Test solution: 10 mg per mL in dioxane.

Mobile phase , System suitability solution, and Chromatographic system—Proceed as directed in the Assay.

Test solution— Dissolve an accurately weighed quantity of Clobetasol Propionate in Mobile phase to obtain a solution containing about 0.1 mg per mL.

Procedure— Inject a volume (about 10 µL) of the Test solution into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the percentage of each impurity in the portion of Clobetasol Propionate taken by the formula: in which ri is the peak response for each impurity, and rs is the sum of the responses of all of the peaks: not more than 1.0% of any individual impurity is found, and the sum of all impurities is not more than 2.5%.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile, 0.05 M monobasic sodium phosphate (adjusted with 85% phosphoric acid to a pH of 2.5), and methanol (95:85:20). Make adjustments if necessary (see System Suitability under Chromatography 621 ).

Internal standard solution— Dissolve an accurately weighed quantity of beclomethasone dipropionate in methanol to obtain a solution having a known concentration of about 0.2 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Clobetasol Propionate RS in methanol and Internal standard solution to obtain a final solution containing about 0.04 mg of USP Clobetasol Propionate RS per mL and 0.08 mg of beclomethasone dipropionate per mL.

System suitability solution— Dissolve suitable quantities of USP Clobetasol Propionate Related Compound A RS and USP Clobetasol Propionate RS in Mobile phase to obtain a solution containing about 0.001 and 0.1 mg per mL, respectively.

Assay preparation— Transfer about 4 mg of Clobetasol Propionate, accurately weighed, to a 100-mL volumetric flask. Add 40.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 240-nm detector and a 4.6-mm × 15-cm column that contains packing L1. The flow rate is about 1.0 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 1.1 for clobetasol propionate related compound A, and 1.0 for clobetasol propionate; the resolution, R, between clobetasol propionate and clobetasol propionate related compound A is not less than 1.5; the column efficiency determined from the clobetasol propionate peak is not less than 5000 theoretical plates; the tailing factor for the clobetasol propionate peak is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 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 1.0 for clobetasol propionate and 1.6 for beclomethasone dipropionate. Calculate the quantity, in mg, of C25H32ClFO5 in the portion of Clobetasol Propionate taken by the formula: in which C is the concentration, in mg per mL, of USP Clobetasol Propionate RS in the Standard preparation; and RU and RS are the ratios of the clobetasol propionate peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.