Accurate quantification of nonesterified and total cholesterol in plasma and urine from crude lipid extracts by stable isotope dilution and field desorption mass spectrometry.
Levels of free and total cholesterol were quantified in control sera and human urine using [2H7]cholesterol or [13C2]cholesterol as internal standard and field desorption mass spectrometry for the quantification. These analyses are performed using a crude chloroform extract without chromatographic separation of the constituents. Due to the extremely soft ionization conditions in field desorption, the constituents of the crude lipid extract predominantly form molecular ion species and only a very low amount of fragment ions of low relative abundance thus minimizing the possibility of ion overlap in the mass spectrum. Mass spectrometric quantification of cholesterol in two control sera resulted in data within the range of assigned concentrations. The standard deviations of the mass spectrometric data were around +/- 2%. Generally, 50 microliters of serum or plasma or 500 microliters of urine are extracted and the mass spectrometric quantification performed using an aliquot of 2-10%. A characteristic pattern of intact cholesteryl esters is also obtained.