Medium: water; 900 mL.

Apparatus 2: 100 rpm.

Time: 60 minutes.

Mobile phase, Internal standard solution, Solvent mixture, Standard stock solution, and Chromatographic system— Prepare as directed in the Assay.

Standard solution— Transfer 20.0 mL of Standard stock solution, 3.0 mL of Internal standard solution, and 20 mL of water to a 50-mL volumetric flask, mix, and allow to stand for about 30 seconds. Dilute with Solvent mixture to volume, and mix. Use within 8 hours.

Test solution— Transfer an aliquot of a filtered portion of the solution under test to a 50-mL volumetric flask in order to obtain an expected concentration of about 0.1 mg per mL of acetaminophen and 0.1J mg per mL of caffeine, where J is defined for the Standard stock solution. Add 3.0 mL of Internal standard solution and 20 mL of Solvent mixture, mix, and allow to stand for 30 seconds. Dilute with Solvent mixture to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay, except to inject the Standard solution and the Test solution. Calculate the quantities, in mg, of acetaminophen (C8H9NO2) and caffeine (C8H10N4O2) dissolved by the formula: in which Vd is the volume, in mL, of Test solution that is transferred to the volumetric flask; C is the concentration, in mg per mL, of the appropriate USP Reference Standard in the Standard solution; and RU and RS are the ratios of the peak responses of the corresponding analyte and internal standard peaks obtained from the Test solution and the Standard solution, respectively.

Tolerances— Not less than 75% (Q) of the labeled amounts of acetaminophen (C8H9NO2) and caffeine (C8H10N4O2) is dissolved in 60 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a suitable mixture of water, methanol, and glacial acetic acid (69:28:3). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of benzoic acid in methanol containing about 6 mg per mL.

Solvent mixture— Prepare a mixture of methanol and glacial acetic acid (95:5).

Standard stock solution— Dissolve accurately weighed quantities of USP Acetaminophen RS and USP Caffeine RS in Solvent mixture to obtain a solution having known concentrations of about 0.25 mg of USP Acetaminophen RS per mL and 0.25J mg of USP Caffeine RS per mL, J being the ratio of the labeled amount, in mg, of caffeine to the labeled amount, in mg, of acetaminophen per Tablet.

Standard preparation— Transfer 20.0 mL of Standard stock solution and 3.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with Solvent mixture to volume, and mix. This solution contains about 0.1 mg of USP Acetaminophen RS and 0.1J mg of USP Caffeine RS per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Acetaminophen and Caffeine Tablets. Transfer an accurately weighed quantity of the well-mixed powder, equivalent to about 250 mg of acetaminophen, to a 100-mL volumetric flask. Add about 75 mL of Solvent mixture, and shake by mechanical means for 30 minutes. Dilute with Solvent mixture to volume, and mix. Transfer 2.0 mL of this solution and 3.0 mL of Internal standard solution to a 50-mL volumetric flask, dilute with Solvent mixture to volume, and mix.

Chromatographic system— The liquid chromatograph is equipped with a 275-nm detector and a 4.6-mm × 10-cm column that contains 5-µm packing L1, and is maintained at 45 ± 1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the tailing factor for each analyte peak is not more than 1.2; the resolution, R, between any of the analyte and internal standard peaks is not less than 1.4; and 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. The relative retention times are about 0.3 for acetaminophen, 0.5 for caffeine, and 1.0 for benzoic acid. Calculate the quantities, in mg, of acetaminophen (C8H9NO2) and caffeine (C8H10N4O2) in the portion of Tablets taken by the 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 responses of the corresponding analyte and internal standard peaks obtained from the Assay preparation and the Standard preparation, respectively.