Determination of double bond position and geometry in linear and highly branched hydrocarbons and fatty acids from gas chromatography - mass spectrometry of epoxides and diols generated by stereospecific resin hydration

Abstract

A method for the structural elucidation of trace unsaturated hydrocarbons and fatty acids in lipid fractions of environmental samples is described. This is based in the sequenced formation of epoxides and diols by initial m-chloroperbenzoic acid oxidation and subsequent catalytic resin hydration (Amberlyst 15). Both derivatizes are analyzed by gas chromatography-mass spectrometry (diols are analyzed as trimethylsilyl ethers) and double bond position is obtained from their mass fragmentation patterns. Double bond stereochemistry is determined from the distinct retention time of the silylated erythro and threo diols produced by the stereospecific resin hydration of E and Z epoxides, respectively. The comparison of this method with alkylthiolation demonstrates its higher capacity for the structural determination of very long chain olefins and highly branched hydrocarbons. It allows the tentative identification of the double bond position in unsaturated 2,6,10,14-tetramethyl-7-(3-methylpentyl)-pentadecene type sesterpenoids, an application that is presented here for the first time.