A: Infrared Absorption 197M.

B: Ultraviolet Absorption 197U—

Test solution: 10 mg per mL, in dioxane.

Format buffer— Dissolve 1.32 g of ammonium formate in 1 L of water, adjust with formic acid to a pH of 3.6, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Formate buffer and acetonitrile (3:2). Make adjustments if necessary (see System Suitability under Chromatography 621).

Test solution— Transfer about 200 mg of Dexamethasone Acetate, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with acetonitrile to volume, and mix. Transfer about 40 mL of this solution to a 100-mL volumetric flask, dilute with Formate buffer to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L11. The flow rate is about 1 mL per minute. Chromatograph the Test solution, and record the peak responses as directed for Procedure: the column efficiency is not less than 5400 theoretical plates.

Procedure— Inject a volume (about 10 µL) of the Test solution into the chromatograph, record the chromatogram, and measure the peak responses. Calculate the percentage of each impurity in the portion of Dexamethasone Acetate taken by the formula: in which ri is the peak response for each impurity; and rS is the sum of the responses of 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.

Solvent— Use dimethyl sulfoxide.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of water and acetonitrile (550:450). Make adjustments if necessary (see System Suitability under Chromatography 621).

pH 6.0 Buffer solution— Transfer 3 mL of 1 N sodium hydroxide, 138 mL of 0.5 N potassium chloride, and 50 mL of 0.5 M monobasic potassium phosphate to a 1-L volumetric flask, dilute with water to volume, and mix.

Diluent— Prepare a mixture of acetonitrile and pH 6.0 Buffer solution (1:1).

Standard preparation— Transfer about 25 mg of USP Dexamethasone Acetate RS, accurately weighed, to a 250-mL volumetric flask. Add 100 mL of Diluent, and sonicate until a clear solution is obtained. Dilute with Diluent to volume, and mix.

Assay preparation— Transfer about 25 mg of Dexamethasone Acetate, accurately weighed, to a 250-mL volumetric flask. Add 100 mL of Diluent, and sonicate until a clear solution is obtained. Dilute with Diluent 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 containing 10-µm packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the capacity factor, k¢, is not less than 2.0; the column efficiency is not less than 1500 theoretical plates; the tailing factor is not more than 2.0; 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 (before and after injections of the Assay 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 C24H31FO6 in the portion of Dexamethasone Acetate taken by the formula: in which C is the concentration, in mg per mL, of USP Dexamethasone Acetate RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.