A: Infrared Absorption 197K.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

Solvent A, Solvent D, Mobile phase, System suitability solution, and Chromatographic system— Proceed as directed in the Assay.

Solvent B— Use acetonitrile.

Solvent C— Use water.

Mobile phase— Use variable mixtures of Solvent A, Solvent B, and Solvent C as directed for Chromatographic system in the Assay. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Dissolve accurately weighed quantities of USP Doxazosin Mesylate RS, USP Doxazosin Related Compound A RS, USP Doxazosin Related Compound B RS, USP Doxazosin Related Compound C RS, USP Doxazosin Related Compound D RS, USP Doxazosin Related Compound E RS, USP Doxazosin Related Compound F RS, USP Terazosin Related Compound A RS, and USP Terazosin Related Compound C RS in approximately 2 mL of Solvent D; and dilute quantitatively, and stepwise if necessary, with Solvent C and Solvent D to obtain a solution having a known concentration of 0.0015 mg per mL of each of the Reference Standards. The final ratio of Solvent C to Solvent D is maintained at 9:1. Sonicate briefly to dissolve completely.

Test solution— Dissolve an accurately weighed quantity of Doxazosin Mesylate in approximately 2 mL of Solvent D, and dilute with Solvent C and Solvent D to obtain a solution having a known concentration of 0.6 mg per mL. The final ratio of Solvent C to Solvent D is maintained at 9:1. Sonicate briefly to dissolve completely.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for all the peaks. Calculate the percentage of each impurity in the portion of Doxazosin Mesylate taken by the formula: in which CS is the concentration, in mg per mL, of each Reference Standard in the Standard solution; CT is the concentration, in mg per mL, of Doxazosin Mesylate in the Test solution; ri is the peak response for each individual impurity obtained from the Test solution; and rS is the peak response for each individual impurity obtained from the Standard solution: not more than 0.3% of terazosin related compound A is found; not more than 0.25% of any other identified individual impurity is found; not more than 0.10% of any other unidentified impurity is found; and not more than 1.0% of total impurities is found. Calculate the percentages of doxazosin related compound G and doxazosin related compound H [note—The doxazosin related compound G is the mesylate salt and has the same retention time as that of the terazosin related compound A. The doxazosin related compound H is the mesylate salt and has the same retention time as that of the doxazosin related compound C.] in the portion of Doxazosin Mesylate taken by the formula: in which the response factor, F, is 0.735 for doxazosin related compound G and 0.769 for doxazosin related compound H; CS is the concentration, in mg per mL, of USP Doxazosin Mesylate RS in the Standard solution; CT is the concentration, in mg per mL, of Doxazosin Mesylate in the Test solution; ri is the peak response of doxazosin related compound G or doxazosin related compound H in the Test solution; and rS is the peak response of USP Doxazosin Mesylate RS in the Standard solution.

Solvent A— Dissolve 5 g of phosphoric acid (84%–86%) in 100 mL of water.

Solvent B— Use acetonitrile.

Solvent C— Use water.

Solvent D— Prepare a mixture of 100 mL of Solvent B and 2 g of phosphoric acid (84%–86%).

Mobile phase— Use variable mixtures of degassed Solvent A, Solvent B, and Solvent C, as directed for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability solution— Dissolve accurately weighed quantities of USP Doxazosin Related Compound A RS and USP Doxazosin Related Compound B RS in approximately 2.5 mL of Solvent D. Further dilute this solution quantitatively, and stepwise if necessary, with Solvent C and Solvent D to obtain a final solution having a known concentration of 12 µg per mL of each of the related compounds. The final ratio of Solvent C to Solvent D is maintained at 9:1. Sonicate briefly to dissolve completely.

Standard preparation— Dissolve an accurately weighed quantity of USP Doxazosin Mesylate RS in approximately 2 mL of Solvent D, and dilute with Solvent C and Solvent D to obtain a solution having a known concentration of 0.6 mg per mL. The final ratio of Solvent C to Solvent D is maintained at 9:1. Sonicate briefly to dissolve completely.

Assay preparation— Dissolve an accurately weighed quantity of Doxazosin Mesylate in approximately 2 mL of Solvent D, and dilute with Solvent C and Solvent D to obtain a solution having a concentration of 0.6 mg per mL, based on the labeled quantity of doxazosin mesylate. The final ratio of Solvent C to Solvent D is maintained at 9:1. Sonicate briefly to dissolve completely.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 210-nm detector and a 4-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 0.8 mL per minute, and the column temperature is maintained at 35. The chromatograph is programmed as follows. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between doxazosin related compound A and doxazosin related compound B is not less than 4.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the doxazosin mesylate peaks. Calculate the percentage of C23H25N5O5·CH4O3S in the portion of Doxazosin Mesylate taken by the formula: in which CS is the concentration, in mg per mL, of USP Doxazosin Mesylate RS in the Standard preparation; CT is the concentration, in mg per mL, of Doxazosin Mesylate in the Assay preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.