Silver ion high-performance liquid chromatography of derivatives of isomeric fatty acids

Abstract

Silver ion high-performance liquid chromatography was used to study the retention characteristics of series of monoenoic, dienoic and polyunsaturated esters. Two columns with stationary phases consisting of bonded phenylsulphonic moieties in the silver ion form were used, with a mobile phase consisting of dichloromethane-dichloroethane (1:1) with various proportions of acetonitrile at fixed temperatures. Accurate capacity factors relative to octadecene as an internal standard were obtained. Each column had distinctive characteristics, and methyl ester derivatives were less well resolved than phenacyl derivatives. For example, baseline resolution of three naturally occurring isomers (6-, 9- and 11-18:1) was possible. The distance of the double bond from the car☐yl group was much more important than the nature of the terminal moiety in its effect on retention values. Fixed temperatures were necessary for reproducible retention times, but lowering the temperature improved the resolution merely by extending the time of chromatography. The results are discussed in terms of a hypothetical mechanism involving simultaneous interaction between one silver ion and either two double bonds, or one double bond and one other electron-rich moiety such as the ester group.