Diluent , Mobile phase, Standard preparation, and Chromatographic system—Proceed as directed in the Assay under Benzocaine, Butamben, and Tetracaine Hydrochloride Topical Solution.

Assay preparation— Transfer an accurately weighed portion of Ointment, 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.

Procedure— Proceed as directed for Procedure in the Assay under Benzocaine, Butamben, and Tetracaine Hydrochloride Topical Solution. Calculate the individual quantities, in mg, of benzocaine (C9H11NO2), butamben (C11H15NO2), and tetracaine hydrochloride (C15H24N2O2·HCl) in the portion of Ointment 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 responses of the corresponding analyte obtained from the Assay preparation and the Standard preparation, respectively.