Acetylcysteine
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C5H9NO3S 163.20
l-Cysteine, N-acetyl-;    
N-Acetyl-l-cysteine     [616-91-1].
DEFINITION
Acetylcysteine contains NLT 98.0% and NMT 102.0% of C5H9NO3S, calculated on the dried basis.
IDENTIFICATION
ASSAY
•  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
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
System suitability 
Sample:  Standard solution
[Note—The relative retention times for acetylcysteine and l-phenylalanine are about 0.5 and 1.0, respectively. ]
Suitability requirements 
Resolution:  NLT 6 between acetylcysteine and l-phenylalanine
Relative standard deviation:  NMT 2.0%
Analysis 
Samples:  Standard solution and Sample solution
Calculate the percentage of acetylcysteine (C5H9NO3S) in the portion of Acetylcysteine taken:
Result = (RU/RS) × (CS/CU) × 100
RU== peak response ratio of acetylcysteine to l-phenylalanine from the Sample solution
RS== peak response ratio of acetylcysteine to l-phenylalanine from the Standard solution
CS== concentration of USP Acetylcysteine RS in the Standard solution (mg/mL)
CU== concentration of acetylcysteine in the Sample solution (mg/mL)
Acceptance criteria:  98.0%–102.0% on the dried basis
IMPURITIES
•  Residue on Ignition 281: NMT 0.5%
•  Heavy Metals, Method II 231
[Caution—Exercise care because explosion may occur. ]
Analysis:  In a dropwise manner, wet the sample with 2 mL of nitric acid, and proceed as directed for the Test preparation.
Acceptance criteria:  NMT 10 ppm
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.
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in tight containers, and store at controlled room temperature.
•  USP Reference Standards 11
USP Acetylcysteine RS Click to View Structure
USP l-Phenylalanine RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Clydewyn M. Anthony, Ph.D.
Senior Scientific Liaison
1-301-816-8139
(SM22010) Monographs - Small Molecules 2
Reference Standards RS Technical Services
1-301-816-8129
rstech@usp.org
USP35–NF30 Page 2069
Pharmacopeial Forum: Volume No. 31(3) Page 726