Preparation, separation, and confirmation of the eight geometrical cis/trans conjugated linoleic acid isomers 8,10-through 11,13-18 : 2

Conjugated linoleic acid (CLA) mixtures were isomerized with p-toluenesulfi nic acid or I-2 catalyst. The resultant mixtures of the eight cis/trans geo metric isomers of 8,10-, 9,11-, 10,12-, and 11,13-octadecadienoic (18:2) ac id methyl esters were separated by silver ion-high-performance liquid chrom atography (Ag+-HPLC) and gas chromatography (GC). Ag+-HPLC allowed the sepa ration of all positional CLA isomers and geometric cis/trans CLA isomers ex cept 10,12-18:2. However, one of the 8,10 isomers (8cis,10trans-18:2) coelu ted with the 9trans,11cis-18:2 isomer. There were differences in the elutio n order of the pairs of geometric CLA isomers resolved by Ag+-HPLC. For the 8,10 and 9,11 CLA isomers, cis,trans eluted before trans,cis, whereas the opposite elution pattern was observed for the 11,13-18:2 geometric isomers (trans,cis before cis,trans). All eight cis/trans CLA isomers were separate d by GC on long polar capillary columns only when their relative concentrat ions were about equal. Large differences in the relative concentration of t he CLA isomers found in natural products obscured the resolution and identi fication of a number of minor CLA isomers. In such cases, GC-mass spectrome try of the dimethyloxazoline derivatives was used to identify and confirm c oeluting CLA isomers. For the same positional isomer, the cis,trans consist ently eluted before the trans,cis CLA isomers by GC High resolution mass sp ectrometry (MS) selected ion recording (SIR) of the molecular ions of the 1 8:1, 18:2, and 18:3 fatty acid methyl esters served as an independent and h ighly sensitive method to confirm CLA methyl ester peak assignments in GC c hromatograms obtained from food samples by flame-ionization detection. The high-resolution MS data were used to correct for the nonselectivity of the flame-ionization detector.