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, mix, and filter. Make adjustments if necessary (see System Suitability under Chromatography 621). [note—Ensure that the concentration of acetonitrile in the Mobile phase is not less than 22% and not more than 25%, in order to retain the correct elution order.]

Resolution solution— Prepare a solution of 4¢-hydroxyacetophenone in acetonitrile containing about 4 mg per mL. Dilute a volume of this solution with Mobile phase to obtain a solution having a concentration of about 0.04 mg per mL. Mix 1 volume of this solution with 3 volumes of Standard preparation.

Standard preparation— Quantitatively dissolve an accurately weighed quantity of USP Clindamycin Phosphate RS in Mobile phase to obtain a solution having a known concentration of about 0.24 mg per mL.

Assay preparation— Transfer an accurately measured volume of Topical Solution, equivalent to about 20 mg of clindamycin, to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

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 1.0 for clindamycin phosphate and 1.2 for 4¢-hydroxyacetophenone; and the resolution, R, between clindamycin phosphate and 4¢-hydroxyacetophenone is not less than 2.0. Chromatograph the Standard preparation, and record the peak responses as directed for 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 each mL of the Topical Solution taken by the formula: in which C is the concentration, in mg per mL, of USP Clindamycin Phosphate RS in the Standard preparation; P is the potency, in µg of C18H33ClN2O5S per mg of USP Clindamycin Phosphate RS; V is the volume, in mL, of Topical Solution taken; and rU and rS are the clindamycin phosphate peak responses obtained from the Assay preparation and the Standard preparation, respectively.