Microdetermination of propofol in plasma by a rapid and sensitive liquid chromatographic method

A direct and sensitive liquid chromatographic method for the determination of propofol in 50 μl of plasma is described. The separation of the drug and internal standard (methyldopa) was achieved using a 4 μm particle size C₁₈ cartridge (10 cm × 8 mm i.d.) in conjunction with a radial compression system and a C₁₈ precolumn module. The mobile phase consisted of 0.01 M sodium acetate solution (adjusted to pH 3 with acetic acid)-acetonitrile-methanol (37:47.25:15.75, v/v/v) at a flow rate of 2 ml min⁻¹. The compounds were detected in the effluent spectrofluorimetrically with excitation and emission wavelengths of 276 and 310 nm, respectively. After the internal standard had been added, the sample was diluted with 50 μl of hydrochloric acid and centrifuged prior to injection into the chromatograph. The peaks of both propofol and internal standard under these conditions were sharp and symmetrical, and the retention times were 8.2 and 5.15 min, respectively. The peak-height ratio (drug/internal standard) varied linearly (r > 0.9959) with concentration in the ranges 0.002–0.1 and 0.1–10 μg ml⁻¹ and the relative standard deviation was consistently < 5.6%. There was no interference in the assay from the endogenous substance or other concomitantly used drug. This method is currently being used for monitoring propofol in intensive care patients and investigating its pharmacokinetics.