Benzocaine, Butamben, and Tetracaine Hydrochloride Topical Solution

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Benzocaine, Butamben, and Tetracaine Hydrochloride Topical Solution contains NLT 90.0% and NMT 110.0% of the labeled amount of benzocaine (C₉H₁₁NO₂), butamben (C₁₁H₁₅NO₂), and tetracaine hydrochloride (C₁₅H₂₄N₂O₂·HCl).

2 IDENTIFICATION

A. The retention times of the major peaks of the Sample solution correspond to those of the Standard solution, as obtained in the Assay.

B. The UV spectrum of the major peaks of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

3 ASSAY

Procedure

Solution A: 0.1% formic acid in water

Solution B: 0.1% formic acid in acetonitrile

Mobile phase: See Table 1.

Diluent: Acetonitrile and water (10:90)

Standard stock solution A: 1750 µg/mL of USP Benzocaine RS prepared as follows. Transfer a suitable amount of USP Benzocaine RS to a suitable volumetric flask and dissolve in 80% of the total volume of Diluent. [Note—Sonication for about 5 min may be necessary.] Dilute with Diluent to volume.

Standard stock solution B: 125 µg/mL each of USP Butamben RS and USP Tetracaine Hydrochloride RS prepared as follows. Transfer a suitable amount of USP Butamben RS and USP Tetracaine Hydrochloride RS to a suitable volumetric flask and dissolve in 80% of the total volume of Diluent. [Note—Sonication for about 10 min may be necessary.] Dilute with Diluent to volume.

Standard solution: 175 µg/mL of USP Benzocaine RS from Standard stock solution A and 25 µg/mL each of USP Butamben RS and USP Tetracaine Hydrochloride RS from Standard stock solution B diluted in Diluent

Sample solution: Nominally 175 µg/mL of benzocaine and 25 µg/mL each of butamben and tetracaine hydrochloride, prepared as follows. Transfer a suitable portion of Topical Solution to a suitable volumetric flask and dissolve in 80% of the total volume of Diluent. [Note—Sonication for about 10 min may be necessary.] Dilute with Diluent to volume.

Chromatographic system

Mode: LC

Detector: UV 300 nm. For Identification B, use a diode array detector in the range of 200–400 nm.

Column: 2.1-mm × 5-cm; 1.7-µm packing L1

Flow rate: 0.6 mL/min

Injection volume: 1 µL

System suitability

Sample: Standard solution

[Note—The relative retention times for benzocaine, tetracaine, and butamben are about 0.71, 0.74, and 1.0, respectively.]

Suitability requirements

Resolution: NLT 2.0 between benzocaine and tetracaine

Relative standard deviation: NMT 2.0% for each of the three analyte peaks

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of benzocaine (C₉H₁₁NO₂), butamben (C₁₁H₁₅NO₂), and tetracaine hydrochloride (C₁₅H₂₄N₂O₂·HCl) in the portion of Topical Solution taken:

Result = (rᵤ/rₛ) × (Cₛ/Cᵤ) × 100

rᵤ = peak response of the corresponding analyte from the Sample solution

rₛ = peak response of the corresponding analyte from the Standard solution

Cₛ = concentration of the corresponding Reference Standard in the Standard solution (µg/mL)

Cᵤ = nominal concentration of the corresponding analyte in the Sample solution (µg/mL)

Acceptance criteria: 90.0%–110.0%

4 PERFORMANCE TESTS

Minimum Fill 〈755〉: Meets the requirements

5 IMPURITIES

Organic Impurities

Mobile phase, Diluent, and Chromatographic system: Proceed as directed in the Assay.

System suitability solution: 10 µg/mL each of USP Benzocaine RS, USP Tetracaine Hydrochloride RS, USP Butamben RS, and USP Ethyl 4-Nitrobenzoate RS in Diluent

Standard solution: 3.4 µg/mL each of USP Benzocaine RS and USP Ethyl 4-Nitrobenzoate RS and 1 µg/mL of USP Tetracaine Hydrochloride RS in Diluent

Sample solution: Nominally 1.68 mg/mL of benzocaine, 0.24 mg/mL of butamben, and 0.24 mg/mL of tetracaine prepared as follows. Accurately weigh a suitable quantity of Topical Solution into a suitable volumetric flask, dissolve with 20% of the total volume of acetonitrile, and dilute with water to volume. [Note—Sonication for about 1 min may be necessary.]

System suitability

Samples: System suitability solution and Standard solution

[Note—See Table 2 for relative retention times.]

Suitability requirements

Resolution: NLT 2 between butamben and ethyl 4-nitrobenzoate; NLT 2 between benzocaine and tetracaine, System suitability solution

Relative standard deviation: NMT 5.0% for each of the analyte peaks, Standard solution

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of 4-aminobenzoic acid in the portion of Topical Solution taken:

Result = (rᵤ/rₛ) × (Cₛ/Cᵤ) × (1/F) × 100

rᵤ = peak response of 4-aminobenzoic acid from the Sample solution

rₛ = peak response of benzocaine from the Standard solution

Cₛ = concentration of USP Benzocaine RS in the Standard solution (µg/mL)

Cᵤ = nominal concentration of benzocaine in the Sample solution (µg/mL)

F = relative response factor (see Table 2)

Calculate the percentage of ethyl 4-nitrobenzoate in the portion of Topical Solution taken:

Result = (rᵤ/rₛ) × (Cₛ/Cᵤ) × 100

rᵤ = peak response of ethyl 4-nitrobenzoate from the Sample solution

rₛ = peak response of ethyl 4-nitrobenzoate from the Standard solution

Cₛ = concentration of USP Ethyl 4-Nitrobenzoate RS in the Standard solution (µg/mL)

Cᵤ = nominal concentration of benzocaine in the Sample solution (µg/mL)

Calculate the percentage of tetracaine related compound B and any individual unspecified degradation product in the portion of Topical Solution taken:

Result = (rᵤ/rₛ) × (Cₛ/Cᵤ) × (1/F) × 100

rᵤ = peak response of tetracaine related compound B or any individual unspecified degradation product from the Sample solution

rₛ = peak response of tetracaine from the Standard solution

Cₛ = concentration of USP Tetracaine Hydrochloride RS in the Standard solution (µg/mL)

Cᵤ = nominal concentration of tetracaine hydrochloride in the Sample solution (µg/mL)

F = relative response factor (see Table 2)

Acceptance criteria: See Table 2. Disregard any impurity peaks less than 0.05%.

a 4-(Butylamino)benzoic acid.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers, and avoid freezing.

USP Reference Standards 〈11〉

USP Benzocaine RS

USP Butamben RS

USP Ethyl 4-Nitrobenzoate RS

Ethyl p-nitrobenzoate. C₉H₉NO₄ 195.17

USP Tetracaine Hydrochloride RS