ADSORPTION OF OXYSTEROLS ON DIFFERENT MICROTUBE MATERIALS DURING SILANYZATION PRIOR TO GAS-CHROMATOGRAPHIC DETERMINATION

The observation of increasing variability in the GC determination of o xysterols when the material of the silanization microtubes was changed led to a comparative study of three kinds of microtubes (Pyrex glass, polypropylene homopolymer and polypropylene copolymer). The character istics of the adsorption of each material on different oxysterols and their influence on the reliability of the determination are reported. The variability of the adsorption in the different materials is high f or 5 alpha-cholestane, one of the most widely used internal standards in the GC determination of oxysterols. This will affect the precision and accuracy of the relative response factors. Significant differences were also found for cholesterol and the five oxysterols analysed, esp ecially between the polypropylene copolymer and the other two material s.