Test specimen: undried.

Buffer solution— Add 14 mL of phosphoric acid to 4000 mL of HPLC grade water. Add 10 mL of ammonium hydroxide, and adjust with ammonium hydroxide to a pH of 3.90 ± 0.05.

Organic solution— Prepare a mixture of acetonitrile and methanol (900:100).

Diluent— Prepare a degassed mixture of Buffer solution and Organic solution (80:20).

Solution A— Prepare a degassed mixture of Buffer solution and Organic solution (920:80).

Solution B— Prepare a degassed mixture of Buffer solution and Organic solution (520:480).

Mobile phase— Use variable mixtures of Solution A and Solution B as directed for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Accurately weigh about 22 mg of USP Clindamycin Phosphate RS. Add 10.0 mL of Diluent, shake briefly, and sonicate for about 5 minutes to dissolve. Allow to cool to ambient temperature.

Assay preparation— Accurately weigh about 22 mg of Clindamycin Phosphate. Add 10.0 mL of Diluent, shake briefly, and sonicate for about 5 minutes to dissolve. Allow to cool to ambient temperature.

Chromatographic system— The liquid chromatograph is equipped with a 214-nm detector and a 4.6-mm × 25-cm column that contains packing L7. The flow rate is about 1.2 mL per minute. Maintain the column at a constant temperature of about 40. The chromatograph is programmed as follows. Chromatograph the Standard preparation, and record the peak areas as directed for Procedure: the resolution between clindamycin phosphate and its nearest related compound is not less than 1.0, determined as follows. Between the peaks, draw a line perpendicular to the base line at the valley that separates the two peaks. The height of the valley from the base line is not more than 40 percent of the height of the related compound peak. Calculate the resolution, R, using peak widths at half height. The relative standard deviation determined from the clindamycin phosphate peak is not more than 2.0%.

Procedure— Separately inject equal volumes (about 25 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas of the clindamycin phosphate peaks. Calculate the quantity, in µg, of clindamycin (C18H33ClN2O5S) in the portion of Clindamycin Phosphate taken by the formula: in which P is the potency, in µg of clindamycin per mg, of USP Clindamycin Phosphate RS; WS is the weight, in mg, of USP Clindamycin Phosphate RS taken to prepare the Standard preparation; WU is the weight, in mg, of Clindamycin Phosphate taken to prepare the Assay preparation; and rU and rS are the peak areas for clindamycin phosphate obtained from the Assay preparation and the Standard preparation, respectively.