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

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

Application volume: 40 µL.

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

Procedure— Proceed as directed in the chapter.

(Official January 1, 2007)

Mobile phase and Chromatographic system— Prepare as directed in the Assay under Betamethasone Dipropionate.

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

Standard preparation— Prepare as directed in the Assay under Betamethasone Dipropionate, except to use chloroform as the solvent. To 10.0 mL of 0.1 N hydrochloric acid in a capped 5-mL centrifuge tube add 4.0 mL of the prepared solution, and treat this solution as follows. Cap, and shake vigorously for about 2 minutes, or disperse on a vortex mixer for about 1 minute. Centrifuge at 2500 rpm for about 3 minutes. 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 4.0 mL of methanol, and swirl to dissolve the residue.

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

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