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

Assay preparation— Fit the valve of the Topical Aerosol container with a cannula, and expel the contents of the container into a 100-mL, glass-stoppered round-bottom flask. Attach the flask to a rotary evaporator, and evaporate under a vacuum of about 600 mm of mercury for about 2.5 hours. Transfer an accurately weighed portion of the material in the round-bottom flask, 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 the residue of Topical Aerosol taken from the round-bottom flask 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.