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A high-pressure liquid chromatographic assay for folic acid in multivitamin-mineral pharmaceutical formulations was developed. The internal standard solution used for sample extraction contained a chelating agent, pentetic acid, for prevention of metal ion-catalyzed degradation of folic acid in th prepared samples. Samples were chromatographed using a paired-ion mobile phase (water-methanol, approximately 76:24; 0.015 M phosphate buffer, pH 7.0; and 0.3% tetrabutylammonium hydroxide) on a column packed with octadecylsilane bonded to microparticulate silica gel. Sample preparation was rapid, and total chromatographic time was approximately 20 min. The method was accurate, precise, and highly specific. Folic acid and the internal standard, methylparaben, were separated from other tablet components and a number of potential impurities and degradation products of folic acid.

Hot-stage microscopy, thermoanalytical methods, and X-ray powder diffraction were used to demonstrate that crystalline erythromycin dihydrate converts to the crystalline anhydrate via a noncrystalline intermediate. X-ray powder diffraction, IR spectral, thermogravimetric, and differential thermal analyses were used to characterize the monohydrate material. The flow interrupt technique, a procedure recently developed to deal with low surface area samples, was employed successfully in obtaining isotherms and specific surface areas for the monohydrate and anhydrate. The relative dissolution rates of the various hydrates were determined in an aqueous solution (0.01 M phosphate buffer, pH 7.5) at 37 degrees. The results showed a significant difference in the dissolution rate of the dihydrate compared to the monohydrate and anhydrate.

Three dissolution apparatus, the rotating basket, the rotating paddle, and the rotating filter--stationary basket, were evaluated for their suitability as production control tools and their relation to blood level studies in dogs. The rotating basket and rotating paddle assemblies were easier to use and less variable than the rotating filter--stationary basket. When relative levels of agitation and the pH of the dissolution medium were held constant, all three apparatus correlated with each other and equally well with the serum drug levels from experimental formulations of an oral hypoglycemic drug after administration to dogs. Such an observed relationship cannot, of course, be used to predict blood levels in other species; however, it does suggest that the choice of one apparatus over another cannot be made a priori.