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

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

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

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 0.8mLper 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 10µL)of the Standard preparationand the Assay preparationinto 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 4minutes for hydroquinone.Calculate the quantity,in mg,of hydroquinone (C6H6O2)in each mLof the Topical Solution taken by the formula: in which Cis the concentration,in mg per mL,of USP Hydroquinone RSin the Standard preparation;Vis the volume,in mL,of Topical Solution taken;and rUand rSare the peak responses of hydroquinone obtained from the Assay preparationand the Standard preparation,respectively.