Internal standard solution— Dissolve cholesteryl benzoate in chloroform to obtain a solution containing about 5mg per mL.

Standard preparation— Transfer about 150mg of USP Clindamycin Palmitate Hydrochloride RS,accurately weighed,to a glass-stoppered,15-mLconical centrifuge tube.Add 5mLof water,5.0mLof Internal standard solution,and 1mLof sodium carbonate solution (3in 10),and mix.Insert the stopper,shake vigorously for not less than 10minutes,and centrifuge.Remove the upper aqueous layer,and transfer 1.0mLof the lower chloroform layer to a 15-mLcentrifuge tube.Add 1.0mLof pyridine and 1.0mLof acetic anhydride.Agitate the tube to ensure complete mixing,cover the top of the centrifuge tube with a plastic cap through which a small hole has been punched,heat at 100for 2.5hours,and allow to cool.Mix,and centrifuge,if necessary.Use the clear solution.

Assay preparation— Transfer about 150mg of Clindamycin Palmitate Hydrochloride,accurately weighed,to a glass-stoppered,15-mLconical centrifuge tube,and proceed as directed for Standard preparation,beginning with “Add 5mLof water.”

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and contains a 0.6-m ×3-mm glass column packed with 1percent phase G36on support S1AB.The column and detector are maintained at about 290and 320,respectively.Dry helium is used as the carrier gas at a flow rate of about 60mLper minute.

Procedure— Separately inject equal volumes of about 1.0µLof the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.In a suitable chromatogram the resolution of the peaks is complete.The elution order is:cholesteryl benzoate,clindamycin palmitate.Calculate the potency,in µg of clindamycin (C18H33ClN2O5S)per mg,in the Clindamycin Palmitate Hydrochloride taken by the formula: in which Fis the potency,in µg of clindamycin per mg,of the USP Clindamycin Palmitate Hydrochloride RS,RUand RSare the ratios of the peak response of clindamycin palmitate to that of cholesteryl benzoate obtained from the Assay preparationand the Standard preparation,respectively,and WSand WUare the amounts,in mg,of USP Clindamycin Palmitate Hydrochloride RSand Clindamycin Palmitate Hydrochloride taken,respectively.