Standard solution— Transfer about 10mg of USP Dexamethasone RS,accurately weighed,to a 10-mLvolumetric flask,dilute with alcohol to volume,and mix.Transfer 1.0mLof this solution to a 100-mLvolumetric flask,dilute with 0.1Nsulfuric acid to volume,and mix to obtain a solution having a known concentration of about 10µg 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-mLportions of 0.1Nsulfuric acid,and transfer the resulting solutions quantitatively to a 50-mLcentrifuge tube containing 15mLof 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 solutionand 10.0mLof the Standard solutioninto separate flasks.Add 2.0mLof 0.1Nsulfuric 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 239nm,with a suitable spectrophotometer,using 0.1Nsulfuric acid as the blank.Calculate the quantity,in µg,of dexamethasone phosphate (C22H30FO8P)contained in the minimum dose by the formula: in which 472.45and 392.47are the molecular weights of dexamethasone phosphate and dexamethasone,respectively;Cis the concentration,in µg per mL,of USP Dexamethasone RSin the Standard solution;Nis the number of sprays discharged to obtain the minimum recommended dose;and AUand ASare the absorbances of the solutions from the Test solutionand the Standard solution,respectively.

Standard preparation— Transfer about 40mg of USP Dexamethasone RS,accurately weighed,to a 50-mLvolumetric flask,dilute with alcohol to volume,and mix.Transfer 5.0mLof this solution to a 500-mLvolumetric flask,dilute with 0.1Nsulfuric acid to volume,and mix to obtain a solution having a known concentration of about 8µg 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 60minutes.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-mLportions of 0.1Nsulfuric 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 105for 2hours,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 mLof 0.1Nsulfuric acid,to a 200-mLvolumetric flask,dilute with 0.1Nsulfuric acid to volume,and mix.Transfer about 20mLof this solution to a centrifuge tube,add 10mLof methylene chloride,shake vigorously for 1minute,and centrifuge.Pipet 10mLof the clear supernatant into a 100-mLvolumetric flask,dilute with 0.1Nsulfuric acid to volume,and mix.

Procedure— Concomitantly determine the absorbances of the Assay preparationand the Standard preparationin 1-cm cells at the wavelength of maximum absorbance at about 239nm,with a suitable spectrophotometer,using 0.1Nsulfuric 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 472.45and 392.47are the molecular weights of dexamethasone phosphate and dexamethasone,respectively,AUand ASare the absorbances of the solutions from the Assay preparationand the Standard preparation,respectively,Cis the concentration,in µg per mL,of USP Dexamethasone RSin the Standard preparation,and W1and W2are the weights,in g,as previously defined.