Use of vinyl methyl ether as a chemical ionization reagent gas for gas chromatographic chemical ionization mass spectrometric discrimination between cyclopropanoid and monoenoic fatty acid methyl esters

Abstract

Methane-, isobutane-and vinyl methyl ether-chemical ionization mass spectra were unsatisfactory for discrimination between cyclopropanoid fatty acid methyl esters and their monoene isomers using packed column gas chromatography mass spectrometry. A methodology was developed whereby these isomers could be distinguished by using a mixture of methane and vinyl methyl ether as the chemical ionization reactant gas. The spectra of the monoene fatty acid methyl esters displayed a series of vinyl methyl ether adduct ions, which were less than one-tenth as abundant in the spectra of the isomeric CycIopropanoid compounds. A greater degree of discrimination was achieved using isobutane/vinyl methyl ether as a mixed reagent gas, when unsaturated fatty acid methyl esters yielded a vinyl methyl ether adduct ion which was essentially absent (reduced to approx. 2%) in the spectrum of the isomeric cyclopropanoids. These adduct ions could be used to clearly define the occurrence of the monoenoic compounds in the presence of isomeric cyclopropanoids. Examples of the use of this method, in conjunction with catalytic hydrogenation, for the identification of cyclopropanoid fatty acid methyl esters in several Thiobacilius spp. and of monounsaturated fatty acid methyl esters in the fungus, Trametes lilacinogilva, are presented.

Extractives of Fungi. VI Gas Chromatographic-Mass Spectrometric Investigations of the Lipids of Trametes Lilacino-Gilva (Berk.) Lloyd

“…The methylated fatty acid fractions were examined using both GC-EIMS and GC-methane-CIMS; the data from these two modes of operation were complementary and were usually sufficient to allow identification of the constituents, although positional and geometric isomers could not be differentiated. Vinyl methyl ether-CIMS was used to establish that monoenoic constituents were indeed so, and not cyclopropanoid in nature (Christopher and Duffield 1980). Quantitative estimates of the proportion of each constituent were derived from gas chromatographic analyses (using a flame ionization detector) in the usual fashion.…”

“…In the light of the occurrence in some bacteria (O'Leary 1962) of C 17 cyclopropane fatty acids, it was desirable to establish positively the nature of the C 17 ' apparently monoenoic, constituent. A technique (Christopher and Duffield 1980) was developed by which, using a vinyl methyl ether-methane mixture as the reactant gas for CIMS, the mass spectra for cyclopropane and monoene fatty acid methyl esters could be differentiated. Using this technique, the C17 constituent, and indeed all of the other monoenes present, were positively confirmed as such.…”