Gas chromatography-mass spectrometry determination of conjugated linoleic acids and cholesterol oxides and their stability in a model system.

A gas chromatography-mass spectrometry (GC-MS) method was developed to simultaneously separate cholesterol, eight cholesterol oxidation products (COPs), and two conjugated linoleic acids (9-cis,11-trans-CLA and 10-trans,12-cis-CLA) and to evaluate their stability in a model system during heating. Among four capillary columns tested, an Equity-5 column with low-polar stationary phase provided better resolution within 30 min. A high-performance liquid chromatography method was also developed to determine cholesterol hydroperoxides by using a YMC C30 column with diphenyl-1-pyrenylphosphine as fluorescence reagent. No formation of COPs or degradation of cholesterol and CLAs occurred at 100 degrees C, but the levels of COPs rose drastically at 150 degrees C. The first-order rate of cholesterol degradation declined following a rise in CLA concentration. For 0-, 100-, and 500-microg/ml CLA levels, the formation profiles of 7-hydroxycholesterol, 7-ketocholesterol, and 5,6-epoxycholesterol at 150 degrees C were fitted as multiple first-order curves, whereas a single first-order model could adequately describe 7-hydroperoxycholesterol and cholestane-3beta,5alpha,6beta-triol formation. A CLA-to-cholesterol mole ratio of 0.49 was required to prevent cholesterol oxidation at 150 degrees C.