Preparation, chromatographic separation and relative retention times of cis/ trans heptadecaenoic (17:1) fatty acids

Abstract

In recent years, several countries have implemented new regulations regarding limitations or labeling of the trans fatty acid ( tFA) content in foods. In order to comply with the new requirements, gas chromatographic methods for fatty acid (FA) analysis have been refined toward the quantitation of a larger number of FAs . Increased attention is also being paid to those present in lower quantities. This article describes a simple procedure for obtaining, pure or in mixtures, geometric and positional isomers of a commercially available monounsaturated FA . cis 10–17:1 Fatty acid methyl ester (FAME) was isomerized into its positional/geometrical isomers by repeated hydrobromination/dehydrobromination of its double bond. Reaction products were fractionated into cis and trans geometric isomers by silver ion HPLC. Pure cis-17:1 FAME positional isomers were obtained by reversed-phase HPLC fractionation and identified by gas chromatography – covalent adduct chemical ionization MS/MS using acetonitrile as the reacting gas. The isomerization with p-toluenesulfinic acid of the purified FAME yielded the corresponding trans isomers; these products were analyzed by GC with flame ionization detection using a Supelco 2560 capillary column in order to determine their elution order and retention times ( t R). A novel procedure was developed to determine t r for 17:1 FAME positional/geometrical isomers relative to that of the commercially available cis 10–17:1 FAME.