Standard solution— Transfer about 10 mg of USP Dexamethasone RS, accurately weighed, to a 10-mL volumetric flask, dilute with alcohol to volume, and mix. Transfer 1.0 mL of this solution to a 100-mL volumetric flask, dilute with 0.1 N sulfuric acid to volume, and mix to obtain a solution having a known concentration of about 10 µg per mL.

Test solution— Discharge the minimum recommended dose into the sampling apparatus, and detach the inhaler as directed. Rinse the apparatus (filter and interior) with two 5.0-mL portions of 0.1 N sulfuric acid, and transfer the resulting solutions quantitatively to a 50-mL centrifuge tube containing 15 mL of methylene chloride that was previously chilled in a dry ice-acetone bath for a few minutes. Insert the stopper in the centrifuge tube, and shake cautiously, releasing the pressure occasionally. Allow the phases to separate, and equilibrate to room temperature. The aqueous phase is the Test solution.

Procedure— Transfer the Test solution and 10.0 mL of the Standard solution into separate flasks. Add 2.0 mL of 0.1 N sulfuric acid to each, and swirl to mix. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 239 nm, with a suitable spectrophotometer, using 0.1 N sulfuric acid as the blank. Calculate the quantity, in µg, of dexamethasone phosphate (C22H30FO8P) contained in the minimum dose by the formula: in which 472.45 and 392.47 are the molecular weights of dexamethasone phosphate and dexamethasone, respectively; C is the concentration, in µg per mL, of USP Dexamethasone RS in the Standard solution; N is the number of sprays discharged to obtain the minimum recommended dose; and AU and AS are the absorbances of the solutions from the Test solution and the Standard solution, respectively.

Standard preparation— Transfer about 40 mg of USP Dexamethasone RS, accurately weighed, to a 50-mL volumetric flask, dilute with alcohol to volume, and mix. Transfer 5.0 mL of this solution to a 500-mL volumetric flask, dilute with 0.1 N sulfuric acid to volume, and mix to obtain a solution having a known concentration of about 8 µg per mL.

Assay preparation— Weigh accurately a filled Inhalation Aerosol container, and record the weight (W1). Place the container in a dry ice-acetone bath, and cool for 60 minutes. Remove the container from the bath, and carefully remove the valve with wire cutters, taking precautions to save all pieces of the valve and cap. With the aid of four 5-mL portions of 0.1 N sulfuric acid, transfer the contents of the container to a beaker previously chilled in the bath. Dry the rinsed empty container and all of its parts in an oven at 105 for 2 hours, cool, and weigh (W2). Allow the contents of the beaker to warm to room temperature. After the bulk of the propellant has evaporated, quantitatively transfer the contents of the beaker, with the aid of several mL of 0.1 N sulfuric acid, to a 200-mL volumetric flask, dilute with 0.1 N sulfuric acid to volume, and mix. Transfer about 20 mL of this solution to a centrifuge tube, add 10 mL of methylene chloride, shake vigorously for 1 minute, and centrifuge. Pipet 10 mL of the clear supernatant into a 100-mL volumetric flask, dilute with 0.1 N sulfuric acid to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Assay preparation and the Standard preparation in 1-cm cells at the wavelength of maximum absorbance at about 239 nm, with a suitable spectrophotometer, using 0.1 N sulfuric acid as the blank. Calculate the quantity, in mg, of dexamethasone phosphate (C22H30FO8P) in each g of Inhalation Aerosol taken by the formula: in which 472.45 and 392.47 are the molecular weights of dexamethasone phosphate and dexamethasone, respectively, AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively, C is the concentration, in µg per mL, of USP Dexamethasone RS in the Standard preparation, and W1 and W2 are the weights, in g, as previously defined.