Phosphate buffer and Mobile phase— Prepare as directed in the Assay.

System suitability stock solution— Dissolve accurately weighed quantities of USP Oxybutynin Related Compound B RS and USP Oxybutynin Related Compound C RS in Mobile phase to obtain a solution having known concentrations of about 100 µg of each USP Reference Standard per mL.

Standard stock solution— Dissolve an accurately weighed quantity of USP Oxybutynin Chloride RS in Mobile phase to obtain a solution having a known concentration of about 1.0 mg per mL.

System suitability solution— Transfer 10.0 mL of the System suitability stock solution to a 100-mL volumetric flask, add 10.0 mL of the Standard stock solution, and dilute with Mobile phase to volume.

Standard solution— Transfer 15.0 mL of the Standard stock solution to a 100-mL volumetric flask, and dilute with Mobile phase to volume. Transfer 5.0 mL of the solution obtained to a separate 100-mL volumetric flask, and dilute with Mobile phase to volume. This solution contains about 7.5 µg of USP Oxybutynin Chloride RS per mL.

Test solution— Transfer about 50 mg of Oxybutynin Chloride, accurately weighed, to a 10-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Chromatographic system— Prepare as directed in the Assay. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between oxybutynin related compound B and oxybutynin related compound C is not less than 1.1; and the relative standard deviation for replicate injections, determined from the oxybutynin peak, is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms for a total time of not less than twice the retention time of the oxybutynin peak, and measure all the peak responses (see Table 1 for known impurities). Calculate the percentage of each impurity in the portion of Oxybutynin Chloride taken by the formula: in which C is the concentration, in µg per mL, of USP Oxybutynin Chloride RS in the Standard solution; W is the weight, in mg, of Oxybutynin Chloride taken to prepare the Test solution; F is the relative response factor for each impurity (see Table 1 for the values); and rU and rS are the peak responses for each impurity obtained from the Test solution and for the oxybutynin peak in the Standard solution, respectively. [note—For unknown impurities, use the relative response factor of 1.0.] In addition to not exceeding the limits for each impurity in Table 1, not more than 0.1% of any other single impurity is found; and not more than 1.0% of total impurities is found.

Phosphate buffer— Dissolve about 6.67 g of monobasic potassium phosphate and 8.55 g of dibasic potassium phosphate in 1 L of water, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Phosphate buffer and acetonitrile (51:49). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Oxybutynin Chloride RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.1 mg per mL.

Assay preparation— Transfer about 50 mg of Oxybutynin Chloride, accurately weighed, to a 10-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix. Transfer 2.0 mL of this solution to a separate 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 210-nm detector and a 3-µm or 3.5-µm, 4.6-mm × 7.5-cm column that contains packing L7. The column temperature is maintained at 45. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

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 major peaks. Calculate the quantity, in mg, of C22H31NO3·HCl in the portion of Oxybutynin Chloride taken by the formula: in which C is the concentration, in mg per mL, of USP Oxybutynin Chloride RS in the Standard preparation; D is the dilution factor for the Assay preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.