Test solution: 40 mg per mL, in dioxane.

Adsorbent: 0.25-mm layer of chromatographic silica gel mixture.

Test solution— Dissolve 100.0 mg in 5.0 mL of methanol.

Standard solution 1— Dissolve a suitable quantity of USP Betamethasone Benzoate RS in methanol to obtain a solution having a known concentration of about 5 mg per mL.

Standard solution 2— Dilute a portion of Standard solution 1, quantitatively and stepwise, with methanol to obtain a solution having a known concentration of about 100 µg per mL.

Application volume: 10 µL.

Developing solvent system: a mixture of toluene, acetone, and methanol (75:25:4).

Procedure— Proceed as directed for Thin-Layer Chromatography under Chromatography 621. Examine the plate under short-wavelength UV light: the principal spot from the Test solution corresponds in RF value to that of Standard solution 1; and the Test solution shows not more than 3 additional spots, the intensity and size of which do not exceed those of the spot from Standard solution 2.

(Official January 1, 2007)

Mobile phase— Prepare a suitable filtered solution of acetonitrile and water (60:40).

Internal standard solution— Prepare a solution of betamethasone dipropionate in methanol containing 0.6 mg per mL.

Standard preparation— Using an accurately weighed quantity of USP Betamethasone Benzoate RS, prepare a solution in methanol having a known concentration of about 0.6 mg per mL. Mix 5.0 mL of this solution and 10.0 mL of the Internal standard solution to obtain a Standard preparation having a known concentration of about 0.2 mg of betamethasone benzoate per mL.

Assay preparation— Transfer about 60 mg of Betamethasone Benzoate, accurately weighed, to a 100-mL volumetric flask. Dilute with methanol to volume, and mix. Mix 5.0 mL of this solution and 10.0 mL of the Internal standard solution.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph three replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%; and the resolution factor between betamethasone benzoate and the internal standard is not less than 3.

Procedure— Separately inject equal volumes (about 15 µL) of the Standard preparation and the Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses for the major peaks. The retention times are about 7 and 5 minutes for betamethasone dipropionate and betamethasone benzoate, respectively. Calculate the quantity, in mg, of C29H33FO6 in the portion of Betamethasone Benzoate taken by the formula: in which C is the concentration, in mg per mL, of USP Betamethasone Benzoate RS in the Standard preparation; and RU and RS are the peak response ratios of the betamethasone benzoate peak and the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.