Mobile phase— Prepare a filtered and degassed mixture of methanol, water, and acetonitrile (23:18:9). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Dissolve suitable quantities of USP Methyltestosterone RS in methanol to obtain a solution containing about 250 µg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Betamethasone Benzoate RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.5 mg per mL. Transfer 5.0 mL of this solution to a 50-mL volumetric flask, add 10.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Assay preparation— Transfer an accurately weighed portion of Gel, equivalent to about 0.5 mg of betamethasone benzoate, to a 125-mL separatory funnel, add 20 mL of water and 2 mL of saturated sodium acetate solution, shake to disperse the Gel, add 2.0 mL of Internal standard solution, and mix. Extract this solution with one 50-mL portion of chloroform followed by three 40-mL portions of chloroform. Discard the aqueous layer. Wash the chloroform extract with 10 mL of water, allow to stand for 10 minutes, then pass through chloroform-wetted glass fiber filter paper and anhydrous sodium sulfate into a suitable container. Evaporate to dryness under vacuum at 30. Dissolve the residue in 10 mL of methanol.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 236-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. The column temperature is maintained at 30. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 1.33 for betamethasone benzoate and 1.0 for methyltestosterone; the resolution, R, between methyltestosterone and betamethasone benzoate is not less than 3.0; and the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of betamethasone benzoate (C29H33FO6) in the portion of Gel taken by the formula: in which C is the concentration, in µg per mL, of USP Betamethasone Benzoate RS in the Standard preparation; and RU and RS are the ratios of the peak responses obtained for betamethasone benzoate and methyltestosterone from the Assay preparation and the Standard preparation, respectively.