Identification of conjugated linoleic acid isomers in cheese by gas chromatography, silver ion high performance liquid chromatography and mass spectral reconstructed ion profiles. Comparison of chromatographic elution sequences

Abstract

Commercial cheese products were analyzed for their composition and content of conjugated linoleic acid (CLA) isomers. The total lipids were extracted from cheese using petroleum ether/diethyl ether and methylated using NaOCH 3. The fatty acid methyl esters (FAME) were separated by gas chromatography (GC), using a 100‐m polar capillary column, into nine minor peaks besides that of the major rumenic acid, 9 c, 11 t‐octadecadienoic acid (18∶2), and were attributed to 19 CLA isomers. By using silver ion‐high performance liquid chromatography (Ag+‐HPLC), CLA isomers were resolved into seven trans, trans (5–9%), three cis/trans (10–13%), and five cis, cis (<1%) peaks, totaling 15, in addition to that of the 9 c, 11 t‐18∶2 (78–84%). The FAME of total cheese lipids were fractionated by semipreparative Ag+‐HPLC and converted to their 4,4‐dimethyloxazoline derivatives after hydrolysis to free fatty acids. The geometrical configuration of the CLA isomers was confirmed by GC‐direct deposition‐Fourier transform infrared, and their double bond positions were established by GC‐electron ionization mass spectrometry. Reconstructed mass spectral ion profiles of the m+2 allylic ion and the m+3 ion (where m is the position of the second double bond in the parent conjugated fatty acid) were used to identify the minor CLA isomers in cheese. Cheese contained 7 t,9 c‐18∶2 and the previously unreported 11 t, 13 c‐18∶2 and 12 c, 14 t‐18∶2, and their trans,trans and cis,cis geometric isomers. Minor amounts of 8,10‐, and 10, 12–18∶2 were also found. The predicted elution orders of the different CLA isomers on long polar capillary GC and Ag*‐HPLC columns are also presented.