(Official January 1, 2007)

Mobile phase— To 225 mg of tetramethylammonium hydroxide pentahydrate add 700 mL of water, 150 mL of methanol, 150 mL of acetonitrile, and 1.0 mL of glacial acetic acid, mix, filter, and degas.

Internal standard solution— Dissolve benzoic acid in methanol to obtain a solution having a concentration of about 8 mg per mL.

Standard preparation— Transfer about 20 mg of USP Salicylic Acid RS, accurately weighed, to a 100-mL volumetric flask, add 10.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer an accurately weighed portion of Topical Foam, equivalent to about 20 mg of salicylic acid, to a 100-mL volumetric flask, add 10.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix. Cool in an ice bath to below room temperature and filter, discarding the first few mL of the filtrate.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph four replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 3.0%, the resolution factor between salicylic acid and benzoic acid is not less than 3.0, and the tailing factors for the salicylic acid and benzoic acid peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 5 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The retention times are about 2.5 minutes for salicylic acid and 5.5 minutes for benzoic acid. Calculate the quantity, in mg, of C7H6O3 in the portion of Topical Foam taken by the formula: in which C is the concentration, in mg per mL, of USP Salicylic Acid RS in the Standard preparation, and RU and RS are the ratios of the peak responses for salicylic acid to the peak responses for benzoic acid obtained from the Assay preparation and the Standard preparation, respectively.