A: Place an amount of the finely powdered contents of Capsules, equivalent to about 65 mg of propoxyphene hydrochloride, in a test tube, add 5 mL of methanol, shake for 5 minutes, and centrifuge. The clear supernatant is the test solution. Dissolve weighed amounts of USP Propoxyphene Hydrochloride RS, USP Aspirin RS, and USP Caffeine RS corresponding, proportionately, to the amounts of propoxyphene hydrochloride, aspirin, and caffeine in the Capsules to obtain a Standard solution having a known concentration of about 13 mg of propoxyphene hydrochloride per mL. Apply 10 µL each of the test solution and the Standard solution to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Develop the chromatograms in a solvent system consisting of a mixture of chloroform, butyl acetate, and formic acid (30:20:10) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, allow it to dry in a fume hood, and examine the plate under short-wavelength UV light: the chromatogram of the test solution exhibits 3 principal spots that correspond in RF value and intensity with those obtained from the Standard solution.

B: The Assay preparation prepared as directed in the Assay for propoxyphene hydrochloride and caffeine is dextrorotatory.

Medium: pH 4.5 acetate buffer; 500 mL.

Apparatus 1: 100 rpm.

Time: 60 minutes.

pH 4.5 acetate buffer— Dissolve 2.99 g of sodium acetate trihydrate in 200 mL of water, add 1.66 mL of glacial acetic acid, dilute with water to 1000 mL, and mix.

Determination of dissolved aspirin— Transfer 5.0 mL of a filtered portion of the solution under test to a 25-mL volumetric flask. Concurrently pipet 5 mL of a standard solution, prepared by dissolving 35 mg of accurately weighed USP Aspirin RS in 50.0 mL of pH 4.5 acetate buffer, into a second 25-mL volumetric flask. Proceed as directed for Procedure in the Assay for aspirin, beginning with “Into each flask pipet 5 mL of Sodium hydroxide reagent.” Concomitantly determine the absorbances of both solutions against a similarly treated blank of 5.0 mL of pH 4.5 acetate buffer. Determine the amount of aspirin (C9H8O4) in solution by comparison with the Standard solution.

Determination of dissolved propoxyphene hydrochloride—

INTERNAL STANDARD SOLUTION —Dissolve n-tricosane in chloroform to obtain a solution containing about 0.06 mg per mL.

STANDARD PREPARATION —Dissolve an accurately weighed quantity of USP Propoxyphene Hydrochloride RS in pH 4.5 acetate buffer to obtain a solution having a known concentration (between 0.06 and 0.13 mg per mL) that corresponds to the concentration of propoxyphene hydrochloride that is estimated for the solution under test, based on the labeled content of the Capsules and the extent of dissolution.

PROCEDURE —Transfer equal, accurately measured, volumes (between 5.0 and 10.0 mL) of filtered portions of the solution under test and the Standard preparation into separate, screw-capped centrifuge tubes. To each tube add 5.0 mL of sodium carbonate solution (1 in 5), and shake. Extract with one 5.0-mL portion of Internal standard solution, and one 5.0-mL portion of chloroform, and shake each extract for 5 minutes. Pass the organic layers through phase-separating paper. Evaporate to about 1 mL, and inject a 10-µL portion into a suitable gas chromatograph equipped with a flame-ionization detector. Proceed as directed for Procedure in the Assay for propoxyphene hydrochloride and caffeine, beginning with “The column is typically 60 cm × 4 mm.” Determine the amount of propoxyphene hydrochloride (C22H29NO2·HCl) in solution by comparison with the Standard preparation.

Tolerances— Not less than 75% (Q) of the labeled amount of C9H8O4 is dissolved in 60 minutes, and not less than 85% (Q) of the labeled amount of C22H29NO2·HCl is dissolved in 60 minutes.

Ferric chloride-urea reagent— Dissolve by swirling, without the aid of heat, 60 g of urea in a mixture of 8 mL of ferric chloride solution (6 in 10) and 42 mL of 0.05 N hydrochloric acid. Adjust this solution, if necessary, with 6 N hydrochloric acid to a pH of 3.2.

Standard preparation— Transfer 15.0 mg of USP Salicylic Acid RS, accurately weighed, to a 100-mL volumetric flask, add chloroform to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask containing 20 mL of methanol, 4 drops of hydrochloric acid, and 20 mL of a 1 in 10 solution of glacial acetic acid in ether, dilute with chloroform to volume, and mix.

Test preparation— Pack a pledget of glass wool in the base of a 25- × 200-mm chromatographic tube. In a beaker, prepare a mixture of 6 g of chromatographic siliceous earth, 2 mL of freshly prepared Ferric chloride-urea reagent, and 40 mL of chloroform. Transfer the mixture to the chromatographic tube. Rinse the beaker with 15 mL of chloroform, transfer to the column, and pack tightly. Place a small amount of glass wool at the top of the column. Weigh accurately a quantity of the finely powdered contents of Capsules, equivalent to about 50 mg of aspirin, mix with 10 mL of chloroform by stirring for 3 minutes, and transfer to the chromatographic column with the aid of 10 mL of chloroform. Pass 40 mL of chloroform through the column, rinse the tip of the chromatographic tube with chloroform, and discard the eluate. Prepare as a receiver a 100-mL volumetric flask containing 20 mL of methanol and 4 drops of hydrochloric acid, and elute any salicylic acid from the column with 20 mL of a 1 in 10 solution of glacial acetic acid in ether that recently has been saturated with water, followed by 30 mL of chloroform. Dilute the eluate with chloroform to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Test preparation in 1-cm cells at the wavelength of maximum absorbance at about 306 nm, with a suitable spectrophotometer, using as the blank a solvent mixture of the same composition as that used for the Standard preparation: the absorbance of the Test preparation does not exceed that of the Standard preparation (3.0%, calculated on the basis of the labeled content of aspirin).

