Determination of pentobarbital and pentobarbital sodium in bulk drug substance and dosage forms by high-performance liquid chromatography.

High performance liquid chromatography (HPLC) is used to determine impurities in pentobarbital (I) and pentobarbital sodium (II) and to determine the strength of the drug substance and dosage forms. Separations were achieved using a Nucleosil C-18 column (5 microns) measuring 4.6 mm x 15 cm and an eluent containing 0.01 M phosphate buffer at pH 3.5:acetonitrile (72:28). The column is eluted isocratically and UV detection is used at 214 nm. Impurities are determinable in the drug substance at levels > or = 0.01%. Assay precision (relative standard deviations) for impurities in I and II ranged from +/- 36% to +/- 1.3% at levels of 0.01-1.46%. The external standard method is used for quantitating impurities in I and II. The determination of strength in drug substances I and II and in dosage forms (elixir, solution, capsules and suppositories) used the internal standard method. Precision for the strength determination ranged from +/- 0.26 to +/- 1.6%. The accuracy of the procedure was evaluated by addition and recovery of I and II to placebos. Recoveries were quantitative at 50-150% addition levels. Variation in parameters of the separation were made to evaluate the robustness of the HPLC separations.