A: Infrared Absorption á197Kñ.

B: Ultraviolet Absorption á197Uñ—

Test solution: 10mg per mL,in dioxane.

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

Mobile phase— Prepare a filtered and degassed mixture of Formate bufferand acetonitrile (67:33).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Test solution— Transfer about 180mg of Dexamethasone,accurately weighed,to a 100-mLvolumetric flask,dissolve in and dilute with acetonitrile to volume,and mix.Transfer about 33mLof 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 5000theoretical plates.

Procedure— Inject a volume (about 10µL)of the Test solutioninto the chromatograph,record the chromatogram,and measure the peak responses.Calculate the percentage of each impurity in the portion of Dexamethasone taken by the formula: in which riis the peak response for each impurity,and rsis the sum of the responses of all 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 degassed solution of water and acetonitrile (about 7:3)such that at an approximate flow rate of 2mLper minute,the retention time of Dexamethasone is about 7minutes.

Standard preparation— Prepare a solution of USP Dexamethasone RSin methanol having a known concentration of about 7.5mg per mL.Dilute an accurately measured volume of this solution with the Mobile phaseto obtain a Standard preparationhaving a known concentration of about 0.3mg per mL.

Assay preparation— Using 30mg of Dexamethasone,proceed as directed for Standard preparation.

Procedure— Introduce equal volumes (between 15and 30µL)of the Assay preparationand the Standard preparationinto a high-pressure liquid chromatograph (see Chromatography á621ñ)operated at room temperature by means of a suitable microsyringe or sampling valve,adjusting the operating parameters such that the peak obtained with the Standard preparationis 60%full-scale.Typically,the apparatus is fitted with a 4-mm ×25-cm column containing packing L7,is equipped with an UVdetector capable of monitoring absorption at 254nm and a suitable recorder,and is operated at about 1000psi.Five replicate injections of the Standard preparationshow a relative standard deviation of not more than 3.0%.Determine the peak responses,at equivalent retention times,obtained with the Assay preparationand the Standard preparation,and calculate the quantity,in mg,of C22H29FO5in the portion of Dexamethasone taken by the formula: in which Cis the concentration,in mg per mL,of USP Dexamethasone RSin the Standard preparation,and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.