On the chemometric deconvolution of gas chromatographically unseparated trans-7,cis-9, cis 9,trans-11 and trans-8,cis-10 octadecadienoic acid isomers in ewe and cow milks

A generally known problem of gas chromatographic (GC) separation of trans-7,cis-9, cis-9,trans-11, and trans-8,cis-10 conjugated linoleic acid (CLA) isomers was studied by GC on a 200 m capillary column coated with a cyanopropyl silicone phase at 130 deg C. The resolution of these CLA isomers obtained at given experimental conditions was not high enough for direct quantitative analysis but it was, however, sufficient for the determination of their peak areas by commercial deconvolution software. Relative retentions and resolution factors of CLA isomers with overlapped peaks determined by the separation of a commercial CLA standard mixture as well as CLA isomer fractions obtained by the HPLC semi-preparation of ewe milk were used as input data in the deconvolution procedure. The milk of pasture-fed ewes with higher cis-9,trans-11 isomer contents showed higher contents of trans-7,cis-9 as well as trans-8,cis-10 CLA isomers in comparison with milk fat of total mixed rations (TMR)-fed ewes. For cow milk samples showing lower CLA contents, no such trends were evident.