Chromatographic purity—
Adsorbent:
0.25-mm layer of chromatographic silica gel mixture.
N-Ethylmaleimide solution—
Prepare a solution of 4% N-ethylmaleimide in alcohol (w/v).
Test solution—
Transfer about 0.2 g of Cysteine Hydrochloride, accurately weighed, to a 10-mL volumetric flask, and dissolve in and dilute with water to volume. To 5.0 mL of this solution, add 5.0 mL of N-Ethylmaleimide solution. Allow the solution to stand for 5 minutes before using. Apply 5 µL.
Standard stock solution—
Dissolve 20 mg of USP L-Cysteine Hydrochloride RS in 10.0 mL of water, add 10.0 mL of N-Ethylmaleimide solution, and mix. Allow the solution to stand for 5 minutes before using.
Standard solution—
Transfer 5.0 mL of the Standard stock solution to a 100-mL volumetric flask, and dilute with water to volume to obtain a solution having a concentration of about 0.05 mg per mL. Apply 5 µL. [NOTE—This solution has a concentration equivalent to about 0.5% of that of the Test solution.]
System suitability solution—
Transfer about 10 mg of USP L-Tyrosine RS and 10 mL of the Standard stock solution to a 25-mL volumetric flask, dilute with water to volume, and mix. Apply 5 µL.
Spray reagent—
Dissolve 0.2 g of ninhydrin in 100 mL of a mixture of butyl alcohol and 2 N acetic acid (95:5).
Developing solvent system—
Prepare a mixture of butyl alcohol, glacial acetic acid, and water (60:20:20).
Procedure—
Proceed as directed for
Thin-Layer Chromatography under
Chromatography 
621
. After air-drying the plate, spray with
Spray reagent, and heat between 100
and 105
for about 15 minutes. Examine the plate under white light. The chromatogram obtained from the
System suitability solution exhibits two clearly separated spots. Any secondary spot in the chromatogram obtained from the
Test solution is not larger or more intense than the principal spot in the chromatogram obtained from the
Standard solution: not more than 0.5% of any individual impurity is found; and not more than 2.0% of total impurities is found.
Assay—
Accurately weigh about 250 mg of Cysteine Hydrochloride into an iodine flask. Add 20 mL of water and 4 g of potassium iodide, and mix to dissolve. Cool the solution in an ice bath, and add 5 mL of 3 N hydrochloric acid and 25.0 mL of 0.1 N iodine VS. Insert the stopper, and allow to stand in the dark for 20 minutes. Titrate the excess iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of
starch TS as the endpoint is approached. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N sodium thiosulfate is equivalent to 15.76 mg of C
3H
7NO
2S·HCl.
;)
