Internal standard solution— Dissolve butyl benzoate in methanol to obtain a solution containing about 17.5 mg per mL.

Mobile phase— Prepare a suitable degassed and filtered mixture of acetonitrile and water (3:2).

Standard solution— Dissolve a suitable quantity of USP Crotamiton RS, accurately weighed, in methanol to obtain a solution having a known concentration of about 1 mg per mL.

Standard preparation— Pipet 10 mL of Standard solution and 5 mL of Internal standard solution into a 50-mL volumetric flask, dilute with methanol to volume, and mix.

Assay preparation— Transfer an accurately weighed portion of Crotamiton Cream, equivalent to about 50 mg of crotamiton, to a tared 50-mL volumetric flask. Add about 25 mL of methanol, and shake and sonicate to disperse the cream. Dilute with methanol to volume, and mix. Filter about 20 mL through moderately retentive filter paper. Pipet 10 mL of the clear filtrate and 5 mL of Internal standard solution into a 50-mL volumetric flask, dilute with methanol to volume, and mix.

Procedure— Inject equal volumes of the Standard preparation and the Assay preparation into a liquid chromatograph (see Chromatography 621) equipped with a 254-nm detector and a 4.6-mm × 25-cm stainless steel column that contains packing L1. In a suitable chromatogram, the resolution, R, between peaks for crotamiton and butyl benzoate is not less than 3.0; and the lowest and highest peak response ratios (RS) of three replicate injections of the Standard preparation agree within 2.0%. Calculate the quantity, in mg, of C13H17NO in the portion of Cream taken by the formula: in which C is the concentration, in mg per mL, of USP Crotamiton RS in the Standard preparation; and RU and RS are the peak response ratios of the crotamiton peak and the butyl benzoate peak obtained from the Assay preparation and the Standard preparation, respectively.