aliphatic chain followed by a rearrangement of a carboxylic hydrogen on to the radical site in the chain. The [M–H–H2O]− ion which is a minor ion in the resonant electron capture spectrum at 7.2 eV is shown to be a precursor for the charge‐remote fragment ions corresponding to formal losses of a hydrogen and elements of alkanes. The [M–H–H2O]− ion corresponding to the second major ion in the resonant electron capture spectrum at 7.2 eV is demonstrated to be consistent with a cyclopentanone anion structure. On the basis of new insights obtained in the present study and taking into account previous results, an updated proposal is presented for the mechanism of charge‐remote fragmentation which operates in resonant electron capture of free fatty acids at 7.2 eV. Copyright © 2002 John Wiley & Sons, Ltd.

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