Change to read:

Solution A— Prepare a filtered and degassed mixture of acetonitrile and glacial acetic acid (1000:1).

Solution B— Prepare a filtered and degassed mixture of water and glacial acetic acid (1000:1).

Mobile phase— Use variable mixtures of Solution Aand Solution Bas directed for Chromatographic system(seeSystem Suitabilityunder Chromatography á621ñ).

System suitability solution— Prepare a solution in acetonitrile containing 100µg of benzoic acid and 60µg of methylparaben per mL.

Test preparation— Transfer an accurately weighed quantity of Gel,equivalent to about 100mg of benzoyl peroxide,to a 50-mLvolumetric flask,add 25mLof acetonitrile,shake vigorously to disperse the specimen,sonicate for 5minutes,dilute with acetonitrile to volume,mix,and filter.

Standard preparation A— Prepare a solution of benzoic acid in acetonitrile containing 500µg per mL.

Standard preparation B— Prepare a solution of ethyl benzoate in acetonitrile containing 20µg per mL.

Standard preparation C— Prepare a solution of benzaldehyde in acetonitrile containing 20µg per mL.

Standard preparation D— Prepare a solution of hydrous benzoyl peroxide,previously subjected to the Assayunder Hydrous Benzoyl Peroxide,in acetonitrile containing the equivalent of 40µg of anhydrous benzoyl peroxide per mL.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 235-nm detector and a 4.6-mm ×25-cmUSP28column containing packing L1.The flow rate is about 1.2mLper minute.The chromatograph is programmed as follows. Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the resolution,R,between benzoic acid and methylparaben is not less than 2.0;USP28and the tailing factors for the benzoic acid and methylparaben peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationsand the Test preparation into the chromatograph,record the chromatograms,and measure the areas for the major peaks.The responses of any peaks obtained from the Test preparation corresponding to benzoic acid,ethyl benzoate,and benzaldehyde are not greater than those of the main peaks obtained from Standard preparation A(25%),Standard preparation B(1%),and Standard preparation C(1%),respectively;the response of any other impurity peak obtained from the Test preparation,other than the main benzoyl peroxide peak,any benzoic acid,ethyl benzoate,benzaldehyde,methylparaben,or propylparaben peak,and any solvent peak,is not more than that obtained from Standard preparation D(2%);and the sum of the responses of all the impurity peaks,other than those of benzoic acid,ethyl benzoate,and benzaldehyde is not more than that obtained from Standard preparation D(2%).

Mobile phase— Prepare a solution of acetonitrile in water (about 5in 10)such that the retention times for ethyl benzoate and benzoyl peroxide are about 7and 14minutes,respectively.

Internal standard solution— Dissolve ethyl benzoate in acetonitrile to obtain a solution having a concentration of about 3.6mg per mL.

Standard preparation— Place a suitable quantity of hydrous benzoyl peroxide,recently subjected to the Assayunder Hydrous Benzoyl Peroxide,in an accurately weighed conical flask fitted with a glass stopper,weigh again to obtain the weight of the specimen,and quantitatively dissolve in acetonitrile to obtain a solution having a known concentration of about 0.8mg of benzoyl peroxide per mL.Pipet 10mLof this solution and 5mLof Internal standard solutioninto a 25-mLvolumetric flask,dilute with acetonitrile to volume,and mix.This Standard preparationcontains about 0.32mg of benzoyl peroxide per mL.

Assay preparation— Transfer an accurately weighed quantity of Gel,equivalent to about 40mg of benzoyl peroxide,to a 50-mLvolumetric flask.Add 40mLof acetonitrile,and shake until the material is thoroughly dispersed.Sonicate the mixture for 5minutes,dilute with acetonitrile to volume,mix,and filter.Pipet 10mLof the filtrate and 5mLof Internal standard solutioninto a 25-mLvolumetric flask,dilute with acetonitrile to volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 4-mm ×30-cm stainless steel column that contains packing L1,and is operated at room temperature.The flow rate is about 1mLper minute.Chromatograph three replicate injections of the Standard preparation,and record the peak responses as directed for Procedure:the lowest and highest peak response ratios (RS)agree within 2.0%;the resolution,R,between ethyl benzoate and benzoyl peroxide is not less than 2.0;and the tailing factors for the ethyl benzoate and benzoyl peroxide peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparation into the chromatograph,record the chromatograms,and measure the peak responses.Calculate the quantity,in mg,of benzoyl peroxide (C14H10O4)in the portion of Gel taken by the formula: in which Cis the concentration,in mg per mL,of benzoyl peroxide in the Standard preparation;and RUand RSare the peak response ratios of benzoyl peroxide to ethyl benzoate obtained from the Assay preparationand the Standard preparation,respectively.