Test specimen: undried.

Buffer solution— Add 14mLof phosphoric acid to 4000mLof HPLCgrade water.Add 10mLof ammonium hydroxide,and adjust with ammonium hydroxide to a pHof 3.90±0.05.

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

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

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

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

Mobile phase— Use variable mixtures of Solution Aand Solution Bas directed for Chromatographic system.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

Assay preparation— Accurately weigh about 22mg of Clindamycin Phosphate.Add 10.0mLof Diluent,shake briefly,and sonicate for about 5minutes 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 1.2mLper 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 40percent 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 25µL)of the Standard preparationand the Assay preparationinto 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 Pis the potency,in µg of clindamycin per mg,of USP Clindamycin Phosphate RS;WSis the weight,in mg,of USP Clindamycin Phosphate RStaken to prepare the Standard preparation;WUis the weight,in mg,of Clindamycin Phosphate taken to prepare the Assay preparation;and rUand rSare the peak areas for clindamycin phosphate obtained from the Assay preparationand the Standard preparation,respectively.