A new method for immediate derivatization of hydroxyl groups by fluoroalkyl chloroformates and its application for the determination of sterols and tocopherols in human serum and amniotic fluid by gas chromatography-mass spectrometry
Abstrakt
A new method has been described for efficient derivatization of secondary alicyclic hydroxyl groups in steroids by alkyl chloroformates (RCFs). Cholesterol, an essential human sterol and a steroid precursor in eukaryotic cells, was used as a model for treatment with various RCFs in an aqueous and non-aqueous environment.
While the cholesterol hydroxyl group did not react completely with any of the tested RCFs reagents in the former case, trifluoroethyl chloroformate (TFECF) or heptafluorobutyl chloroformate (HFBCF) fully converts cholesterol and related metabolites into the corresponding mixed carbonates under anhydrous conditions in seconds. The acylation reaction was combined with liquid-liquid microextraction (LLME) between isooctane and acetonitrile phase.
The sample preparation requires just a stepwise addition of 50 mu l isooctane with 5 mu l of a pyridine catalyst, 100 mu l acetonitrile and 100 mu l isooctane with dissolved 5 mu l of the fluoroalkyl chloroformate reagent to a dried sample. The protocol developed in this study was successfully tested for GC-MS analysis of 12 important model steroids and four main tocopherols.
Each analyte provided a single peak with excellent GC separation properties, well defined El spectra containing diagnostic fragment ions suitable for their identification and quantitation. The new method was further validated for the determination of six diagnostic non-cholesterol sterols and four main tocopherols in human serum and in amniotic fluid.
Satisfactory data were obtained in terms of calibration, quantitation limits (for sterols and tocopherols, 0.05 and 0.15 mu g/ml, respectively), within-run precision (0.9-19.5%) and between-run precision (0.2-19.0%), accuracy (82-115%) and recovery (90-110%). The validated method was successfully applied to GC-MS analysis of the analytes in woman sera and amniotic fluids and the results are well-comparable with those reported by other authors.