Test solution: 10mg per mL,in dehydrated alcohol.

Mobile phase— Prepare a filtered and degassed mixture of a 9in 100solution of tetrahydrofuran in methanol and a 1in 2000solution of phosphoric acid in water (33:67).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Resolution solution— Dissolve USP Captopril RS,USP Captopril Disulfide RS,and 3-acetylthio-2-methylpropanoic acid in methanol to obtain a stock solution containing about 0.1mg of each per mL.Quantitatively dilute a portion of this stock solution with methanol to obtain a solution containing about 10µg of each Reference Standard per mL.

Standard solution— [NOTE—Use low-actinic glassware.]Dissolve an accurately weighed quantity of USP Captopril Disulfide RSin methanol and dilute quantitatively,and stepwise if necessary,with methanol to obtain a solution having a known concentration of 10µg per mL.

Test solution— [NOTE—Use low-actinic glassware.]Transfer 50mg of Captopril,accurately weighed,to a 25-mLvolumetric flask.Dissolve the specimen in methanol,dilute with methanol to volume,mix,and use the solution promptly.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 220-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 1mLper minute.Chromatograph the Resolution solution(about 20µL),and record the peak responses as directed for Procedure:the relative retention times are about 0.32for captopril,0.42for 3-acetylthio-2-methylpropanoic acid,and 1.0for captopril disulfide,and the resolution,R,between captopril and 3-acetylthio-2-methylpropanoic acid is not less than 3.0.

Procedure— [NOTE—Use the peak areas where peak responses are indicated.]Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the percentage of captopril disulfide in the portion of Captopril taken by the formula: in which CSis the concentration,in µg per mL,of USP Captopril Disulfide RSin the Standard solution,CUis the concentration,in µg per mL,of Captopril in the Test solution,and rUand rSare the captopril disulfide peak responses obtained from the Test solutionand the Standard solution,respectively:not more than 1.0%of the captopril disulfide is found.Compare the peak responses,excluding those of the solvent,captopril,and captopril disulfide,in the chromatogram of the Test solutionwith the main peak response in the chromatogram of the Standard solution:the peak response of each impurity does not exceed 40%of the main peak response in the chromatogram of the Standard solution(0.2%),and the sum of the impurity peak responses does not exceed the main peak response in the chromatogram of the Standard solution(0.5%).

0.1N Potassium iodate titrant— Dissolve 3.567g of potassium iodate,previously dried at 110to constant weight,in water to make 1000.0mL.

Procedure— Dissolve about 300mg of Captopril,accurately weighed,in 100mLof water in a suitable glass-stoppered flask,add 10mLof 3.6Nsulfuric acid,1g of potassium iodide,and 2mLof starch TS.Titrate with 0.1N Potassium iodate titrantto a faint blue endpoint that persists for not less than 30seconds.Perform a blank determination (see Titrimetry á541ñ),and make any necessary correction.Each mLof 0.1N Potassium iodate titrantis equivalent to 21.73mg of C9H15NO3S.