Isolation and quantification of cholesterol oxides in dairy products by selected ion monitoring mass spectrometry.

A method for isolation, detection and quantification of cholesterol oxidation products based on solid phase extraction in combination with preparative HPLC and gas chromatography-mass spectrometry selected ion monitoring has been developed for dairy products. The isolation procedure had a high recovery and artifact formation was minimal, as shown by isotope labelling. The limits of detection ranged from 0.3 to 35 pg/microliters of the isomeric forms of 7-hydroxycholesterol, 20 alpha-hydroxycholesterol, the isomeric forms of cholesterol-5,6-epoxides, cholestanetriol, 25-hydroxycholesterol and 7-ketocholesterol corresponding to a limit of quantification of 2-6 ng oxysterol/g lipid in the dairy product, depending on the nature of the cholesterol oxidation product.