MECHANISTIC ASPECTS OF FATTY-ACID RETENTION IN SILVER ION CHROMATOGRAPHY

The retention properties of various benzyl, phenacyl, and normal- and branched-chain alkyl esters of 6-18:1, 9-18:1 and 11-18:1 fatty acids have been studied by silver ion high-performance liquid chromatography . The weak electron-donating effect of the alkyl substituents on the c arbonyl oxygen in the aliphatic alkyl esters ensures baseline resoluti on of isopropyl and tert.-butyl esters, but not of n-alkyl esters. Com parison between benzyl and phenacyl derivatives confirmed that the sil ver ion interacted simultaneously with the double bond of the fatty ac id and the carbonyl oxygen of the phenacyl moiety. This dual interacti on enabled the separation of positional isomers. Introduction of elect ron-withdrawing or electron-donating substituents in the p-position in the benzene ring decreased or increased retention, respectively. The substituents strongly affect the resolution, and p-methoxyphenacyl est ers have not only the highest k' values but also the best resolution o f the three positionally isomeric fatty acids, established so far.