Medium: 0.01 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of doxazosin mesylate (C23H25N5O5·CH4SO3) dissolved by employing UV absorption at the wavelength of maximum absorbance at about 246 nm on filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Doxazosin Mesylate RS in the same Medium.

Tolerances— Not less than 70%(Q) of the labeled amount of C23H25N5O5·CH4SO3 is dissolved in 30 minutes.

Buffer solution— Transfer 3.4 g of monobasic potassium phosphate into a 1-L flask, and add 800 mL of water and 4.0 mL of triethylamine to dissolve. Adjust with phosphoric acid to a pH of 4.5, and dilute with water to volume.

Mobile phase— Prepare a filtered and degassed mixture of methanol and Buffer solution (11 : 9). Make adjustments if necessary (see System Suitability under Chromatography 621.)

Diluent: A mixture of methanol and 0.1 N hydrochloric acid (9 : 1).

Standard preparation— Dissolve an accurately weighed quantity of USP Doxazosin Mesylate RS in Diluent, and dilute quantitatively and stepwise if necessary, with Diluent to obtain a solution having a final concentration of about 49 µg per mL.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 245-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute, and the column temperature is maintained at 40. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the capacity factor, k¢, for doxazosin is not less than 2.0; the column efficiency is not less than 1000 theoretical plates; the tailing factor is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Assay preparation— Transfer 10 Tablets, whole or ground, into a 250-mL volumetric flask, add 10 mL of water, and sonicate until the Tablets are disintegrated. Add 150 mL of Diluent, sonicate for 30 minutes, dilute with Diluent to volume, and mix. Quantitatively dilute a portion of the supernatant with Diluent to obtain a solution having a concentration of about 0.04 mg of doxazosin per mL.

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 doxazosin peaks. Calculate the quantity, in mg, of doxazosin (C23H25N5O5) in the portion of Tablets taken by the formula: in which 451.48 and 547.58 are the molecular weights of doxazosin and doxazosin mesylate, respectively; C is the concentration, in mg per mL, of USP Doxazosin Mesylate RS in the Standard preparation; D is the dilution volume, in mL, considering the initial 250-mL flask and any subsequent dilution used to prepare the Assay preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.