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,and mix.Pass through a filter having a porosity of 0.5µm or less,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Resolution solution— Prepare a solution in Mobile phasecontaining in each mLabout 0.6mg of USP Clindamycin Phosphate RSand about 0.6mg of USP Clindamycin Hydrochloride RS.

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

Assay preparation— Transfer an accurately weighed quantity of Gel,equivalent to about 20mg of clindamycin (C18H33ClN2O5S),to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and shake by mechanical means for 30minutes.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 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 under Procedure:the relative retention times are about 1for clindamycin phosphate and 1.5for 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 1700theoretical 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 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 the portion of Gel taken by the formula: in which Cis the concentration,in mg per mL,of USP Clindamycin Phosphate RSin the Standard preparation,Eis the designated clindamycin (C18H33ClN2O5S)content,in µg per mg,of USP Clindamycin Phosphate RS,and rUand rSare the clindamycin phosphate peak responses obtained from the Assay preparationand the Standard preparation,respectively.