Simultaneous determination of phenylbutazone and oxyphenbutazone in plasma by high-speed liquid chromatography

A sensitive, specific, high-speed liquid chromatographic procedure is described for the simultaneous determination of phenylbutazone and its metabolite, oxyphenbutazone, in plasma. Acidified plasma is partitioned with cyclohexane-ether (1:1) containing the 2,4-dinitrophenylhydrazone of 3,4-dimethoxybenzaldehyde as an internal standard. The organic extract is reduced to dryness, the resulting residue is redissolved in chloroform, and aliquots of this solution are chromatographed on an adsorption column, using a mobile phase of 0.002% acetic acid and 23.0% tetrahydrofuran in n-hexane at 35 degrees. Use of a UV detector permits quantitative analysis of samples containing less than 0.25 mug/ml of phenylbutazone or oxyphenbutazone.