Liquid chromatography in pharmaceutical analysis XI: determination of muscle relaxant--analgesic mixture using reversed-phase and ion-pair techniques.

High pressure liquid chromatography using reversed-phase and/or ion-pair techniques was used to optimize resolution of aspirin-containing muscle relaxant mixtures as well as other therapeutic agents commonly found in muscle relaxant-analgesic mixtures. The compounds were chromatographed on an octadecylsilane column using methanol--water solvent systems, some of which contained tetrabutylammonium cation as counterion. Mixtures of methocarbamol--aspirin and chlorzoxazone--acetaminophen were selected to demonstrate separation and quantification. The methocarbamol--aspirin mixture was chromatographed with methanol--water (40:60, pH 6.8) containing 0.01 M tetrabutylammonium cation at a flow rate of 2.0 ml/min. The chlorzoxazone--acetaminophen mixture was chromatographed with methanol--water (50:50) at a 2.0 ml/min flow rate. The separation and quantitation of each mixture were achieved in approximately 8 min with accuracy in the 2--3% range.