Test solution— Place the container in a dry ice–methanol bath for about 5 minutes. Open the can by means of a tube-cutter, and allow the propellant to evaporate under a gentle stream of nitrogen for about 1 hour. Transfer about 3 mL of the residue to a 50-mL centrifuge tube. Add 10 mL of a mixture of methanol and water (4:1), and shake vigorously. Centrifuge to clarify.

Standard solution: USP Betamethasone Dipropionate RS in methanol containing 3.2 mg per mL.

Application volume: 25 µL.

Developing solvent system: a mixture of toluene and ethyl acetate (1:1).

Procedure— Proceed as directed in the chapter. Spray the plate with a mixture of sulfuric acid, methanol, and nitric acid (10:10:1), and heat at 105 for 15 minutes.

(Official January 1, 2007)

Mobile phase— Prepare as directed for Mobile phase in the Assay under Betamethasone Dipropionate.

Internal standard solution— Prepare a solution of USP Beclomethasone Dipropionate RS, having a known concentration of about 0.90 mg per mL, in isopropyl alcohol containing glacial acetic acid (1 in 1000).

Standard preparation— Prepare a solution of USP Betamethasone Dipropionate RS, having a known concentration of about 0.642 mg per mL, in isopropyl alcohol containing acetic acid (1 in 1000). Transfer 10.0 mL of this solution and 10.0 mL of Internal standard solution to a 100-mL volumetric flask, add isopropyl alcohol containing acetic acid (1 in 1000) to volume, and mix, to obtain a solution having known concentrations of about 0.09 mg of beclomethasone dipropionate and about 0.0642 mg of betamethasone dipropionate per mL.

Assay preparation— Discharge the entire contents of the container of Topical Aerosol into a 100-mL volumetric flask. Allow the solution to warm to room temperature slowly to prevent it from boiling out of the flask, then evaporate the propellant by swirling the flask in a water bath at about 25 until the solution stops bubbling. Add 10.0 mL of Internal standard solution, and dilute with glacial acetic acid in isopropyl alcohol (1 in 1000) to volume. Pass the solution through a 0.45-µm filter.

Procedure— Proceed as directed for Procedure in the Assay under Betamethasone Dipropionate. Calculate the quantity, in mg, of betamethasone (C22H29FO5) equivalent to the quantity of betamethasone dipropionate (C28H37FO7) in the container of the Topical Aerosol taken by the formula: in which 392.46 and 504.60 are the molecular weights of betamethasone and betamethasone dipropionate, respectively; C is the concentration, in mg per mL, of USP Betamethasone Dipropionate RS in the Standard preparation; and RU and RS are the peak height ratios of the betamethasone dipropionate and beclomethasone dipropionate peaks in the Assay preparation and the Standard preparation, respectively.