(Official January 1, 2007)

Diluent— Prepare a mixture of methanol and water (60:40).

Mobile phase— Prepare a mixture of methanol, water, and 0.25 M sodium 1-heptanesulfonate (500:500:20). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Transfer about 140 mg of USP Benzocaine RS, accurately weighed, to a 100-mL volumetric flask with the aid of 25 mL of methanol, and swirl. Transfer about 140 J mg of USP Butamben RS, accurately weighed, to the same volumetric flask with the aid of 25 mL of water, J being the ratio of the labeled amount, in percent, of butamben to the labeled amount, in percent, of benzocaine in the Topical Solution. Transfer about 140 J¢ mg of USP Tetracaine Hydrochloride RS, accurately weighed, to the same volumetric flask with the aid of 25 mL of water, J¢ being the ratio of the labeled amount, in percent, of tetracaine hydrochloride to the labeled amount, in percent, of benzocaine in the Topical Solution. Sonicate for about 1 minute, dilute with Diluent to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Topical Solution, equivalent to about 1400 mg of benzocaine, to a 100-mL volumetric flask. Add about 75 mL of methanol, and mix. Sonicate for about 1 minute, dilute with methanol to volume, and mix. Transfer 10.0 mL of this solution to a second 100-mL volumetric flask, add about 75 mL of Diluent, shake, dilute with Diluent to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 313-nm detector, and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.3 for benzocaine, 0.8 for butamben, and 1.0 for tetracaine; the resolution, R, between the benzocaine peak and the butamben peak and between the butamben peak and the tetracaine peak is not less than 2; and the relative standard deviation for replicate injections is not more than 2.0% for each of the three analyte peaks.

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 peak areas for the major peaks. Calculate the individual quantities, in mg, of benzocaine (C9H11NO2), butamben (C11H15NO2), and tetracaine hydrochloride (C15H24N2O2·HCl) in the portion of Topical Solution taken by the formula: in which C is the concentration, in mg per mL, of the appropriate Reference Standard in the Standard preparation, and rU and rS are the peak areas in the corresponding analyte obtained from the Assay preparation and the Standard preparation, respectively.