A: Transfer a quantity of Topical Suspension,equivalent to about 20mg of resorcinol,to a 15-mLcentrifuge tube,add 5mLof 5Nsodium hydroxide,mix,and centrifuge the mixture for 5minutes.Decant the supernatant into a test tube,and retain the residue for Identificationtest B.Add 0.5mLof chloroform,mix,and heat on a steam bath:an intense crimson color is produced.Add a slight excess of hydrochloric acid:the color changes to pale yellow(presence of resorcinol).

B: Place a small portion of the residue from the centrifuge tube in Identificationtest Aon the tip of a spatula,and burn it:sulfur dioxide,which turns moistened starch-iodate paper blue,is formed (presence of sulfur).

Mobile phase— Prepare a suitable degassed solution of water,acetonitrile,and methanol (about 55:7:6)such that the retention times of resorcinol and caffeine are about 3minutes and 4minutes,respectively.

Internal standard solution— Dissolve about 140mg of caffeine in 2mLof chloroform,add methanol to make 100mL,and mix.

Standard preparation— Transfer 50mg of USP Resorcinol RS,accurately weighed,to a 25-mLvolumetric flask,dilute with methanol to volume,and mix.Transfer 10.0mLof this solution and 5.0mLof Internal standard solutionto a 100-mLvolumetric flask,dilute with methanol to volume,and mix.

Assay preparation— Transfer an accurately weighed portion of Topical Suspension,equivalent to about 20mg of resorcinol,to a 150-mLbeaker.Add 40mLof methanol and 5.0mLof Internal standard solution,and heat on a steam bath for 5minutes.Cool the mixture to room temperature,and decant the liquid into a 100-mLvolumetric flask.Wash the residue in the beaker by adding 20mLof methanol to the beaker.Heat on a steam bath for 5minutes,cool the mixture to room temperature,and decant the liquid into the volumetric flask.Repeat the washing,heating,cooling,and decanting.Dilute the contents of the volumetric flask with methanol to volume,and mix.

Procedure— Introduce equal volumes (about 10µL)of the Assay preparationand the Standard preparationinto a high-pressure liquid chromatograph (see Chromatography á621ñ),operated at room temperature,by means of a suitable microsyringe or sampling valve,adjusting the specimen size and other operating parameters such that the peak obtained from the Standard preparationis about 0.6full scale.Typically,the apparatus is fitted with a 4-mm ×30-cm column containing packing L1and is equipped with an UVdetector capable of monitoring absorption at 280nm,and a suitable recorder.In a suitable chromatogram the coefficient of variation for five replicate injections of the Standard preparationis not more than 3.0%.Measure the peak responses at equivalent retention times,obtained from the Assay preparationand the Standard preparation,and calculate the quantity,in mg,of resorcinol (C6H6O2)in the portion of Topical Suspension taken by the formula: in which Cis the concentration,in mg per mL,of USP Resorcinol RSin the Standard preparation;and RUand RSare the ratios of the responses of the resorcinol and caffeine peaks obtained from the Assay preparationand the Standard preparation,respectively.