» Dexamethasone Gel contains not less than 90.0 percent and not more than 110.0 percent of the labeled amount of C22H29FO5.
Packaging and storage Preserve in collapsible tubes. Keep tightly closed. Avoid exposure to temperatures exceeding 30.
Identification Evaporate 25 mL of the Assay preparation, prepared as directed in the Assay, on a steam bath just to dryness, and dissolve the residue in 0.5 mL of chloroform. The chloroform extract responds to the Thin-Layer Chromatographic Identification Test 201, 10 µL of the chloroform extract and 10 µL of a Standard solution containing about 500 µg per mL of USP Dexamethasone RS being applied, and a mixture of chloroform and diethylamine (2:1) being used for development. Locate the spots on the plate by lightly spraying with dilute sulfuric acid (1 in 2) and heating.
Minimum fill 755: meets the requirements.
Mobile solvent Dilute 100 mL of methylene chloride with isooctane to one L.
Chromatographic columns Tamp a pledget of glass wool at the constriction of a glass chromatographic tube measuring about 30- × 1.5-cm, equipped with a polytef stopcock. Fill the tube about half-full with Mobile solvent. Mix 8 g of chromatographic siliceous earth with 8 mL of methanol and water (1:1). Transfer successive portions of the mixture to the column, emptying and adding Mobile solvent after each addition to pack the column. Finally drain the column to a layer of Mobile solvent about 1 cm above the absorbant. Pack a second tube to provide a blank and proceed as directed for Assay preparation, but omit the specimen.
Standard preparation Prepare a solution of USP Dexamethasone RS in alcohol to obtain a solution having a known concentration of about 10 µg per mL.
Assay preparation Accurately weigh an amount of Gel, equivalent to about 0.5 mg of Dexamethasone in a 100-mL beaker, add 1 g of chromatographic siliceous earth, and mix. Transfer the mixture to the column, wash the beaker with small portions of Mobile solvent, adding them to the column. Adjust the flow rate to about 2 mL per minute, discarding the eluate. Elute with four additional 25-mL portions of Mobile solvent, and discard. Rinse the sample beaker with two 25-mL portions of methylene chloride and transfer to the column, collecting the eluate in a suitable beaker. Elute the column with six additional 25-mL portions of methylene chloride, combining the eluates and evaporating with a gentle stream of air to dryness, dissolve the residue in alcohol, and transfer to a 50-mL volumetric flask. Wash the beaker with successive 5-mL portions of alcohol, collecting the washings in the flask, dilute with alcohol to volume, and mix. Centrifuge or filter and then pipet 10 mL of this solution, 10 mL of the Standard preparation, 10 mL of the column blank solution, and 10 mL of alcohol to provide a reagent blank to separate flasks. Proceed as directed under Assay for Steroids 351, except to allow to stand in the dark for 45 minutes. Calculate the quantity, in mg, of C22H29FO5 in the portion of Gel taken by the formula:
(0.05C)(AU ACB / AS ARB)in which C is the concentration, in µg per mL, of USP Dexamethasone RS in the Standard preparation; and AU, ACB, AS, and ARB are the absorbances of the solutions from the Assay preparation, column blank preparation, Standard preparation, and reagent blank preparation, respectively.
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USP32NF27 Page 2085