Gas–liquid chromatography–mass spectrometry of hydroxy fatty acids as their methyl esters tert.-butyldimethylsilyl ethers

Abstract

tert.-Butyldimethylsilyl ethers of secondary hydroxy fatty acid methyl esters ( tBDMS-O-FAMEs) produce stable derivatives amenable to gas–liquid chromatography (GLC) and mass spectrometry (MS). Derivatives produce prominent molecular mass minus 57 [M−57] + fragment ions and unique marker fragment ions indicating the locations of the secondary hydroxyl groups along the aliphatic chain from the ω-2 carbon to carbon number 5 from the carboxylic terminus, in addition to yielding information regarding carbon chain length, and degree of unsaturation. The tBDMS-derivatives of C-2, C-3 hydroxy fatty acids and the unique GLC–MS data of γ- and δ-lactones are also presented. Though several tBDMS-O-FAMEs with centrally located hydroxyl groups were not chromatographically resolved, the combination of GLC retention times and monitoring of key diagnostic fragment ions of each tBDMS-derivative, when applied to mixtures containing all hydroxy isomers of palmitic through arachidic acid methyl esters, and to several monounsaturated, monohydroxylated fatty acid methyl esters, allowed for their unambiguous identification. Coupled with derivative stability, permitting their purification and concentration, this method was applied to the identification of trace lipids isolated from bovine skim milk which contained a complex mixture of hydroxy fatty acids of which 19 were newly identified.