A:Infrared Absorption á197Mñ.

B:Ultraviolet Absorption á197Uñ—

Test solution: 10mg per mL,in dioxane.

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

Mobile phase— Prepare a filtered and degassed mixture of Formate bufferand acetonitrile (3:2).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

Chromatographic system (see Chromatography á621ñ)— 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 1mLper minute.Chromatograph the Test solution,and record the peak responses as directed for Procedure:the column efficiency is not less than 5400theoretical plates.

Procedure— Inject a volume (about 10µL)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 riis the peak response for each impurity;and rSis 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.

Mobile phase— Prepare a filtered and degassed mixture of water and acetonitrile (550:450).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

pH6.0Buffer solution— Transfer 3mLof 1Nsodium hydroxide,138mLof 0.5Npotassium chloride,and 50mLof 0.5Mmonobasic potassium phosphate to a 1-Lvolumetric flask,dilute with water to volume,and mix.

Diluent— Prepare a mixture of acetonitrile and pH6.0Buffer solution(1:1).

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

Assay preparation— Transfer about 25mg of Dexamethasone Acetate,accurately weighed,to a 250-mLvolumetric flask.Add 100mLof Diluent,and sonicate until a clear solution is obtained.Dilute with Diluentto volume,and mix.

Chromatographic system(see Chromatography á621ñ)— 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 2mLper 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 1500theoretical 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 20µL)of the Standard preparation(before and after injections of the Assay preparation)and the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C24H31FO6in the portion of Dexamethasone Acetate taken by the formula: in which Cis the concentration,in mg per mL,of USP Dexamethasone Acetate RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.