Benzocaine and Menthol Topical Aerosol

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 and Menthol Topical Aerosol is a solution of Benzocaine and Menthol with suitable propellants in a pressurized container. It contains NLT 90.0% and NMT 110.0% of the labeled amounts of benzocaine (C₉H₁₁NO₂) and menthol (C₁₀H₂₀O).

2 IDENTIFICATION

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

B. The retention time of the major peak for benzocaine of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

C. The retention time of the major peak for menthol of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

3 ASSAY

3.1 BENZOCAINE

Solution A: Dilute 1.0 mL of trifluoroacetic acid with water to 1 L.

Solution B: Acetonitrile

Mobile phase: See Table 1.

Table 1

Diluent: Solution A and Solution B (1:1)

Standard solution: 0.1 mg/mL of USP Benzocaine RS in Diluent. Sonicate for 2-5 min to dissolve before diluting to final volume.

Sample solution: Nominally 0.1 mg/mL of benzocaine in Diluent prepared as follows. Spray the contents of the Topical Aerosol into a flask with a stopper. Heat and stir the sprayed Topical Aerosol at 100° for 30 min in an oil bath to obtain a viscous liquid sample. Cool the sample to room temperature. Transfer an amount of Topical Aerosol equivalent to 10 mg of benzocaine to a 100-mL volumetric flask. Dissolve the sample in Diluent and dilute with Diluent to volume.

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: UV 280 nm. For Identification test A, use a diode array detector in the range of 200-400 nm.

Column: 4.6-mm × 25-cm; 5-µm packing L7

Flow rate: 1.5 mL/min

Injection volume: 20 µL

System suitability

Sample: Standard solution

Suitability requirements

Tailing factor: NMT 1.5

Relative standard deviation: NMT 2.0%

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of benzocaine (C₉H₁₁NO₂) in the portion of Topical Aerosol taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response from the Sample solution

r_S = peak response from the Standard solution

C_S = concentration of USP Benzocaine RS in the Standard solution (mg/mL)

C_U = nominal concentration of benzocaine in the Sample solution (mg/mL)

Acceptance criteria: 90.0%-110.0%

3.2 MENTHOL

Internal standard solution: 1 mg/mL of decanol in isopropyl alcohol

Standard stock solution: 1 mg/mL of USP Menthol RS in isopropyl alcohol

Standard solution: 0.5 mg/mL each of USP Menthol RS and decanol in isopropyl alcohol from Internal standard solution and Standard stock solution

Sample solution: Nominally 0.5 mg/mL of menthol prepared as follows. Spray the contents of the Topical Aerosol into a flask. Transfer an amount of Topical Aerosol equivalent to 5 mg of menthol to a 10-mL volumetric flask and add 5.0 mL of Internal standard solution. Dilute with isopropyl alcohol to volume.

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: GC

Detector: Flame ionization

Column: 0.53 - mm * 30 - m fused-silica capillary column bonded with a 1.0-µm film of phase G16

Carrier gas: Hydrogen

Temperatures

Injection port: 250°

Detector: 250°

Column: See Table 2.

Table 2

Flow rate: 10 mL/min

Injection volume: 1 µL

Injection type: Split, split ratio 10:1

System suitability

Sample: Standard solution

[NOTE-The relative retention times for menthol and decanol are 1.0 and 1.6, respectively.]

Suitability requirements

Resolution: NLT 2.5 between the menthol and decanol peaks

Relative standard deviation: NMT 1.0% of the ratio of the peak response of menthol to that of decanol

Analysis

Samples: Standard solution and Sample solution Calculate the percentage of the labeled amount of menthol (C₁₀H₂₀O) in the portion of Topical Aerosol taken:

Result = (R_U/R_S) × (C_S/C_U) × 100

R_U = peak response ratio of menthol to decanol from the Sample solution

R_S = peak response ratio of menthol to decanol from the Standard solution

C_S = concentration of USP Menthol RS in the Standard solution (mg/mL)

C_U = nominal concentration of menthol in the Sample solution (mg/mL)

Acceptance criteria: 90.0%-110.0%

4 PERFORMANCE TESTS

MINIMUM FILL (755): Meets the requirements

5 IMPURITIES

5.1 ORGANIC IMPURITIES

Solution A: Dilute 1.0 mL of trifluoroacetic acid with water to 1 L.

Solution B: Acetonitrile

Mobile phase: See Table 3.

Table 3

Diluent: Solution A and Solution B (1:1)

Standard stock solution: 0.1 mg/mL each of USP Benzocaine RS, USP Aminobenzoic Acid RS, and USP Ethyl 4-Nitrobenzoate RS in Diluent. Sonicate for about 5 min to dissolve before diluting to final volume.

Standard solution: 0.001 mg/mL each of USP Benzocaine RS, USP Aminobenzoic Acid RS, and USP Ethyl 4-Nitrobenzoate RS in Diluent from the Standard stock solution

Sample solution: Nominally 0.5 mg/mL of benzocaine in Diluent prepared as follows. Spray the contents of the Topical Aerosol into a flask. Heat and stir the sprayed Topical Aerosol at 100° for 30 min in an oil bath to obtain a viscous liquid sample. Cool the sample to room temperature. Transfer an amount of Topical Aerosol equivalent to 50 mg of benzocaine to a 100-mL volumetric flask. Dissolve the sample in Diluent and dilute with Diluent to volume.

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: UV 280 nm

Column: 4.6-mm x 25-cm; 5-µm packing L7

Flow rate: 1.5 mL/min

Injection volume: 20 µL

System suitability

Sample: Standard solution

Suitability requirements

Analysis

Relative standard deviation: NMT 2.0% for each peak corresponding to benzocaine, aminobenzoic acid, and ethyl 4-nitrobenzoate

Samples: Standard solution and Sample solution

Calculate the percentage of aminobenzoic acid and ethyl 4-nitrobenzoate in the portion of Topical Aerosol taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response of aminobenzoic acid or ethyl 4-nitrobenzoate from the Sample solution

r_S = peak response of the corresponding Reference Standard from the Standard solution 

C_S = concentration of USP Aminobenzoic Acid RS or USP Ethyl 4-Nitrobenzoate RS in the Standard solution (mg/mL)

C_U = nominal concentration of benzocaine in the Sample solution (mg/mL)

Calculate the percentage of each unspecified degradation product in the portion of Topical Aerosol taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response of each unspecified degradation product from the Sample solution

r_S = peak response of benzocaine from the Standard solution 

C_S = concentration of USP Benzocaine RS in the Standard solution (mg/mL)

C_U = nominal concentration of benzocaine in the Sample solution (mg/mL)

Acceptance criteria: See Table 4. Disregard peaks less than 0.05%.

Table 4

6 SPECIFIC TESTS

MICROBIAL ENUMERATION TESTS (61) and TESTS FOR SPECIFIED MICROORGANISMS (62): It meets the requirements for the absence of Staphylococcus

aureus and Pseudomonas aeruginosa.

PRESSURE TEST: It meets the requirements in Topical Aerosols (603).

LEAKAGE TEST: It meets the requirements in Leak Rate (604).

7 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight, pressurized containers, and avoid exposure to excessive heat.

USP REFERENCE STANDARDS (11)

USP Aminobenzoic Acid RS

Benzoic acid, 4-amino.

C₇H₇NO₂  137.14

USP Benzocaine RS

USP Ethyl 4-Nitrobenzoate RS

Benzoic acid, 4-nitro-, ethyl ester.

C₉H₉NO₄  195.17

USP Menthol RS