A: Infrared Absorption 197M.

B: Thin-Layer Chromatographic Identification Test 201—

Test solution: 10 mg per mL, in dioxane.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile and water (65:35). Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability solution— Dissolve accurately weighed quantities of USP Betamethasone Dipropionate RS and USP Betamethasone Valerate RS in Mobile phase to obtain a solution having final concentrations of about 0.05 mg of each per mL.

Test solution— Transfer about 3 mg of Betamethasone Dipropionate, accurately weighed, to a suitable flask. Add 10 mL of Mobile phase, and shake until dissolved.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 15-cm column that contains packing L1. The flow rate is about 1 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between betamethasone valerate and betamethasone dipropionate is not less than 4.0; and the column efficiency is not less than 8000 theoretical plates.

Procedure— Inject a volume (about 10 µL) of the Test solution into the chromatograph, record the chromatogram, and measure all the peak responses. Calculate the percentage of each impurity in the portion of Betamethasone Dipropionate taken by the formula: in which ri is the peak response for each impurity; and rs is the sum of the responses for all the peaks: not more than 1.0% of any individual impurity is found; and not more than 2.0% of total impurities is found.

Mobile phase— Prepare a suitable acetonitrile solution (about 1 in 2), degassed by ultrasonic vibration for 5 to 10 minutes, such that the retention time of betamethasone dipropionate is approximately 14 minutes and that of beclomethasone dipropionate is approximately 18 minutes. [note—Do not leave the mobile phase in the column overnight, but flush the system after use with water for 15 minutes, followed by methanol for 15 minutes.]

Internal standard solution— Prepare a solution of USP Beclomethasone Dipropionate RS in a solution of acetic acid in methanol (1 in 1000) having a known concentration of about 0.9 mg per mL.

Standard preparation— Prepare a solution of USP Betamethasone Dipropionate RS in a solution of acetic acid in methanol (1 in 1000) having a known concentration of about 0.6 mg per mL. Transfer 5.0 mL of this solution to a suitable vial, and add 5.0 mL of Internal standard solution to obtain a solution having known concentrations of about 0.3 mg of betamethasone dipropionate and about 0.45 mg of beclomethasone dipropionate per mL.

Assay preparation— Accurately weigh about 60 mg of Betamethasone Dipropionate. Dilute quantitatively and stepwise with a solution of acetic acid in methanol (1 in 1000) to obtain a solution containing about 0.6 mg per mL. Transfer 5.0 mL of this solution to a suitable vial, and add 5.0 mL of Internal standard solution.

Procedure— Separately inject equal volumes (between 5 µL and 25 µL) of the Assay preparation and the Standard preparation into a high-pressure liquid chromatograph (see Chromatography 621) operated at room temperature, by means of a suitable microsyringe or sampling valve, adjusting the specimen size and other operating parameters such that the peak obtained from the internal standard in the Standard preparation is about 0.6 full-scale. Typically, the apparatus is fitted with a 4-mm × 30-cm column that contains packing L1, and is equipped with a UV detector capable of monitoring absorption at 254 nm or 240 nm and a suitable recorder, and is capable of operating at a column pressure of up to 3500 psi. In a suitable chromatogram, the lowest and highest peak area ratios (RS) of three successive injections of the Standard preparation agree within 2.0%. Determine the ratio of the peak heights, at equivalent retention times, obtained with the Assay preparation and the Standard preparation, and calculate the quantity, in mg, of C28H37FO7 in the portion of Betamethasone Dipropionate taken by the formula: in which 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 betamethasone dipropionate to the internal standard obtained from the Assay preparation and the Standard preparation, respectively.