Double Bond Migration to Methylidene Positions During Electron Ionization Mass Spectrometry of Branched Monounsaturated Fatty Acid Derivatives

Electron ionization mass spectra of several monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters were examined. These spectra exhibited some intensive fragment ions, whose formation could be explained after double-bond migration to methylidene position. This preferential migration (substantiated by deuterium labeling) acts significantly in the case of monounsaturated fatty acid methyl and trimethylsilyl esters possessing a methyl branch localized between the penultimate and the C4 positions (relative to the ester group), whatever the position of the double-bond. Allylic cleavage and γ-hydrogen rearrangement of the ionized methylidene group thus formed afforded very interesting fragment ions, which could be particularly useful to determine branching positions of monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters without additional treatment.

Graphical AbstractEI mass spectra of monounsaturated methyl-branched fatty acid esters exhibit some intensive fragment ions resulting from double bond migration to methylidene position.Figure optionsDownload full-size imageDownload high-quality image (113 K)Download as PowerPoint slide