Medium: water;900mL.

Apparatus 2: 100rpm.

Time: 60minutes.

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

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

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

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

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

Mobile phase— Prepare a suitable mixture of water,methanol,and glacial acetic acid (69:28:3).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Prepare a solution of benzoic acid in methanol containing about 6mg 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 RSand USP Caffeine RSin Solvent mixtureto obtain a solution having known concentrations of about 0.25mg of USP Acetaminophen RSper mLand 0.25Jmg of USP Caffeine RSper mL,Jbeing the ratio of the labeled amount,in mg,of caffeine to the labeled amount,in mg,of acetaminophen per Tablet.

Standard preparation— Transfer 20.0mLof Standard stock solutionand 3.0mLof Internal standard solutionto a 50-mLvolumetric flask,dilute with Solvent mixtureto volume,and mix.This solution contains about 0.1mg of USP Acetaminophen RSand 0.1Jmg of USP Caffeine RSper mL.

Assay preparation— Weigh and finely powder not fewer than 20Acetaminophen and Caffeine Tablets.Transfer an accurately weighed quantity of the well-mixed powder,equivalent to about 250mg of acetaminophen,to a 100-mLvolumetric flask.Add about 75mLof Solvent mixture,and shake by mechanical means for 30minutes.Dilute with Solvent mixtureto volume,and mix.Transfer 2.0mLof this solution and 3.0mLof Internal standard solutionto a 50-mLvolumetric flask,dilute with Solvent mixtureto 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 2mLper 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 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.3for acetaminophen,0.5for caffeine,and 1.0for benzoic acid.Calculate the quantities,in mg,of acetaminophen (C8H9NO2)and caffeine (C8H10N4O2)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of the appropriate USP Reference Standard in the Standard preparation;and RUand RSare the ratios of the peak responses of the corresponding analyte and internal standard peaks obtained from the Assay preparationand the Standard preparation,respectively.