Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C22H31NO3·HCl dissolved using the method set forth in the Assay, making any necessary modifications to the concentration of the Standard preparation to correspond to that of the solution under test.

Tolerances— Not less than 80% (Q) of the labeled amount of C22H31NO3·HCl is dissolved in 30 minutes.

Solvent A— Add about 0.9 mL of triethylamine to a filtered and deaerated mixture of water and methanol (3200:800). Adjust with phosphoric acid to a pH of 3.5 ± 0.05.

Mobile phase— Prepare a degassed and filtered mixture of Solvent A and acetonitrile (80:20).

Standard preparation— Prepare a solution of USP Oxybutynin Chloride RS in Mobile phase having an accurately known concentration of about 0.05 mg per mL.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 50 mg of oxybutynin chloride, to a 1000-mL volumetric flask, add about 400 mL of Mobile phase, sonicate for about 10 minutes, shake by mechanical means for about 45 minutes, dilute with Mobile phase to volume, and mix.

Chromatographic system— The liquid chromatograph is equipped with a 203-nm detector, and a 4-mm × 30-cm column that contains packing L10. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the chromatogram as directed for Procedure: the tailing factor is not more 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 C22H31NO3·HCl in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Oxybutynin Chloride RS in the Standard preparation, and rU and rS are the oxybutynin peak responses obtained from the Assay preparation and the Standard preparation, respectively.