A: Transfer the finely ground contents of 1 Tablet to a test tube, add 5 mL of methanol, shake for 5 minutes, and centrifuge. Use the clear supernatant as the test solution. Prepare a Standard solution in methanol containing, in each mL, about 20 mg of USP Propoxyphene Napsylate RS and 130 mg of USP Acetaminophen RS. Apply 10 µL of the test solution on a line parallel to and about 2 cm from the bottom edge of a 20- × 5-cm thin-layer chromatographic plate (see Chromatography 621) coated with chromatographic silica gel mixture, and apply 10 µL of the Standard solution separately on the starting line. Place the plate in a developing chamber containing a mixture of chloroform, butyl acetate, and formic acid (50:50:20), and develop the chromatogram until the solvent front has moved about 15 cm above the line of application. Remove the plate, allow to dry in a hood, and view under short-wavelength UV light: the solution under test exhibits two principal spots having intensities and RF values identical to those of the two principal spots obtained from the Standard solution.

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

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

Medium: pH 4.5 acetate buffer, prepared as directed in the test for Dissolution under Propoxyphene Hydrochloride, Aspirin, and Caffeine Capsules; 500 mL.

Apparatus 1: 100 rpm.

Time: 60 minutes.

Diethylamine phosphate buffer, Mobile phase, and Chromatographic system— Proceed as directed in the Assay under Propoxyphene Napsylate Tablets.

Standard solution— Transfer about 10 mg of USP Propoxyphene Napsylate RS and about 10J mg of USP Acetaminophen RS, both accurately weighed, to a 100-mL volumetric flask, J being the ratio of the labeled amount, in mg, of acetaminophen to the labeled amount, in mg, of propoxyphene napsylate in each Tablet. Dissolve in and dilute with Dissolution Medium to volume, and mix. This solution contains about 0.1 mg of USP Propoxyphene Napsylate RS and 0.1J mg of USP Acetaminophen RS per mL. [note—Retain a portion of this solution for use in preparing the Standard solution when determining the amount of acetaminophen dissolved.]

Test solution and Procedure— Proceed as directed in the Dissolution test under Propoxyphene Napsylate Tablets.

Diethylamine phosphate buffer— Prepare as directed in the Assay under Propoxyphene Napsylate Tablets.

Mobile phase, Chromatographic system, and Procedure— Proceed as directed in the Assay for acetaminophen.

Standard solution A— Transfer 10.0 mL of the Standard solution to a 100-mL volumetric flask, dilute with Dissolution Medium to volume, and mix. This solution contains about 0.01J mg of USP Acetaminophen RS per mL.

Test solution A— Quantitatively dilute a filtered portion of the solution under test with Dissolution Medium to obtain a solution having a concentration of about 0.01J mg of acetaminophen per mL.

Tolerances— Not less than 75% (Q) of the labeled amounts of C22H29NO2·C10H8O3S·H2O and C8H9NO2 is dissolved in 60 minutes.

Diethylamine phosphate buffer, Diluent, Mobile phase, and Chromatographic system— Proceed as directed in the Assay under Propoxyphene Napsylate Tablets.

Standard preparation— Transfer about 10 mg of USP Propoxyphene Napsylate RS and about 10J mg of USP Acetaminophen RS, both accurately weighed, to a 100-mL volumetric flask, J being the ratio of the labeled amount, in mg, of acetaminophen to the labeled amount, in mg, of propoxyphene napsylate in each Tablet. Dissolve in and dilute with Diluent to volume, and mix. This solution contains about 0.1 mg of USP Propoxyphene Napsylate RS and 0.1J mg of USP Acetaminophen RS per mL. [note—Retain a portion of the Standard preparation for use in the Assay for acetaminophen.]

Assay preparation and Procedure— Proceed as directed in the Assay under Propoxyphene Napsylate Tablets. [note—Retain a portion of the Assay preparation for use in the Assay for acetaminophen.]

Diethylamine phosphate buffer— Prepare as directed in the Assay under Propoxyphene Napsylate Tablets.

Mobile phase— Prepare a mixture of Diethylamine phosphate buffer and acetonitrile (4:1). Sonicate for 15 minutes, and pass through a filter having a 0.5-µm or finer porosity. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Transfer 10.0 mL of the Standard preparation prepared as directed in the Assay for propoxyphene napsylate to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 0.01J mg of USP Acetaminophen RS per mL.

Assay preparation— Transfer 10.0 mL of the Assay preparation prepared as directed in the Assay for propoxyphene napsylate to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 245-nm detector and a 3.9-mm × 30-cm column containing packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the tailing factor is not more than 1.5; 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 areas for the acetaminophen peaks. Calculate the quantity, in mg, of acetaminophen (C8H9NO2) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Acetaminophen RS in the Standard preparation; D is the dilution factor used to prepare the Assay preparation in the Assay for propoxyphene napsylate; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.