Test solution— Transfer a portion of powdered Tablets,equivalent to about 100mg of captopril,to a conical flask.Add 25mLof methanol,stir for 30minutes using a magnetic stirrer,and centrifuge.Use the clear supernatant.

Standard solution: 4mg per mL,in methanol.

Application volume: 50µL,as streaks.

Developing solvent system: a mixture of toluene,glacial acetic acid,and methanol (75:25:1).

Procedure— Proceed as directed in the chapter.Locate the spots on the plate by lightly spraying with a freshly prepared mixture of 1volume of ammonium hydroxide and 6volumes of a solution of 0.04%5,5¢-dithiobis(2-nitrobenzoic acid)in methanol.

Medium: 0.01Nhydrochloric acid;900mL.

Apparatus 1: 50rpm.

Time: 20minutes.

Procedure— Determine the amount of C9H15NO3Sdissolved by employing UVabsorption at the wavelength of maximum absorbance at about 205nm on filtered portions of the solution under test,suitably diluted with Dissolution Medium,if necessary,in comparison with a Standard solution having a known concentration of USP Captopril RSin the same Medium.

Tolerances— Not less than 80%(Q)of the labeled amount of C9H15NO3Sis dissolved in 20minutes.

Mobile phase— Proceed as directed in the Assay.

System suitability solution— Dissolve suitable quantities of USP Captopril RSand USP Captopril Disulfide RSin Mobile phaseto obtain a solution having known concentrations of about 1mg per mLand 0.05mg per mL,respectively.

Standard solution— Dissolve an accurately weighed quantity of USP Captopril Disulfide RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.05mg per mL.

Test solution— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 25mg of captopril,to a suitable centrifuge tube.Add 25.0mLof Mobile phase,sonicate for 15minutes,and centrifuge.Use the clear supernatant as the Test solution.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm ×25-cm column that contains packing L1with about 15%hydrocarbon load.The flow rate is about 1mLper minute.Chromatograph the System suitability solutionand the Standard solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.5for captopril and 1.0for captopril disulfide;the resolution,R,between captopril and captopril disulfide in the System suitability solutionis not less than 2.0;and the relative standard deviation for replicate injections of the Standard solutionis not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the percentage of captopril disulfide in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Captopril Disulfide RSin the Standard solution;Wis the quantity,in mg,of captopril in the portion of Tablets taken to prepare the Test solution,based on the labeled amount per Tablet;and rUand rSare the captopril disulfide peak responses obtained from the Test solutionand the Standard solution,respectively:not more than 3.0%is found.

Mobile phase— Prepare a filtered and degassed mixture of 550mLof methanol and 450mLof water containing 0.50mLof phosphoric acid.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve suitable quantities of USP Captopril RSand USP Captopril Disulfide RSin Mobile phaseto obtain a solution having known concentrations of about 1mg per mLand 0.05mg per mL,respectively.

Assay preparation— Transfer not fewer than 20Tablets to a suitable volumetric flask,add Mobile phaseto fill the flask to about half of its capacity,and sonicate for 15minutes.Dilute with Mobile phaseto volume,shake by mechanical means for 15minutes,and filter.Dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a concentration of about 1mg of captopril per mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm ×25-cm column that contains packing L1with about 15%hydrocarbon load.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.5for captopril and 1.0for captopril disulfide;the resolution,R,between captopril and captopril disulfide is not less than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of captopril (C9H15NO3S)in the portion of Tablets taken by the formula: in which Lis the labeled amount,in mg,of captopril in each Tablet;Dis the concentration,in mg per mL,of captopril in the Assay preparationbased on the labeled quantity per Tablet and the extent of dilution;Cis the concentration,in mg per mL,of USP Captopril RSin the Standard preparation;and rUand rSare the captopril peak responses obtained from the Assay preparationand the Standard preparation,respectively.