Mobile phase— Prepare as directed in the Assay.

Standard solution— Dissolve accurately weighed quantities of USP Lincomycin Hydrochloride RS and USP Clindamycin Hydrochloride RS quantitatively in Mobile phase to obtain a solution having known concentrations of about 0.5 mg per mL of USP Lincomycin Hydrochloride RS and 1 mg per mL of USP Clindamycin Hydrochloride RS per mL. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Test solution— Transfer about 125 mg of Clindamycin Hydrochloride, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.4 for lincomycin, 0.65 for clindamycin B, 0.8 for 7-epiclindamycin, and 1.0 for clindamycin.

Procedure— Separately inject equal volumes (about 10 µL) of the Test solution and the Standard solution, and record the chromatograms for a period of time that is six times the retention time of clindamycin. Calculate the percentage of lincomycin in the Clindamycin Hydrochloride taken by the formula: in which CL is the concentration, in mg per mL, of USP Lincomycin Hydrochloride RS in the Standard solution; PL is the potency, in µg of lincomycin (C18H34N2O6S) per mg, of USP Lincomycin Hydrochloride RS; W is the weight, in mg, of Clindamycin Hydrochloride taken to prepare the Test solution; and rU and rS are the lincomycin peak responses obtained from the Test solution and the Standard solution, respectively. Calculate the percentage of all other related compounds in the Clindamycin Hydrochloride taken by the formula: in which C is the concentration, in mg per mL, of USP Clindamycin Hydrochloride RS in the Standard solution; P is the potency, in µg, of clindamycin (C18H33ClN2O5S) per mg, of USP Clindamycin Hydrochloride RS; W is the weight, in mg, of Clindamycin Hydrochloride taken to prepare the Test solution; ri is the response of an individual related compound, other than lincomycin, in the chromatogram obtained from the Test solution; and rC is the clindamycin peak response in the chromatogram obtained from the Standard solution. Not more than 4.0% of 7-epiclindamycin and not more than 2.0% of clindamycin B is found, the percentage of any other individual related compound is not more than 1.0%, and the total of all related compounds, including lincomycin, is not more than 6.0%.

Mobile phase— Dissolve 6.8 g of monobasic potassium phosphate in 1 L of water, and adjust with 8 N potassium hydroxide to a pH of 7.5. Prepare a filtered and degassed mixture of this buffer solution and acetonitrile (550:450). Make adjustments if necessary (see System Suitability under Chromatography 621). [note—Increasing the proportion of acetonitrile in the Mobile phase decreases the retention time, and decreasing it increases the resolution between 7-epiclindamycin and clindamycin.]

Standard preparation— Dissolve an accurately weighed quantity of USP Clindamycin Hydrochloride RS quantitatively in Mobile phase to obtain a solution having a known concentration of about 1 mg per mL.

Assay preparation— Transfer about 125 mg of Clindamycin Hydrochloride, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix. Transfer 5.0 mL of this solution to a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm × 25-cm column that contains 5-µm 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 resolution, R, between the clindamycin B peak and the 7-epiclindamycin peak is not less than 2.4; the resolution, R, between the 7-epiclindamycin peak and the clindamycin peak is not less than 3.0; the column efficiency calculated from the clindamycin peak is not less than 4000; the tailing factor for the clindamycin peak is not more than 1.2; and the relative standard deviation for the clindamycin peak is not more than 1.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms for a period of time that is about twice the retention time of the clindamycin peak, and measure the clindamycin peak areas. Calculate the potency, in µg of clindamycin (C18H33ClN2O5S) per mg, of the Clindamycin Hydrochloride taken by the formula: in which C is the concentration, in mg per mL, of USP Clindamycin Hydrochloride RS in the Standard preparation; P is the potency, in µg of clindamycin per mg, of USP Clindamycin Hydrochloride RS; W is the weight, in mg, of Clindamycin Hydrochloride taken to prepare the Assay preparation; and rU and rS are the clindamycin peak area responses obtained from the Assay preparation and the Standard preparation, respectively.