Mobile phase— Prepare a suitable filtered and degassed solution containing methanol,water,and glacial acetic acid (85:15:0.5).

Standard preparation— Dissolve an accurately weighed quantity of USP Padimate O RSin isopropyl alcohol and dilute quantitatively,and stepwise if necessary,with isopropyl alcohol to obtain a solution having a known concentration of about 100µg per mL.

Assay preparation— Transfer an accurately weighed quantity of Lotion,equivalent to about 100mg of Padimate O,to a 100-mLvolumetric flask,and add about 75mLof isopropyl alcohol.Heat gently with swirling until the specimen is dispersed.Cool to room temperature,dilute with isopropyl alcohol to volume,and mix.Pipet 10.0mLof this solution into a 100-mLvolumetric flask,dilute with isopropyl alcohol to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 308-nm detector and a 4.6-mm ×25-cm column that contains 5-µm base-deactivatedpacking L1.The flow rate is about 1.5mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed under Procedure:the tailing factor is not more than 2.5and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C17H27NO2in the portion of Lotion taken by the formula: in which Cis the concentration,in µg per mL,of USP Padimate O RSin the Standard preparation,and rUand rSare the peak responses for padimate Oobtained from the Assay preparationand the Standard preparation,respectively.