Mobile phase— Dissolve 10.54g of monobasic potassium phosphate in 775mLof water,and adjust with phosphoric acid to a pHof 2.5.Add 225mLof acetonitrile,mix,and filter.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).[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 4mg per mL.Dilute a volume of this solution with Mobile phaseto obtain a solution having a concentration of about 0.04mg per mL.Mix 1volume of this solution with 3volumes of Standard preparation.

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

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

Chromatographic system (see Chromatography á621ñ)— 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 1mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the relative retention times are about 1.0for clindamycin phosphate and 1.2for 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 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of clindamycin (C18H33ClN2O5S)in each mLof the Topical Solution taken by the formula: in which Cis the concentration,in mg per mL,of USP Clindamycin Phosphate RSin the Standard preparation;Pis the potency,in µg of C18H33ClN2O5Sper mg of USP Clindamycin Phosphate RS;Vis the volume,in mL,of Topical Solution taken;and rUand rSare the clindamycin phosphate peak responses obtained from the Assay preparationand the Standard preparation,respectively.