Methionine

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

Methionine contains NLT 98.5% and NMT 101.5% of l-methionine (C₅H₁₁NO₂S), calculated on the dried basis.

2 IDENTIFICATION

A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K

3 ASSAY

Procedure

Sample: 140 mg of Methionine

Blank: Mix 3 mL of formic acid and 50 mL of glacial acetic acid.

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Direct titration

Titrant: 0.1 N perchloric acid VS

Endpoint detection: Potentiometric

Analysis: Dissolve the Sample in 3 mL of formic acid and 50 mL of glacial acetic acid, and titrate with the Titrant.

Calculate the percentage of l-methionine (C₅H₁₁NO₂S) in the portion of the Sample taken:

Result = {[(V_S− V_B) × N × F]/W} × 100

V_S = Titrant volume consumed by the Sample (mL)

V_B = Titrant volume consumed by the Blank (mL)

N = actual normality of the Titrant (mEq/mL)

F = equivalency factor, 149.2 mg/mEq

W = Sample weight (mg)

Acceptance criteria: 98.5%–101.5% on the dried basis

4 IMPURITIES

Residue on Ignition 〈281〉: NMT 0.4%

Chloride and Sulfate 〈221〉, Chloride

Standard solution: 0.50 mL of 0.020 N hydrochloric acid

Sample: 0.73 g of Methionine

Acceptance criteria: NMT 0.05%

Chloride and Sulfate 〈221〉, Sulfate

Standard solution: 0.10 mL of 0.020 N sulfuric acid

Sample: 0.33 g of Methionine

Acceptance criteria: NMT 0.03%

Change to read:

Iron 〈241〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 30 ppm

Related Compounds

Phosphoric acid solution: 23 g/L of phosphoric acid

Solution A: Acetonitrile, Phosphoric acid solution, and water (0.5:12.5:87)

Solution B: Acetonitrile, Phosphoric acid solution, and water (47.5:12.5:40)

Mobile phase: Gradient elution. See Table 1.

Table 1

System suitability solution: Transfer 5 mg each of USP l-Methionine RS and l-methionine sulfoxide to a 50-mL volumetric flask, and dissolve in and dilute with Solution A to volume.

Standard solution: 30 μg/mL of USP l-Methionine RS in Solution A

N-Acetyl-d,l-methionine standard solution: 0.06 mg/mL of USP N-Acetyl-d,l-methionine RS in Solution A

Sample solution: 30 mg/mL of Methionine in Solution A

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 205 nm

Column: 4.6-mm × 25-cm; 5-μm packing L1

Column temperature: 30°

Flow rate: 1.0 mL/min

Injection volume: 50 μL

System suitability

Sample: System suitability solution

[Note—The relative retention times for l-methionine and l-methionine sulfoxide are 1.0 and 0.5, respectively.]

Suitability requirements

Resolution: NLT 5.0 between l-methionine and l-methionine sulfoxide peaks

Analysis

Samples: Standard solution, N-Acetyl-d,l-methionine standard solution, and Sample solution

Calculate the percentage of N-acetyl-d,l-methionine in the portion of Methionine taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response of N-acetyl-d,l-methionine from the Sample solution

r_S = peak response of N-acetyl-d,l-methionine from the N-Acetyl-d,l-methionine standard solution

C_S = concentration of USP N-Acetyl-d,l-methionine RS in the N-Acetyl-d,l-methionine standard solution (mg/mL)

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

Separately calculate the percentage of methionine sulfoxide and any unspecified impurity in the portion of Methionine taken:

Result = (r_U/r_S) × (C_S/C_U) × (1/F) × 100

r_U = peak response of methionine sulfoxide or any unspecified impurity from the Sample solution

r_S = peak response of methionine from the Standard solution

C_S = concentration of USP l-Methionine RS in the Standard solution (mg/mL)

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

F = relative response factor (see Table 2)

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

Table 2

5 SPECIFIC TESTS

Optical Rotation 〈781S〉, Procedures, Specific Rotation

Sample solution: 20 mg/mL in 6 N hydrochloric acid

Acceptance criteria: +22.4° to +24.7°

pH 〈791〉

Sample solution: 10 mg/mL of solution

Acceptance criteria: 5.6–6.1

Loss on Drying 〈731〉

Analysis: Dry at 105° for 3 h.

Acceptance criteria: NMT 0.3%

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers.

USP Reference Standards 〈11〉

USP N-Acetyl-d,l-methionine RS

USP l-Methionine RS