(Official January 1, 2007)

Internal standard solution— Dissolve n-tricosane in chloroform to obtain a solution having a concentration of about 0.6 mg per mL.

Standard preparation— Transfer to a 50-mL volumetric flask accurately weighed quantities of about 32 mg of USP Propoxyphene Hydrochloride RS, about 32J mg of USP Aspirin RS, and about 32J¢ mg of USP Caffeine RS, where J is the ratio of the labeled amount, in mg, of aspirin to the labeled amount, in mg, of propoxyphene hydrochloride per Capsule, and J¢ is the ratio of the labeled amount, in mg, of caffeine to the labeled amount, in mg, of propoxyphene hydrochloride per Capsule. Add 10 mL of acetone, and swirl to dissolve the reference standards completely. Dilute with water to volume, and mix.

Assay preparation— Remove as completely as possible the contents of 20 Capsules, and transfer an accurately weighed portion of the powder, equivalent to 65 mg of propoxyphene hydrochloride, to a 100-mL volumetric flask containing 20 mL of acetone. If the Capsules contain a pellet (propoxyphene hydrochloride) as well as a powder, finely grind the pellets, then mix with the powder before proceeding. Sonicate for about 1 minute. Dilute the milky solution with water to volume, and mix. Filter, discarding the first 20 mL of the filtrate.

Procedure for propoxyphene hydrochloride and caffeine— Transfer 5.0-mL aliquots of the Assay preparation and the Standard preparation to separate 60-mL separators. To each add 5.0 mL of sodium carbonate solution (1 in 5) and 5.0 mL of Internal standard solution. Shake vigorously for 5 minutes, and allow the layers to separate. Drain the chloroform layer through phase-separating paper, suitably supported in a funnel, into a screw-capped test tube. Extract with one 5-mL portion of chloroform, and drain the chloroform layer through phase-separating paper. Evaporate the combined chloroform extracts, using a stream of dry nitrogen, to a final volume of about 2 mL. Inject separately a suitable volume, equivalent to about 6.4 µg of propoxyphene, of the chloroform extracts from the Assay preparation and the Standard preparation into a suitable gas chromatograph equipped with a flame-ionization detector. The column is typically 60 cm × 3 mm and is packed with 3% methyl phenyl silicone, liquid phase on 80- to 100-mesh chromatographic siliceous earth. The temperature of the injection port is 200, the column temperature is 175, and the carrier gas, nitrogen, has a flow rate of about 60 mL per minute. Relative retention times are about 0.65 for caffeine, 1.0 for the internal standard, and 1.7 for propoxyphene. In a suitable chromatogram, the resolution factor is not less than 1.0 between any two peaks, the relative standard deviation for five replicate injections of the Standard preparation is not more than 2.0, and the tailing factor for caffeine is not greater than 1.5. Calculate the quantities, in mg, of propoxyphene hydrochloride (C22H29NO2·HCl) and caffeine (C8H10N4O2), respectively, in the portion taken for the Assay preparation by the same formula: in which C is the concentration, in mg per mL, of the appropriate USP Reference Standard in the Standard preparation, and RU and RS are the ratios of the peak areas of the corresponding analyte to those of the internal standard obtained from the Assay preparation and the Standard preparation, respectively.

Sodium hydroxide reagent— Dissolve 1 g of polyoxyethylene (23) lauryl ether in about 100 mL of hot water contained in a 1000-mL volumetric flask. Dilute with water to about 600 mL, and dissolve 10 g of sodium hydroxide in this solution. Dilute with water to volume, and mix.

Ferric nitrate reagent— Mix 70 mL of nitric acid with about 600 mL of water contained in a 1000-mL volumetric flask. Dissolve 40 g of ferric nitrate [Fe(NO3)3·9H2O] in this solution, dilute with water to volume, and mix.

Standard preparation and Assay preparation—Prepare as directed in the Assay for propoxyphene hydrochloride and caffeine to obtain solutions having concentrations of about 4 mg of aspirin per mL.

Procedure— Into separate 25-mL volumetric flasks pipet 2 mL each of the Standard preparation and the Assay preparation, and 2 mL of dilute acetone (1 in 5) to provide the blank. Into each flask pipet 5 mL of Sodium hydroxide reagent, mix by gentle swirling, and allow to stand at room temperature for 8 minutes. Dilute with Ferric nitrate reagent to volume, and mix. Concomitantly determine the absorbances of both solutions against the blank in 1-cm cells at the wavelength of maximum absorbance at about 530 nm, taking care to allow the solutions to reach an equilibrium temperature in the cell compartment. The color intensity is temperature-dependent. Calculate the quantity, in mg, of aspirin (C9H8O4) in the portion taken for the Assay preparation by the formula: in which C is the concentration, in mg per mL, of USP Aspirin RS in the Standard preparation, and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.