Identification of the position and the stereochemistry of the double bond in monounsaturated fatty acid methyl esters by gas chromatography/mass spectrometry of dimethyl disulfide derivatives

A single-step derivatization procedure using dimethyl disulfide (DMDS) is described for monounsaturated fatty acid esters (MUFAME). The analysis of DMDS adducts by gas chromatography/mass spectrometry is detailed: The mass spectra (electron impact 70 eV) show molecular ions (M/sup +/) and give key fragments that permit determination of the position of the original double bond. Alkylthiolation of Z- and E-MUFAME as a specific antiaddition leads, respectively, to the threo and erythro adducts. The two isomers are well-separated by gas chromatography, permitting precise stereochemistry of the initial double bond. Nevertheless, for a complex environmental mixture of MUFAME, the stereochemical identification is restricted to the linear series. This procedure is suitable for the analysis of trace compounds at the nanogram level such as encountered in the natural environment. An application is presented for the analysis of planktonic and bacterially derived fatty acids in recent marine sediments.