A: The solution employed for measurement of absorbance in the Assay for tetracaine exhibits a maximum at 310 ± 2 nm (presence of tetracaine).

B: Dissolve 5 g in 50 mL of ether, extract the ether solution with 5 mL of 3 N hydrochloric acid, and filter the acid extract. Add 2 mL of potassium thiocyanate solution (1 in 2) to the filtrate: a crystalline precipitate is formed, and when recrystallized from water and dried at 80 for 2 hours, it melts between 130 and 132 (see Melting Range or Temperature 741) (presence of tetracaine).

C: When chromatographed as directed in the Assay for menthol, the Assay preparation exhibits a major peak for menthol, the retention time of which corresponds to that exhibited by menthol in the Standard preparation.

Standard preparation— Prepare as directed in the Assay under Tetracaine Ointment.

Assay preparation— Using Ointment, proceed as directed in the Assay under Tetracaine Ointment.

Procedure— Proceed as directed for Procedure in the Assay under Tetracaine Ointment.

Internal standard solution— Dissolve decanol in n-hexane to obtain a solution having a concentration of about 1 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Menthol RS in n-hexane to obtain a solution having a known concentration of about 1 mg per mL. Transfer 5.0 mL of this solution and 5.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with ether to volume, and mix. Combine 2.0 mL of this solution and 2.0 mL of ether in a suitable container, and mix. This Standard preparation has a known concentration of about 0.05 mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ointment, equivalent to about 5 mg of menthol, to a 50-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with n-hexane to volume, mix, and sonicate. Using a suitable syringe attached firmly to a 25- × 12.5-mm chromatographic cartridge containing packing L4, force 2.0 mL of the solution through the cartridge at a rate of 1 mL per 12 seconds. Wash the cartridge at the same rate with two 5-mL portions of n-hexane, and discard the washings. Force two 2.0-mL portions of ether through the cartridge, combine the ether eluates in a suitable container, and mix.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm × 1.8-m column packed with 10% phase G16 on support S1AB. The column temperature is maintained isothermally at about 170, the injection port temperature is maintained at about 260, and the detector block temperature is maintained at about 240. Dry helium is used as the carrier gas at a flow rate of about 50 mL per minute.

System suitability— Chromatograph three injections of the Standard preparation, and record the peak responses as directed for Procedure: the retention time of menthol is about 0.7 relative to decanol; the resolution, R, between the 2 peaks is not less than 2.5; and the relative standard deviation of the ratio of the peak response obtained with menthol to that obtained with decanol is not more than 2%.

Procedure— Separately inject equal volumes (about 2 µL) of the Assay preparation and the Standard preparation into the gas chromatograph, and measure the peak responses for menthol and decanol in each chromatogram. Calculate the quantity, in mg, of C10H20O in the portion of Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Menthol RS in the Standard preparation; and RU and RS are the peak response ratios of menthol to decanol obtained from the Assay preparation and the Standard preparation, respectively.