Medium: pH 6.8 phosphate buffer (see Buffer Solutions in the section Reagents, Indicators, and Solutions); 900 mL.

Apparatus 1: 100 rpm.

Time: 30 minutes.

Procedure— Determine the amount of clindamycin (C18H33ClN2 O5S) dissolved by employing the following method.

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

Standard solution— Prepare a solution of USP Clindamycin Hydrochloride RS in Dissolution Medium having 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 Medium if necessary.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a refractive index detector and a column that contains 3-µm packing L1. The flow rate is about 2 mL per 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 50 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the amount of C18H33ClN2O5S dissolved.

Tolerances— Not less than 80% (Q) of the labeled amount of C18H33ClN2O5S is dissolved in 30 minutes.

Mobile phase— Add 2 g of dl-10-camphorsulfonic acid, 1 g of ammonium acetate, and 1 mL of glacial acetic acid to 200 mL of water in a 500-mL volumetric flask, and mix to dissolve. Dilute with methanol to volume, and mix. Adjust, if necessary, with hydrochloric acid or a sodium hydroxide solution (1 in 2) to a pH of 6.0 ± 0.1. Make adjustments, if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Add 0.5 mL of phenylethyl alcohol to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

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

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

Chromatographic system (see Chromatography 621)— 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 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.6 for the internal standard and 1.0 for 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 25 µL) of the Standard preparation and the Assay preparation into 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 WS is the amount, in mg, of USP Clindamycin Hydrochloride RS taken to prepare the Standard preparation; P is the potency, in µg, of clindamycin per mg of USP Clindamycin Hydrochloride RS; and RU and RS are the ratios of the response of the clindamycin peak to that of the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.