Mobile phase— Dissolve 3.4 g of monobasic potassium phosphate in about 850 mL of water, adjust with phosphoric acid to a pH of 3.0 ± 0.05, dilute with water to 1000 mL, mix, and pass through a suitable filter having a 0.5-µm or finer porosity. Prepare a mixture of methanol and this solution (650:350), and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution in Mobile phase containing about 0.25 mg of hexanophenone per mL.

Standard preparation— Transfer about 40 mg of USP Hexylresorcinol RS, accurately weighed, to a 100-mL volumetric flask, dissolve in Mobile phase, dilute with Mobile phase to volume, and mix. Transfer 10.0 mL of this solution and 10.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 0.08 mg of USP Hexylresorcinol RS per mL.

Assay preparation— Weigh and pulverize not fewer than 20 Lozenges. Transfer an accurately weighed portion of the powder, equivalent to about 4 mg of hexylresorcinol, to a 50-mL volumetric flask, add 10.0 mL of Internal standard solution and 20 mL of Mobile phase, and shake until dissolved. Dilute with Mobile phase to volume, and mix. Filter a portion of this solution through a suitable filter of 0.5 µm or finer porosity, and use the filtrate as the Assay preparation.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 15-cm column that contains packing L7 and is maintained at 37 ± 2. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the relative retention times are about 0.6 for hexylresorcinol and 1.0 for hexanophenone, the tailing factor is not less than 0.9 and not more than 1.4, the column efficiency is not less than 1500 theoretical plates, and the relative standard deviation for replicate injections is not more than 2.0%. Inject the Assay preparation, and record the peak responses as directed under Procedure: the resolution, R, between the hexylresorcinol peak and the nearest adjacent peak is not less than 1.2.

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 C12H18O2 in the portion of Lozenges taken by the formula: in which C is the concentration, in mg per mL, of USP Hexylresorcinol RS in the Standard preparation; and RU and RS are the ratios of the responses of the hexylresorcinol peak and the hexanophenone peak obtained from the Assay preparation and the Standard preparation, respectively.