Medium: pH6.8phosphate buffer (see Buffer Solutions in the sectionReagents,Indicators,and Solutions);900mL.

Apparatus 1: 100rpm.

Time: 30minutes.

Procedure— Determine the amount of clindamycin (C18H33ClN2O5S)dissolved by employing the following method.

Mobile phase— Dissolve 16g of dl-10-camphorsulfonic acid,8g of ammonium acetate,and 8mLof glacial acetic acid in 1600mLof water,and mix.Add 2400mLof methanol to this solution,mix,and adjust with hydrochloric acid or 5Nsodium hydroxide to a pHof 6.0±0.05.

Standard solution— Prepare a solution of USP Clindamycin Hydrochloride RSin Dissolution Mediumhaving an accurately known concentration similar to that expected in the Test solution.

Test solution— Use a filtered portion of the solution under test,diluted with Dissolution Mediumif necessary.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a refractive index detector and a column that contains 3-µm packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard solution,and record the peak responses as directed for Procedure:the tailing factor is not more than 2.0;and the relative standard deviation for replicate injections is not more than 3.0%.

Procedure— Separately inject equal volumes (about 50µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the amount of C18H33ClN2O5Sdissolved.

Tolerances— Not less than 80%(Q)of the labeled amount of C18H33ClN2O5Sis dissolved in 30minutes.

Mobile phase— Add 2g of dl-10-camphorsulfonic acid,1g of ammonium acetate,and 1mLof glacial acetic acid to 200mLof water in a 500-mLvolumetric flask,and mix to dissolve.Dilute with methanol to volume,and mix.Adjust,if necessary,with hydrochloric acid or a sodium hydroxide solution (1in 2)to a pHof 6.0±0.1.Make adjustments,if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Add 0.5mLof phenylethyl alcohol to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Standard preparation— Transfer about 90mg of USP Clindamycin Hydrochloride RS,accurately weighed,to a suitable container.Add 5.0mLof Internal standard solution,and swirl to dissolve.

Assay preparation— Remove as completely as possible the contents of not fewer than 20Capsules,accurately counted,weigh,and mix.Transfer an accurately weighed portion of the powder,equivalent to about 75mg of clindamycin,to a suitable container.Add 5.0mLof Internal standard solution,and shake for about 30minutes.Centrifuge or filter,if necessary,to obtain a clear solution.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a refractive index detector and a 4-mm ×30-cm stainless steel column containing packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.6for the internal standard and 1.0for clindamycin;the resolution,R,between the analyte and the internal standard is not less than 5.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 25µL)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 Capsules taken by the formula: in which WSis the amount,in mg,of USP Clindamycin Hydrochloride RStaken to prepare the Standard preparation;Pis the potency,in µg,of clindamycin per mg of USP Clindamycin Hydrochloride RS;and RUand RSare the ratios of the response of the clindamycin peak to that of the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.