Test solution— Transfer about 1.5 g of Cream to a glass-stoppered, 50-mL centrifuge tube. Add 15 mL of a methanol–hydrochloric acid solution prepared by mixing 1 volume of dilute hydrochloric acid (1 in 120) with 4 volumes of methanol. Shake to obtain a homogeneous mixture. Add 30 mL of solvent hexane, mix for 10 minutes, and centrifuge. Using a suitable syringe, transfer the lower aqueous phase to a second centrifuge tube, add about 20 mL of water, and mix. Extract this aqueous mixture with chloroform by shaking, centrifuging, and removing the lower, chloroform phase with a syringe. Evaporate the chloroform on a steam bath with the aid of a stream of nitrogen to dryness, cool, and dissolve the residue in chloroform to obtain a solution containing about 150 µg of betamethasone dipropionate per mL.

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— Prepare as directed in the Assay under Betamethasone Dipropionate.

Internal standard solution— Prepare a solution of USP Beclomethasone Dipropionate RS in methanol containing acetic acid (1 in 1000) having a known concentration of about 0.45 mg per mL.

Standard preparation— Prepare a solution of USP Betamethasone Dipropionate RS in methanol containing acetic acid (1 in 1000) having a known concentration of about 0.2 mg per mL. Transfer 10.0 mL of this solution to a suitable vial, and add 5.0 mL of Internal standard solution, to obtain a Standard preparation having known concentrations of about 0.133 mg of betamethasone dipropionate and about 0.15 mg of beclomethasone dipropionate per mL.

Assay preparation— Transfer an accurately weighed quantity of Cream, equivalent to about 2 mg of betamethasone dipropionate, into a capped 50-mL centrifuge tube. Add 5.0 mL of Internal standard solution and 10.0 mL of methanol containing acetic acid (1 in 1000). Heat in a water bath at 60, shaking intermittently, until the assay specimen melts. Remove from the bath, and shake vigorously until the specimen has resolidified. Repeat the heating and shaking. Freeze in an ice-methanol bath for about 15 minutes, and centrifuge at 2500 rpm for about 5 minutes. Transfer a portion of the supernatant to a suitable vial.

Procedure— Proceed as directed for Procedure in the Assay under Betamethasone Dipropionate. Calculate the quantity, in mg, of betamethasone (C22H29FO5) in the portion of Cream 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.