Acetylcysteine

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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C₅H₉NO₃S 163.19

l-Cysteine, N-acetyl-;

N-Acetyl-l-cysteine CAS RN®: 616-91-1.

1 DEFINITION

Acetylcysteine contains NLT 98.0% and NMT 102.0% of C H NO S, calculated on the dried basis.

2 IDENTIFICATION

Change to read:

A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K (CN 1-May-2020)

3 ASSAY

3.1 Procedure

Mobile phase: 6.8 g/L of monobasic potassium phosphate. Adjust with phosphoric acid to a pH of 3.0.

Sodium metabisulfite solution: 0.5 mg/mL of sodium metabisulfite in water, freshly prepared

Internal standard solution: 5 mg/mL of USP l-Phenylalanine RS in Sodium metabisulfite solution

Standard stock solution: 10 mg/mL of USP Acetylcysteine RS in Sodium metabisulfite solution

Standard solution: 0.5 mg/mL of USP Acetylcysteine RS and 0.25 mg/mL of USP l-Phenylalanine RS in Sodium metabisulfite solution from Standard stock solution and Internal standard solution

Sample stock solution: 10 mg/mL of Acetylcysteine in Sodium metabisulfite solution

Sample solution: 0.5 mg/mL of Acetylcysteine and 0.25 mg/mL of USP l-Phenylalanine RS in Sodium metabisulfite solution from Sample stock solution and Internal standard solution

3.2 Chromatographic system

Mode: LC

Detector: UV 214 nm

Column: 3.9-mm × 30-cm; packing L1

Flow rate: 1.5 mL/min

Injection size: 5 μL

3.3 System suitability

Sample: Standard solution

[Note—The relative retention times for acetylcysteine and l-phenylalanine are about 0.5 and 1.0, respectively.]

3.4 Suitability requirements

Resolution: NLT 6 between acetylcysteine and l-phenylalanine

Relative standard deviation: NMT 2.0%

3.5 Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of acetylcysteine (C₅H₉NO₃S) in the portion of Acetylcysteine taken:

Result = (R_u/R_s) × (C_s/C_u) × 100

R_u = peak response ratio of acetylcysteine to l-phenylalanine from the Sample solution

R_s = peak response ratio of acetylcysteine to l-phenylalanine from the Standard solution

C_s = concentration of USP Acetylcysteine RS in the Standard solution (mg/mL)

C_u = concentration of acetylcysteine in the Sample solution (mg/mL)

Acceptance criteria: 98.0%–102.0% on the dried basis

4 IMPURITIES

Residue on Ignition 〈281〉: NMT 0.5%

5 SPECIFIC TESTS

Optical Rotation, Specific Rotation〈781S〉

Buffer: Mix 29.5 mL of 1 N sodium hydroxide, 50 mL of 1 M monobasic potassium phosphate, and sufficient water to make 100 mL. Adjust to a pH of 7.0 ± 0.1 by adding more of either solution, as necessary.

Sample solution: In a 25-mL volumetric flask, mix 1.25 g with 1 mL of edetate disodium solution (1 in 100), add 7.5 mL of sodium hydroxide solution (1 in 25), and mix to dissolve. Dilute with Buffer to volume.

Acceptance criteria: +21° to +27°

pH 〈791〉: 2.0–2.8 in a solution (1 in 100)

Loss on Drying 〈731〉: Dry a sample at a pressure of about 50 mm of mercury at 70° for 4 h: it loses NMT 1.0% of its weight.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers, and store at controlled room temperature.

USP Reference Standards 〈11〉

USP Acetylcysteine RS

USP l-Phenylalanine RS