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

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

Application volume: 25µL.

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 105for 15minutes.

Mobile phase— Prepare as directed for Mobile phasein the Assayunder Betamethasone Dipropionate.

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

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

Assay preparation— Discharge the entire contents of the container of Topical Aerosol into a 100-mLvolumetric 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 25until the solution stops bubbling.Add 10.0mLof Internal standard solution,and dilute with glacial acetic acid in isopropyl alcohol (1in 1000)to volume.Pass the solution through a 0.45-µm filter.

Procedure— Proceed as directed for Procedurein the Assayunder 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 392.46and 504.60are the molecular weights of betamethasone and betamethasone dipropionate,respectively;Cis the concentration,in mg per mL,of USP Betamethasone Dipropionate RSin the Standard preparation;and RUand RSare the peak height ratios of the betamethasone dipropionate and beclomethasone dipropionate peaks in the Assay preparationand the Standard preparation,respectively.