A: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, both relative to the internal standard, as obtained in the Assay.

B: Place a quantity of Ointment, equivalent to about 1.25 mg of alclometasone dipropionate, in a 50-mL centrifuge tube, add 10 mL of 2,2,4-trimethylpentane, insert a stopper securely into the tube, and disperse the specimen using a vortexing mixer. Add 5.0 mL of a solution of methanol in water (45 in 50), insert the stopper securely, shake vigorously for 2 minutes, and centrifuge at 2500 rpm for 3 minutes. Remove the lower, aqueous alcohol phase, and transfer this test solution to a stoppered vial. Apply separately 20 µL of the test solution and 20 µL of a Standard solution of USP Alclometasone Dipropionate RS in methanol containing about 0.25 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture, and dry the applications with the aid of a stream of nitrogen. Position the plate in a saturated, unlined chromatographic chamber, and develop the chromatograms in a solvent system consisting of a mixture of chloroform and acetone (7:1) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Observe the plate under short-wavelength UV light: the RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

Methanol solution— Dilute 450 mL of methanol with water to 500 mL, and mix.

0.05 M Monobasic potassium phosphate— Transfer 3.40 g of monobasic potassium phosphate to a 500-mL volumetric flask, add water to volume, and mix.

Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.05 M Monobasic potassium phosphate (2:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Transfer about 30 mg of betamethasone dipropionate to a 200-mL volumetric flask, add Methanol solution to volume, and mix.

Standard preparation— Transfer about 20 mg of USP Alclometasone Dipropionate RS, accurately weighed, to a 200-mL volumetric flask, add Methanol solution to volume, and mix. Transfer 5.0 mL of this solution to a small stoppered flask, add 5.0 mL of Internal standard solution, and mix to obtain a Standard preparation having a known concentration of about 0.05 mg of USP Alclometasone Dipropionate RS per mL.

Assay preparation— Transfer an accurately weighed quantity of Ointment, equivalent to about 0.5 mg of alclometasone dipropionate, to a 50-mL centrifuge tube, add 10 mL of 2,2,4-trimethylpentane, insert a stopper securely into the tube, and disperse the specimen using a vortexing mixer. Add 5.0 mL of Internal standard solution and 5.0 mL of Methanol solution, insert the stopper securely, shake vigorously for 2 minutes, and centrifuge at 2500 rpm for 3 minutes. Remove the lower, aqueous alcohol phase, and transfer this Assay preparation to a stoppered vial.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 1.2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between the analyte and internal standard peaks is not less than 3.0; and the relative standard deviation for replicate injections is not more than 2%.

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 responses for the major peaks. The relative retention times are about 0.7 for alclometasone dipropionate and 1.0 for betamethasone dipropionate. Calculate the quantity, in mg, of alclometasone dipropionate (C28H37ClO7) in the portion of Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Alclometasone Dipropionate RS in the Standard preparation; and RU and RS are the peak height ratios obtained from the Assay Preparation and the Standard preparation, respectively.