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

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

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

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

Chromatographic system (see Chromatography á621ñ)—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 2mLper 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 5µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The retention times are about 2.5minutes for salicylic acid and 5.5minutes for benzoic acid.Calculate the quantity,in mg,of C7H6O3in the portion of Topical Foam taken by the formula: in which Cis the concentration,in mg per mL,of USP Salicylic Acid RSin the Standard preparation,and RUand RSare the ratios of the peak responses for salicylic acid to the peak responses for benzoic acid obtained from the Assay preparationand the Standard preparation,respectively.