Determination of trans double bonds in polyunsaturated fatty acid methyl esters from their electron impact mass spectra

The geometrical isomers of unsaturated fatty acid methyl esters were analysed by gas chromatography — electron impact mass spectrometry. Distinct differences in the electron impact mass spectra of methylene‐interrupted tri‐ and tetra‐unsaturated fatty acid methyl esters were observed. The differences were particularly related to the geometry of the central double bond in trienes or the two central double bonds in tetraenes. Trans geometry in these positions led to a significant decrease of m / z 79 [C 6 H 7 ] + , which is the base peak in the all‐ cis isomers. Disappearance or significant reductions of ions that are used for the determination of double bond positions in the corresponding cis isomers were also observed in the same spectra. The observed differences may be of help in the identification of double bond geometry in fatty acid isomers.