Determination of double-bond positions in methylene-interrupted dienoic fatty acids by GC-MS as their dimethyl disulfide adducts

Abstract

A method using GC-MS for determining the double-bond positions in methylene-interrupted dienoic fatty acids as their dimethyl disulfide adducts is described. As a standard procedure for the single-step preparation of the adducts, methyl linoleate was subjected to the I 2-catalyzed reaction with dimethyl disulfide for 30 min. After the addition of a hexane/ether mixture and an aqueous Na 2S 2O 3 solution to reduce I 2, the resulting adducts in the upper phase were analyzed directly by GC-MS. The adducts consisted of an equimolar mixture of methyl 9,10-bis(methylthio)octadec-12-enoate and 12,13-bis(methylthio)octadec-9-enoate. The cleavage between the methylthio-substituted carbons yielded sets of key fragment ions showing the original double-bond positions in the starting ester. Standard experiments using other authentic methyl dienoates revealed the accuracy of this convenient method. The analysis of [2,2- 2H 2]linoleic acid and naturally-occurring dienoic fatty acids using this method is also described.