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 RS in isopropyl alcohol and dilute quantitatively, and stepwise if necessary, with isopropyl alcohol to obtain a solution having a known concentration of about 100 µg per mL.

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

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 308-nm detector and a 4.6-mm × 25-cm column that contains 5-µm base-deactivated packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the tailing factor is not more than 2.5 and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C17H27NO2 in the portion of Lotion taken by the formula: in which C is the concentration, in µg per mL, of USP Padimate O RS in the Standard preparation, and rU and rS are the peak responses for padimate O obtained from the Assay preparation and the Standard preparation, respectively.