High-performance liquid chromatographic determination of modafinil and its two metabolites in human plasma using solid-phase extraction

A simple procedure for the simultaneous determination of modafinil, its acid and sulfone metabolites in plasma is described. The assay involved an extraction of the drug, metabolites and internal standard from plasma with a solid-phase extraction using C18 cartridges. These compounds were eluted by methanol. The extract was evaporated to dryness at 40 degrees C under a gentle stream of nitrogen. The residue was redissolved in 250 microl of mobile-phase and a 30 microl aliquot was injected via an automatic sampler into the liquid chromatograph and eluted with the mobile-phase (26%, v/v acetonitrile in 0.05 M orthophosphoric acid buffer adjusted to pH 2.6) at a flow-rate of 1.1 ml/min on a C8 Symmetry cartridge column (5 microm, 150 mm x 3.9 mm, Waters) at 25 degrees C. The eluate was detected at 225 nm. Intra-day coefficients of variation ranged from 1.0 to 2.9% and inter-day coefficients from 0.9 to 6.1%. The limits of detection and quantitation of the assay were 0.01 microg/ml and 0.10 microg/ml respectively.