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In the later case, a known amount of an internal standard (C17:0) is added before further treatment. The separation dry skin of free and esterified sterols may be also effected by column chromatography (SPE). Oil samples are transferred by aid of hexane onto a silicagel column. Elution is started with hexane/ethyl acetate (90/10, v/v) to dry skin collect the dry skin steryl esters, followed by elution with hexane/ether/ethanol (25/25/50, v/v) for collection of free sterols. After evaporation of the solvent, both fractions can be derivatized according to the selected procedure (after saponification for the steryl ester fraction) (Verleyen T et al., JAOCS 2002, 79, 117). The fatty acid composition of cholesterol esters is determined by gas chromatography after either saponification or direct transmethylation. During hydrolysis and methylation, contaminating residues are formed and were shown to interfere with the GC analysis (Smuts CM et al, J Chromatogr 1991, 564, 272).
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