Quantitative determination of cholesterol sulphate in plasma by stable isotope dilution fast atom bombardment mass spectrometry.

A stable isotope dilution assay has been developed for the quantitative determination of cholesterol sulphate in plasma using negative ion fast atom bombardment (FAB) mass spectrometry. The assay is highly selective and avoids problems of contamination from free cholesterol and other conjugates of cholesterol present in plasma. (6,7,7-2H3)Cholesterol sulphate is used as the internal standard and solvent extraction and silica Sep-Paks are employed to isolate plasma cholesterol sulphate. Limited-range acceleration voltage scanning in FAB mass spectrometric analyses leads to sub-microgram detection limits. Comparison of results obtained by FAB mass spectrometry of the intact cholesterol sulphate, and by gas chromatography/mass spectrometry selected ion monitoring of the free cholesterol, released by solvolysis of the cholesterol sulphate, showed that the latter approach probably overestimates plasma levels of cholesterol sulphate.