Medium: 0.2 M acetic acid; 500 mL.

Apparatus 1: 150 rpm.

Time: 45 minutes.

Mobile phase— Prepare a suitable degassed mixture of acetonitrile and 0.05 M monobasic potassium phosphate (65:35).

Standard solution— Dissolve an accurately weighed quantity of USP Diphenoxylate Hydrochloride RS in methanol to obtain a solution having a known concentration of about 250 µg per mL. Pipet 10 mL of this solution into a 500-mL volumetric flask, dilute with Dissolution Medium to volume, mix, and filter.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 210-nm detector and a 3.9-mm × 30-cm column that contains packing L11. The flow rate is about 1.0 mL per minute. Chromatograph replicate injections of the Standard solution, and record the peak responses as directed for Procedure: the tailing factor is not more than 1.5; and the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard solution and of filtered portions of the solution under test into the chromatograph, record the chromatograms, measure the response for the major peak, and determine the amount of C30H32N2O2·HCl dissolved.

Tolerances— Not less than 75% (Q) of the labeled amount of C30H32N2O2·HCl is dissolved in 45 minutes.

Diluent— Use a mixture of acetonitrile and water (1:1).

Solution A— Transfer 192 mg of sodium 1-pentanesulfonate monohydrate to a suitable container, add 200 mL of water, and sonicate to dissolve. Add 800 mL of water and 1.0 mL of phosphoric acid, and mix.

Solution B— Transfer 192 mg of sodium 1-pentanesulfonate monohydrate to a suitable container, add 200 mL of water, and sonicate to dissolve. Add 800 mL of acetonitrile and 1.0 mL of phosphoric acid, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Solution B and Solution A (66:34). Make adjustments if necessary (see System Suitability under Chromatography 621).

Atropine stock preparation— Dissolve an accurately weighed quantity of USP Atropine Sulfate RS in Diluent, and dilute quantitatively, and stepwise if necessary, with Diluent to obtain a solution having a known concentration of about 0.04 mg per mL.

Standard preparation— Transfer about 20 mg of USP Diphenoxylate Hydrochloride RS to a 200-mL volumetric flask, add about 100 mL of Diluent, and sonicate to dissolve. Accurately add 5.0 mL of Atropine stock preparation, and mix. Allow the solution to reach room temperature, and then dilute with Diluent to volume. This solution contains about 0.1 mg of diphenoxylate hydrochloride and about 0.001 mg of atropine sulfate per mL.

Assay preparation— Transfer an accurately counted number of Tablets, equivalent to about 25 mg of diphenoxylate hydrochloride, based on the label claim, to a 250-mL volumetric flask, add approximately 100 mL of Diluent, and shake by mechanical means for at least 15 minutes or until the Tablets are completely disintegrated. Sonicate for an additional 15 minutes, allow the solution to reach room temperature, dilute with Diluent to volume, and mix. Pass a portion of the solution obtained through a 0.45-µm PTFE filter, discarding the first few mL, and use the clear filtrate.

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 5-µm packing L10. The flow rate is about 1.7 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.35 for atropine and 1.0 for diphenoxylate; the resolution, R, between atropine and diphenoxylate is not less than 5.0; the tailing factor is not more than 1.5 for atropine; and the relative standard deviation for replicate injections is not more than 2.0% for diphenoxylate and not more than 5.0% for atropine. [note—If a significant tailing of the diphenoxylate peak is observed (greater than 2.5), it is recommended to maintain the column temperature at 25, to stabilize the system.]

Procedure— Separately inject equal volumes (about 50 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 diphenoxylate hydrochloride (C30H32N2O2·HCl) in the portion of Tablets taken by the formula: in which 250 is the volume, in mL, of the Assay preparation; CD is the concentration, in mg per mL, of USP Diphenoxylate Hydrochloride RS in the Standard preparation; and rU and rS are the diphenoxylate peak responses obtained from the Assay preparation and the Standard preparation, respectively.