Plasma concentrations of alphaxalone by gas chromatography: comparison with other gas chromatographic methods and gas chromatography-mass spectrometry

Existing methods for the measurement of plasma or tissue alphaxalone concentrations by gas chromatography are not wholly satisfactory because of the problems of interfering peaks and detector contamination by the silylation reagent. This paper describes an alternative gas chromatographic method using the nitrogen selective alkali flame ionization detector. Plasma samples from patients given Althesin (alphaxalone-alphadolone acetate) were extracted with light petroleum. O-Methyl oximes were formed using methoxyamine hydrochloride, and the O-methyloxime-3-acetate prepared by heating the extract with acetic anhydride-pyridine. Gas liquid chromatography was carried out on a column of 3% OV 17 on Celite 545 (mesh 80–100), initial temperature 235 °C and increasing by 1 °C min−1. Comparison has been made of the O-methyl oxime acetates with trimethylsilyl ethers using both gas chromatography and gas chromatography-mass spectrometry. The results of the two methods were not significantly different. The coefficient of variation for the O-methyloxime-3-acetates at a plasma concentration of 1·09 μl−1 was 6·9%. This method has an improved sensitivity and selectivity over the existing method, and allows determination of the plasma alphaxalone concentrations found in patients receiving Althesin-supplemented anaesthesia.