A: Transfer a quantity of Topical Suspension, equivalent to about 20 mg of resorcinol, to a 15-mL centrifuge tube, add 5 mL of 5 N sodium hydroxide, mix, and centrifuge the mixture for 5 minutes. Decant the supernatant into a test tube, and retain the residue for Identification test B. Add 0.5 mL of 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 Identification test A on 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 3 minutes and 4 minutes, respectively.

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

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

Assay preparation— Transfer an accurately weighed portion of Topical Suspension, equivalent to about 20 mg of resorcinol, to a 150-mL beaker. Add 40 mL of methanol and 5.0 mL of Internal standard solution, and heat on a steam bath for 5 minutes. Cool the mixture to room temperature, and decant the liquid into a 100-mL volumetric flask. Wash the residue in the beaker by adding 20 mL of methanol to the beaker. Heat on a steam bath for 5 minutes, 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 10 µL) of the Assay preparation and the Standard preparation into a high-pressure liquid chromatograph (see Chromatography 621), 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 preparation is about 0.6 full scale. Typically, the apparatus is fitted with a 4-mm × 30-cm column containing packing L1 and is equipped with an UV detector capable of monitoring absorption at 280 nm, and a suitable recorder. In a suitable chromatogram the coefficient of variation for five replicate injections of the Standard preparation is not more than 3.0%. Measure the peak responses at equivalent retention times, obtained from the Assay preparation and the Standard preparation, and calculate the quantity, in mg, of resorcinol (C6H6O2) in the portion of Topical Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Resorcinol RS in the Standard preparation; and RU and RS are the ratios of the responses of the resorcinol and caffeine peaks obtained from the Assay preparation and the Standard preparation, respectively.