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 A and Solution B as directed for Chromatographic system (see System Suitability under Chromatography 621).

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

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

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

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

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

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

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 235-nm detector and a 4.6-mm × 25-cm column containing packing L1. The flow rate is about 1.2 mL per 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; and the tailing factors for the benzoic acid and methylparaben peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparations and 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 5 in 10) such that the retention times for ethyl benzoate and benzoyl peroxide are about 7 and 14 minutes, respectively.

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

Standard preparation— Place a suitable quantity of hydrous benzoyl peroxide, recently subjected to the Assay under 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 containing a known concentration of about 0.8 mg of benzoyl peroxide per mL. Pipet 10 mL of this solution and 5 mL of Internal standard solution into a 25-mL volumetric flask, dilute with acetonitrile to volume, and mix. This Standard preparation contains about 0.32 mg of benzoyl peroxide per mL.

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

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1, and is operated at room temperature. The flow rate is about 1 mL per 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 10 µL) of the Standard preparation and the Assay preparation, 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 C is the concentration, in mg per mL, of benzoyl peroxide in the Standard preparation; and RU and RS are the peak response ratios of benzoyl peroxide to ethyl benzoate obtained from the Assay preparation and the Standard preparation, respectively.