Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Procedure— Determine the amount of acetaminophen (C8H9NO2) and of diphenhydramine citrate (C17H21NO·C6H8O7) dissolved, employing the procedures set forth in the Assay for acetaminophen and the Assay for diphenhydramine citrate, respectively, making any necessary volumetric adjustments.

Tolerances— Not less than 75% (Q) of the labeled amounts of C8H9NO2 and C17H21NO·C6H8O7 are dissolved in 45 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a suitable degassed and filtered mixture of water and methanol (60:40), making adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of guaifenesin in a mixture of water and methanol (4:1) to obtain a solution containing 8.0 mg per mL.

Standard preparation— Transfer about 50 mg of USP Acetaminophen RS, accurately weighed, to a 100-mL volumetric flask. Dissolve in 2.5 mL of methanol, dilute with water to volume, and mix. Transfer 2.0 mL of this solution to a 50-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 500 mg of acetaminophen, to a 100-mL volumetric flask, add 25 mL of methanol, and shake by mechanical means for 10 minutes. Dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 2.0 mL of this solution to a 50-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute. The column temperature is maintained at about 35 ± 0.5. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the column efficiency as determined from the analyte peak is not less than 1000 theoretical plates; the tailing factor for the analyte peak is not more than 2; the resolution, R, between the analyte and internal standard peaks is not less than 6.0; and the relative standard deviation of the peak response ratios for replicate injections is not more than 2.5%.

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. The relative retention times are about 0.5 for acetaminophen and 1.0 for guaifenesin. Calculate the quantity, in mg, of acetaminophen (C8H9NO2) in the portion of Tablets taken by the formula: in which WS is the weight, in mg, of USP Acetaminophen RS taken; and RU and RS are the ratios of the peak response of acetaminophen to that of the internal standard obtained from the Assay preparation and the Standard preparation, respectively.

Mobile phase— Prepare a suitable degassed and filtered mixture of methanol, water, and glacial acetic acid (61:38:1) containing 1.0813 g of sodium 1-octanesulfonate in each 1000 mL of solution, making adjustments if necessary (see System Suitability under Chromatography 621).

Solvent mixture— Prepare a mixture of methanol and water (1:1).

Internal standard solution— Prepare a solution of xylometazoline hydrochloride in water having a concentration of about 8 mg per mL.

Standard preparation— Transfer about 38 mg of USP Diphenhydramine Citrate RS, accurately weighed, to a 100-mL volumetric flask containing 500 mg of acetaminophen. Add 5.0 mL of Internal standard solution and about 50 mL of Solvent mixture, and mix until solution is complete. Dilute with Solvent mixture to volume, and mix.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 38 mg of diphenhydramine citrate, to a 100-mL volumetric flask, add about 65 mL of Solvent mixture, and shake by mechanical means for about 15 minutes. Add 5.0 mL of Internal standard solution, dilute with Solvent mixture to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 265-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. The column temperature is maintained at about 35 ± 0.5. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the column efficiency as determined from the analyte peak is not less than 1000 theoretical plates; the tailing factor for the analyte peak is not more than 1.7; the resolution, R, between the analyte and internal standard peaks is not less than 2.5; and the relative standard deviation of the peak response ratios for replicate injections is not more than 2.0%.

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 peak responses for the diphenhydramine citrate and xylometazoline hydrochloride peaks. The relative retention times are about 0.3 for acetaminophen, 0.7 for diphenhydramine citrate, and 1.0 for xylometazoline hydrochloride, respectively. Calculate the quantity, in mg, of diphenhydramine citrate (C17H21NO·C6H8O7) in the portion of Tablets taken by the formula: in which WS is the weight, in mg, of USP Diphenhydramine Citrate RS taken; and RU and RS are the ratios of the peak response of diphenhydramine citrate to that of the internal standard obtained from the Assay preparation and the Standard preparation, respectively.