Assay of oxazepam in human plasma by reversed-phase high-performance liquid chromatography

A rapid, selective and sensitive method for determination of oxazepam in plasma is described, based on reversed-phase high-performance liquid chromatography. The separation was performed on an analytical 300 x 3.9 mm i.d m-bondapack C18 column using UV detector at 230 nm. A mixture of methanol-phosphate buffer (50:50 v/v) adjusted to pH 6 at a flow rate of 1.5 mL min-1 was used as mobile phase. The proteins were precipitated with a 24% perchloric acid (HClO4) solution and analytical recovery was complete. Diazepam was used as an internal standard. The standard curve was linear over the range 100-1600 ng mL-1 and the detection limit for oxazepam was 30 ngmL-1. The inter- and intra-assay coefficients of variation were found to be less than 10%. This method is simple, rapid and sensitive enough for monitoring of oxazepam levels in clinical studies and pharmacokinetic aspects.