Test solution: 20 mg per mL, in water.

Mobile phase— Add 13.5 mL of phosphoric acid to 1000 mL of water, and adjust with ammonium hydroxide to a pH of 6.0. Prepare a filtered and degassed mixture of this solution, acetonitrile, and methanol (780:150:150). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Lincomycin Hydrochloride RS in Mobile phase to obtain a solution having a known concentration of about 1.2 mg per mL, using sonication if necessary to effect solution.

Assay preparation— To about 12 mg of Lincomycin Hydrochloride, accurately weighed, add 10.0 mL of Mobile phase. Shake by mechanical means for 5 minutes, and sonicate if necessary to effect solution.

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 L7 and is maintained at a temperature of 46. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the tailing factor for the main lincomycin peak is not more than 1.3; the column efficiency determined from the main lincomycin peak is not less than 4000 theoretical plates; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. The relative retention times are about 0.5 for lincomycin B and 1.0 for lincomycin. Calculate the quantity, in µg, of lincomycin (C18H34N2O6S) in each mg of the Lincomycin Hydrochloride taken by the formula: in which C is the concentration, in mg per mL, of USP Lincomycin Hydrochloride RS in the Standard preparation; P is the designated potency, in µg of lincomycin per mg, of USP Lincomycin Hydrochloride RS; W is the weight, in mg, of the portion of Lincomycin Hydrochloride taken to prepare the Assay preparation; and rU and rS are the lincomycin peak responses obtained from the Assay preparation and the Standard preparation, respectively.