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

摘要

Gas chromatography/electron ionization mass spectrometry of methyl (for a review see [1])and trimethylsilyl [2] esters constitutes a particularly powerful technique for the identification of fatty acids. Unfortunately, the mass spectra of methyl and trimethylsilyl esters of monoenoic fatty acids have no information that helps to locate the position of double bonds. While there have been suggestions that such information can be obtained from close examination of certain minor peaks in the spectrum, the value of such techniques seems doubtful. There is no feature that permits location of the double-bond, because this can migrate to any position when the alkyl chain is ionized in the mass spectrometer. To get around the problem of location of double bonds, it is possible to prepare specific derivatives of unsaturated fatty acids that ‘fix’ the double-bond. Very many have been described. The more commonly employed are dimethyldisulfide adducts (which have excellent mass spectrometric properties and are prepared in a simple one-pot reaction) [3, 4] and vicinal trimethylsilyl ethers arising from stereospecific OsO4 oxidation of double bonds [5]. Alternatively, picolinyl esters [6, 7] or DMOX [8] or pyrrolidine [9] derivatives can be utilized to locate double bonds. In these last cases, the carboxyl group is derivatized with a reagent containing a nitrogen atom. When the molecule is ionized in the mass spectrometer, the nitrogen atom, not the alkyl chain, carries the charge, and double-bond migration is minimized.