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

Standard preparation— Transfer about 150 mg of USP Clindamycin Palmitate Hydrochloride RS, accurately weighed, to a glass-stoppered, 15-mL conical centrifuge tube. Add 5 mL of water, 5.0 mL of Internal standard solution, and 1 mL of sodium carbonate solution (3 in 10), and mix. Insert the stopper, shake vigorously for not less than 10 minutes, and centrifuge. Remove the upper aqueous layer, and transfer 1.0 mL of the lower chloroform layer to a 15-mL centrifuge tube. Add 1.0 mL of pyridine and 1.0 mL of 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 100 for 2.5 hours, and allow to cool. Mix, and centrifuge, if necessary. Use the clear solution.

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

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and contains a 0.6-m × 3-mm glass column packed with 1 percent phase G36 on support S1AB. The column and detector are maintained at about 290 and 320, respectively. Dry helium is used as the carrier gas at a flow rate of about 60 mL per minute.

Procedure— Separately inject equal volumes of about 1.0 µL of the Standard preparation and the Assay preparation into 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 F is the potency, in µg of clindamycin per mg, of the USP Clindamycin Palmitate Hydrochloride RS; RU and RS are the ratios of the peak response of clindamycin palmitate to that of cholesteryl benzoate obtained from the Assay preparation and the Standard preparation, respectively; and WS and WU are the amounts, in mg, of USP Clindamycin Palmitate Hydrochloride RS and Clindamycin Palmitate Hydrochloride taken, respectively.