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

Standard solution: 4 mg per mL, in methanol.

Application volume: 50 µL, 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 1 volume of ammonium hydroxide and 6 volumes of a solution of 0.04% 5,5¢-dithiobis(2-nitrobenzoic acid) in methanol.

Medium: 0.01 N hydrochloric acid; 900 mL.

Apparatus 1: 50 rpm.

Time: 20 minutes.

Procedure— Determine the amount of C9H15NO3S dissolved by employing UV absorption at the wavelength of maximum absorbance at about 205 nm 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 RS in the same Medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C9H15NO3S is dissolved in 20 minutes.

Mobile phase— Proceed as directed in the Assay.

System suitability solution— Dissolve suitable quantities of USP Captopril RS and USP Captopril Disulfide RS in Mobile phase to obtain a solution having known concentrations of about 1 mg per mL and 0.05 mg per mL, respectively.

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

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

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm × 25-cm column that contains packing L1 with about 15% hydrocarbon load. The flow rate is about 1 mL per minute. Chromatograph the System suitability solution and the Standard solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.5 for captopril and 1.0 for captopril disulfide; the resolution, R, between captopril and captopril disulfide in the System suitability solution is not less than 2.0; and the relative standard deviation for replicate injections of the Standard solution is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into 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 C is the concentration, in mg per mL, of USP Captopril Disulfide RS in the Standard solution; W is 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 rU and rS are the captopril disulfide peak responses obtained from the Test solution and the Standard solution, respectively: not more than 3.0% is found.

Mobile phase— Prepare a filtered and degassed mixture of 550 mL of methanol and 450 mL of water containing 0.50 mL of phosphoric acid. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve suitable quantities of USP Captopril RS and USP Captopril Disulfide RS in Mobile phase to obtain a solution having known concentrations of about 1 mg per mL and 0.05 mg per mL, respectively.

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

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm × 25-cm column that contains packing L1 with about 15% hydrocarbon load. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.5 for captopril and 1.0 for 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 20 µL) of the Standard preparation and the Assay preparation into 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 L is the labeled amount, in mg, of captopril in each Tablet; D is the concentration, in mg per mL, of captopril in the Assay preparation based on the labeled quantity per Tablet and the extent of dilution; C is the concentration, in mg per mL, of USP Captopril RS in the Standard preparation; and rU and rS are the captopril peak responses obtained from the Assay preparation and the Standard preparation, respectively.