1: HPLC of ranitidine/famotidine/fusidic acid and Danazol. 2: Computed LC injection. 3: Reversed phase retention in IE columns

High performance liquid chromatographic methods were developed for the determination of ranitidine, famotidine, fusidic acid and danazol in biological fluids. These drugs were extracted from the biological fluid with actetonitrile and an inorganic salt. Ranitidine and danazol were extracted from plasma and famotidine was extracted from urine, all using ammonium sulfate. For danazol and fusidic acid prior to salting out, cadmium sulfate was mixed with the acetonitrile-plasma mixture, to remove interfering constituents. In the case of danazol the extract was evaporated to concentrate the drug. The chromatographic conditions consisted of the following columns and mobile phases (v:v): ranitidine, octadecylsilyl and 30:70, acetonitrile:7mM triethylammonium ion (pH = 3.0); famotidine, cyanopropyl and 8:92, acetonitrile:10mM sodium dihydrogen phosphate; fusidic acid, cyanopropyl and 39:61, acetonitrile:20mM sodium dihydrogen phosphate (pH = 3.5); danazol, octadecylsilyl and 71:29, v:v, methanol:20mM potassium dihydrogen phosphate. An UV-visible detector was used. The limits of detection were found to be 3, 70, 200 and 2ng/mL for ranitidine, famotidine, fusidic acid and danazol, respectively. A number of clinically important drugs did not show any chromatographic interference with the four drugs. Equilibrium distribution theory was applied to the injection of eluite into several theoretical plates in a column. A microcomputer spreadsheet calculation showed quick generation of a symmetric peak despite an initial skewed distribution of the eluite. Eluite peak distortion due to injection solvent strength was also simulated using the spreadsheet. For a significant difference between the injection solvent and the mobile phase strengths, calculation indicated increasing peak distortion with higher injection volume. A microcomputer simulation of the dilution of an injection plug as it traversed the chromatograph resulted in peak multiplicity. A polymer based cation exchange column was used to study retention mechanisms during ion-exchange. Organic cations were used and their retention in ion-exchange was studied with varying concentrations of different types of electrolytes in the mobile phase. A secondary reversed phase retention mechanism was supported by these studies. A model was proposed to rationalize the secondary retention mechanism in the ion-exchange.