Gas chromatography-high resolution selected-ion mass spectrometric identification of trace 21 : 0 and 20 : 2 fatty acids eluting with conjugated linoleic acid isomers

High-resolution selected-ion recording (SIR) of the exact molecular ion mas s was used to confirm unambiguously the presence of conjugated linoleic aci d (CLA) derivatives in biological matrices and standard mixtures and to dif ferentiate non-CLA derivatives from CLA derivatives in the CLA region of th e gas chromatogram. The success of this method was based on the selectivity of the SIR technique and its sensitivity, which was comparable to that of flame-ionization detection. A minor fatty acid methyl ester (FAME) was iden tified as methyl heneicosanoate (21:0), and six isomers of 20:2 FAME were f ound to elute in the CLA region. Isomerization of a standard CLA mixture re sulted in a non-CLA flame-ionization response eluting in the CLA region of the gas chromatogram. it is therefore recommended that the identification o f minor CLA isomers in natural products or biological matrices should inclu de their direct confirmation by mass spectrometry.