Mobile phase— Dissolve 10.54 g of monobasic potassium phosphate in 775 mL of water, and adjust with phosphoric acid to a pH of 2.5. Add 225 mL of acetonitrile, and mix. Pass through a filter having a porosity of 0.5 µm or less, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Resolution solution— Prepare a solution in Mobile phase containing in each mL about 0.6 mg of USP Clindamycin Phosphate RS and about 0.6 mg of USP Clindamycin Hydrochloride RS.

Standard preparation— Dissolve an accurately weighed quantity of USP Clindamycin Phosphate RS in Mobile phase to obtain a solution having a known concentration of about 0.25 mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Gel, equivalent to about 20 mg of clindamycin (C18H33ClN2O5S), to a 100-mL volumetric flask, dilute with Mobile phase to volume, and shake by mechanical means for 30 minutes. Centrifuge a portion of the solution thus obtained, and if necessary, filter a portion of the supernatant. Use the clear filtrate as the Assay preparation.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm × 25-cm column that contains packing L7. The flow rate is about 1 mL per minute. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the relative retention times are about 1 for clindamycin phosphate and 1.5 for clindamycin, the resolution, R, between the clindamycin phosphate peak and the clindamycin peak is not less than 6.0, the column efficiency is not less than 1700 theoretical plates when calculated by the formula: and the tailing factor is not more than 1.3. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation for replicate injections is not more than 2.5%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of clindamycin (C18H33ClN2O5S) in the portion of Gel taken by the formula: in which C is the concentration, in mg per mL, of USP Clindamycin Phosphate RS in the Standard preparation; E is the designated clindamycin (C18H33ClN2O5S) content, in µg per mg, of USP Clindamycin Phosphate RS; and rU and rS are the clindamycin phosphate peak responses obtained from the Assay preparation and the Standard preparation, respectively.