Determination of cholesterol oxides in processed food using high-performance liquid chromatography–mass spectrometry with atmospheric pressure chemical ionisation

The present work describes the development and application of an on-line atmospheric pressure ionisation (APCI) LC–MS interface for the simultaneous determination of seven toxicologically relevant cholesterol oxides (7α-hydroxycholesterol, 7β-hydroxycholesterol, 25-hydroxycholesterol, 7-ketocholesterol, 5,6α-, 5,6β-epoxycholesterol and cholestan-3β,5α,6β-triol). The HPLC method has been optimised to reach better separation of all tested compounds. The influences of APCI parameters (nebulising temperature, cone voltage, source temperature) on signal intensity and fragmentation pattern were investigated for all tested cholesterol oxides compounds. This is the first report on optimisation and determination of two compounds 7α-hydroxycholesterol and 5,6β-epoxycholesterol in processed food using LC–MS. After extraction with hexane, clean-up was carried out using solid-phase extraction on a silica column. For the chromatographic separation of cholesterol oxides an Aquasil C18 column was used with acetonitrile–methanol (60:40) as mobile phase. For the first time we report the use of such a C18 column with a relatively hydrophilic nature for the separation of cholesterol oxides. APCI-MS detection was then applied in selected ion monitoring and positive ion modes by using the molecular ions and the main fragments. The developed method shows good linearity, high repeatability and good recovery for all tested cholesterol oxides. The method was applied for determination of seven selected cholesterol oxidation products in different foodstuffs such as butter, butteroil, lard and egg powder.