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

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.