Thimerosal Topical Aerosol »Thimerosal Topical Aerosol is an alcoholic solution of Thimerosal mixed with suitable propellants in a pressurized container.It contains not less than 85.0percent and not more than 115.0percent of the labeled amount of C9H9HgNaO2S. NOTE—Thimerosal Topical Aerosol is sensitive to some metals. Packaging and storage— Preserve in tight,light-resistant,pressurized containers,and avoid exposure to excessive heat. USP Reference standards á11ñ USP Thimerosal RS. Identification— To about 10mLof solution sprayed into a suitable container from Topical Aerosol add 10mLof water,and heat on a steam bath until the odor of alcohol is no longer perceptible.Cool,and pass hydrogen sulfide through the solution:no black discoloration or black precipitate is formed.To 15mLof solution sprayed into a suitable container from Topical Aerosol add 15mLof water,heat on a steam bath until the odor of alcohol is no longer perceptible,and add 2drops of bromine.Mix with 1.5mLof 3Nhydrochloric acid,filter,evaporate the excess bromine with a current of air,and pass hydrogen sulfide through the filtrate:a black precipitate is formed. Alcohol content— Weigh,chill,and open 1Topical Aerosol container,and remove the propellant as directed for Assay preparationin the Assay,continuing until the bulk of the propellant has evaporated.Determine the alcohol content of the specimen thus prepared by the Gas-Liquid Chromatographic Method(see Method IIunder Alcohol Determination á611ñ),using methyl ethyl ketone as the internal standard in place of acetone:between 18.7%and 25.3%(w/w)of C2H5OHis found. Other requirements— It meets the requirements for Pressure Test,Minimum Fill,and Leakage Testunder Aerosols,Nasal Sprays,Metered-Dose Inhalers,and Dry Powder Inhalers á601ñ. Assay— [NOTE—The Standard preparations and Assay preparation may be diluted quantitatively with water,if necessary,to yield solutions,of suitable concentration,adaptable to the linear or working range of the instrument.] Stannous chloride solution— Dissolve 50g of stannous chloride in 100mLof hydrochloric acid on a steam bath,cool,dilute with water to 500mL,and mix.Use within 3months. Standard solutions— Prepare aqueous solutions of USP Thimerosal RSof known concentrations of about 1.8,2.0,and 2.2µg per mL. Standard preparations— Pipet 20mLof each Standard solutioninto separate 100-mLvolumetric flasks,and treat each flask as follows.Add 5mLof sulfuric acid,cool,add 3mLof nitric acid,and mix.Add potassium permanganate crystals,while mixing,until the purple color persists for not less than 15minutes.Add about 200mg of potassium persulfate,mix,and heat on a steam bath for 2hours.Cool,dilute with water to volume,and mix. Assay solution— Weigh accurately a filled Topical Aerosol container,and record the weight.Place the container in a dry ice-alcohol bath,and cool for 60minutes.Remove the container from the bath,and carefully remove the spray cap with wire cutters,taking precautions to save all pieces of the spray head and cap.With the aid of three 5-mLportions of water,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.Calculate the weight of the container contents.Add a few boiling chips to the beaker,and carefully stir to help evaporate the propellant.After the bulk of the propellant has evaporated,place the beaker on a steam bath,evaporate the volatile solvents,and cool.Transfer the residual liquid with the aid of 35.0mLof alcohol to a 50-mLvolumetric flask,dilute with water to volume,and mix.Dilute quantitatively with water a volume,vmL,of this solution to VmLof Assay solutioncontaining about 2µg of thimerosal per mL. Assay preparation— Pipet 20mLof Assay solutioninto a 100-mLvolumetric flask,and proceed as directed for Standard preparations,beginning with “Add 5mLof sulfuric acid.” Blank preparation— Pipet 20mLof water into a 100-mLvolumetric flask,and proceed as directed for Standard preparation,beginning with “Add 5mLof sulfuric acid.” Procedure— Proceed with each of the Standard preparations,the Assay preparation,and the Blank preparationas follows:Pipet 3mLinto the scrubbing chamber of a suitable system designed for determination of mercury by flameless atomic absorption,using a mercury hollow-cathode lamp,dilute with water to about 150mL,and add hydroxylamine hydrochloride solution (1in 10)just to reduce the excess permanganate.Add 5mLof Stannous chloride solution,and immediately attach the scrubbing chamber to the system.Concomitantly determine the absorbance of the vapor from each solution at an integration time of 15seconds.Use the absorbance of the Blank preparationto correct the absorbances of the Standard preparationsand the Assay preparation.Plot the corrected absorbances of the standards versus the respective concentrations of the Standard solutions,in µg per mL,and from the curve so obtained determine the concentration,C,in µg per mL,of the Assay solution.Calculate the quantity,in mg,of C9H9HgNaO2Sin the weight of the container contents taken by the formula: 50C(V/v), in which the terms are as defined therein. Auxiliary Information— Staff Liaison:Behnam Davani,Ph.D.,MBA,Senior Scientist Expert Committee:(PA7)Pharmaceutical Analysis 7 USP28–NF23Page 1912 Phone Number:1-301-816-8394