Quantitative determination of naproxen in plasma by a simple high-performance liquid chromatographic method

A high-performance liquid chromatographic method for the determination of naproxen in plasma is described. The technique is based on the single extraction of the drug from acidified plasma with chloroform using 2-naphthalene acetic acid as internal standard. The chromatographic system consisted of a column packed with Spherisorb ODS (5 micrometer); the mobile phase was acetonitrile--phosphoric acid (pH 3) (45:55, v/v). The method can accurately measure plasma naproxen concentrations down to 1 microgram/ml using 100 microliter of sample, with no interference from endogenous compounds. The coefficients of variation of the method at 120 microgram/ml and 1 microgram/ml are 2.8 and 21.6%, respectively, and the calibration curve is linear. The method described is very suitable for routine clinical and pharmacokinetic studies.