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  • Title: 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.
    Author: Sehat N, Kramer JK, Mossoba MM, Yurawecz MP, Roach JA, Eulitz K, Morehouse KM, Ku Y.
    Journal: Lipids; 1998 Oct; 33(10):963-71. PubMed ID: 9832075.
    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 NaOCH3. 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, 9c,11t-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 9c,11t-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 7t,9c-18:2 and the previously unreported 11t,13c-18:2 and 12c,14t-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.
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