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.]

Standard preparation— 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 Injection,equivalent to about 300mg of clindamycin,to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.Transfer 7.0mLof the resulting solution to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Resolution solution— Prepare a solution of benzyl alcohol in Mobile phaseto obtain a solution having a concentration of about 0.1mg per mL.Add about 25mLof this solution to a 100-mLvolumetric flask containing about 25mg of USP Clindamycin Phosphate RS,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 resolution,R,between clindamycin phosphate and benzyl alcohol is not less than 2.0.The relative retention times are 1.0for clindamycin phosphate and about 1.2for benzyl alcohol.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 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 C18H33ClN2O5Sin each mLof the Injection 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 Injection taken,and rUand rSare the clindamycin phosphate peak responses obtained from the Assay preparationand the Standard preparation,respectively.