Liquid chromatography in pharmaceutical analysis IX: Determination of muscle relaxant--analgesic mixtures using normal phase chromatography.

High-pressure liquid chromatography was used to optimize the resolution of eight widely prescribed therapeutic agents commonly found in muscle relaxant--analgesic mixtures. The compounds were chromatographed on normal phase porpous silica or cyanopropylsilane columns, using various solvent systems paired on the basis of Synder's solvent selectivity scheme to give a polarity index for each system of 3.3. A carisoprodol, phenacetin, and caffeine mixture was selected to demonstrate the utility of the separation and quantification method. The mixture was chromatographed on a porous silica column, using tetrahydrofuran--toluene (50:50) at a flow rate of 2.0 ml/min. Each determination can be achieved in approximately 8 min with an accuracy of 3--5%.