Test solution— Transfer a quantity of Lotion,equivalent to about 0.6mg of betamethasone dipropionate,to a 50-mLvial.Add 10mLof 0.1Nhydrochloric acid,then add 4mLof chloroform.Disperse on a vortex mixer for about 1minute,then shake vigorously for 10minutes,and centrifuge at 2000rpm for about 5minutes.Transfer the chloroform layer to a suitable vial.

Standard solution: USP Betamethasone Dipropionate RSin chloroform containing 150µg per mL.

Application volume: 40µL.

Developing solvent system: a mixture of chloroform and acetone (7:1).

Procedure— Proceed as directed in the chapter.

Mobile phaseand Chromatographic system— Prepare as directed in the Assayunder Betamethasone Dipropionate.

Internal standard solution— Prepare as directed in the Assayunder Betamethasone Dipropionate,except to use chloroform as the solvent.

Standard preparation— Prepare as directed in the Assayunder Betamethasone Dipropionate,except to use chloroform as the solvent.To 10.0mLof 0.1Nhydrochloric acid in a capped 5-mLcentrifuge tube add 4.0mLof the prepared solution,and treat this solution as follows.Cap,and shake vigorously for about 2minutes,or disperse on a vortex mixer for about 1minute.Centrifuge at 2500rpm for about 3minutes.Transfer the chloroform phase to a suitable vial.Evaporate the chloroform under a stream of nitrogen at a slightly elevated temperature to dryness.Cool the vial to room temperature,add 4.0mLof methanol,and swirl to dissolve the residue.

Assay preparation— Transfer an accurately weighed quantity of Lotion,equivalent to about 1.2mg of betamethasone dipropionate,to a capped 50-mLcentrifuge tube.Add 10.0mLof 0.1Nhydrochloric acid,shake to disperse,then add 2.0mLof Internal standard solutionand 2.0mLof chloroform.Proceed as directed for Standard preparationbeginning with “Cap,and shake.”

Procedure— Proceed as directed for Procedurein the Assayunder Betamethasone Dipropionate.Calculate the quantity,in mg,of betamethasone (C22H29FO5)in the portion of Lotion 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 peak and the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.