A: Dilute 2 mL with 2 mL of methanol. Apply 10 µL of this solution and 10 µL of a methanol and water solution (1:1) of USP Betamethasone Sodium Phosphate RS containing 2 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621), coated with a 0.25-mm layer of chromatographic silica gel. Proceed as directed for Identification test B under Betamethasone Sodium Phosphate, beginning with “Allow the spots to dry.”

B: Apply 10 µL of the solution prepared for Identification test A and 10 µL of a methanol and water solution (1:1) of USP Betamethasone Acetate RS containing 1.5 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621), coated with a 0.25-mm layer of chromatographic silica gel. Proceed as directed for Identification test B under Betamethasone, beginning with “Allow the spots to dry.”

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.075 M monobasic potassium phosphate (7:5). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Transfer about 50 mg of methyltestosterone to a 50-mL volumetric flask, add methanol to volume, and mix.

Standard preparation— Transfer about 63 mg of USP Betamethasone Sodium Phosphate RS, accurately weighed, to a 25-mL volumetric flask, add Mobile phase to volume, and mix (Solution 1). Transfer about 45 mg of USP Betamethasone Acetate RS, accurately weighed, to a 25-mL volumetric flask, add methanol to volume, and mix (Solution 2). Pipet 5 mL each of Solution 1 and Solution 2 into a 100-mL volumetric flask. Add 10.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix to obtain a Standard preparation having known concentrations of about 126 µg of USP Betamethasone Sodium Phosphate RS per mL and 90 µg of USP Betamethasone Acetate RS per mL.

Assay preparation— Using a “To contain” pipet transfer an accurately measured volume of the well-mixed Injectable Suspension, equivalent to about 9 mg of betamethasone acetate, to a 100-mL volumetric flask. Rinse the pipet with about 10 mL of Mobile phase, collecting the rinse in the volumetric flask. Add 10.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between the betamethasone phosphate and betamethasone acetate peaks is not less than 5.0, and the resolution, R, between the betamethasone acetate and internal standard peaks is not less than 3.0, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.5 for betamethasone phosphate, 1.7 for methyltestosterone, and 1.0 for betamethasone acetate. Calculate the quantity, in mg, of betamethasone acetate (C24H31FO6) in each mL of the Injectable Suspension taken by the formula: in which C is the concentration, in µg per mL, of USP Betamethasone Acetate RS in the Standard preparation; V is the volume, in mL, of Injectable Suspension taken; and RU and RS are the peak response ratios obtained for betamethasone acetate and methyltestosterone from the Assay preparation and the Standard preparation, respectively. Calculate the quantity, in mg, of betamethasone (C22H29FO5) equivalent to the quantity of betamethasone sodium phosphate (C22H28FNa2O8P), in each mL of the Injectable Suspension taken by the formula: in which 392.46 and 516.41 are the molecular weights of betamethasone and betamethasone sodium phosphate, respectively; C is the concentration, in µg per mL, of USP Betamethasone Sodium Phosphate RS in the Standard preparation; V is the volume, in mL, of Injectable Suspension taken; and RU and RS are the peak response ratios obtained for betamethasone phosphate and methyltestosterone from the Assay preparation and the Standard preparation, respectively.