Mobile phase— Mix 55 volumes of methanol and 45 volumes of water.

Standard preparation— Transfer about 250 mg of USP Hydroquinone RS, accurately weighed, to a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix. Transfer 3.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Topical Solution, equivalent to about 30 mg of hydroquinone, to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

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 0.8 mL per minute. Chromatograph three replicate injections of the Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation is not more than 3.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses for the major peaks. The retention time is about 4 minutes for hydroquinone. Calculate the quantity, in mg, of hydroquinone (C6H6O2) in each mL of the Topical Solution taken by the formula: in which C is the concentration, in mg per mL, of USP Hydroquinone RS in the Standard preparation; V is the volume, in mL, of Topical Solution taken; and rU and rS are the peak responses of hydroquinone obtained from the Assay preparation and the Standard preparation, respectively.