Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

Mobile phase— Prepare as directed under Assay.

Solvent mixture— Prepare as directed under Assay.

Internal standard solution— Prepare a solution of benzoic acid in methanol having a concentration of about 1mg per mL.

Standard preparation I— Dissolve an accurately weighed quantity of USP Salicylic Acid RSin the Solvent mixtureto obtain a solution having a known concentration of about 70µg per mL.Combine 4.0mLof this solution and 1.0mLof the Internal standard solution,and mix.

Standard preparation II— Dissolve accurately weighed quantities of USP Acetaminophen RSand USP Aspirin RSin the Solvent mixtureto obtain a solution having known concentrations of about 360µg of acetaminophen and about 360µg of aspirin per mL.Combine 4.0mLof this solution and 1.0mLof the Internal standard solution,and mix.

Test preparation— Combine 4.0mLof a filtered portion of the solution under test and 1.0mLof the Internal standard solution,and mix.

Chromatographic system— Proceed as directed under Assay.

Procedure— Separately inject equal volumes (about 20µL)of the two Standard preparationsand the Test preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.3for acetaminophen,0.4for salicylic acid,0.6for aspirin,and 1.0for benzoic acid.Determine the amount of acetaminophen (C8H9NO2)dissolved by the formula: in which Cis the concentration,in µg per mL,of USP Acetaminophen RSin Standard preparation II;RUand RSare the relative peak response ratios obtained from the Test preparationand Standard preparation II,respectively;and Wis the labeled amount,in mg,of acetaminophen.Determine the amount of aspirin (C9H8O4)dissolved by the formula: in which C1and C2are the concentrations,in µg per mL,of USP Aspirin RSin Standard preparation IIand USP Salicylic Acid RSin Standard preparation I,respectively;RU1and RS1are the relative peak response ratios for the aspirin peak and the internal standard peak obtained from the Test preparationand Standard preparation II,respectively;RU2and RS2are the relative peak response ratios for the salicylic acid peak and the internal standard peak obtained from the Test preparationand Standard preparation I,respectively;and Wis the labeled amount,in mg,of aspirin.

Tolerances— Not less than 75%(Q)of the labeled amounts of C8H9NO2and C9H8O4is dissolved in 45minutes.

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

Procedure— Dissolve a suitable quantity of USP Salicylic Acid RS,accurately weighed,in Solvent mixtureto obtain a solution having a known concentration of about 1.0mg per mL.Transfer 1.0-mL,5.0-mL,and 10.0-mLportions,respectively,of this solution to separate 100-mLvolumetric flasks,add 10.0mLof Internal standard solutionto each flask,dilute with Solvent mixtureto volume,and mix.Chromatograph these three Standard solutions as directed in the Assay.Plot the ratios of the peak responses for salicylic acid and benzoic acid for each of the Standard solutions versus concentrations,in mg per mL,of salicylic acid,and draw the straight line best fitting the three plotted points.From the graph so obtained,and from the ratio of the peak responses for salicylic acid and benzoic acid in the chromatogram of the Assay preparationas obtained in the Assay,determine the concentration,in mg per mL,of salicylic acid (C7H6O3)in the Assay preparation,and calculate the percentage of salicylic acid in relation to the concentration of aspirin in the Assay preparation,as determined in the Assay.Not more than 3.0%is found.

Solvent mixture— Prepare a mixture of chloroform,methanol,and glacial acetic acid (78:20:2).

Mobile phase— Transfer 225mg of tetramethylammonium hydroxide pentahydrate to a 1000-mLflask,and add 750mLof water,125mLof methanol,125mLof acetonitrile,and 1.0mLof glacial acetic acid.Stir for 3minutes,pass through a membrane filter having a 0.5-µm or finer porosity,and degas.

Internal standard solution— Dissolve benzoic acid in Solvent mixtureto obtain a solution having a concentration of about 20mg per mL.

Standard preparation— Transfer about 325mg of USP Acetaminophen RSand about 325mg of USP Aspirin RS,each accurately weighed,to a 100-mLvolumetric flask,add 10.0mLof Internal standard solution,dilute with Solvent mixtureto volume,and mix.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 325mg of acetaminophen,to a 100-mLvolumetric flask,add 10.0mLof Internal standard solutionand about 50mLof Solvent mixture,and sonicate for about 3minutes.Dilute with Solvent mixtureto volume,and mix.Pass a portion of this solution through a filter having a 2.5-µm or finer porosity,and use the filtrate as the Assay preparation.

Chromatographic system— The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph four replicate injections of the Standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation for either analyte is not more than 3.0%.

Procedure— Separately inject equal volumes (about 5µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The retention times are about 2,3,5,and 8minutes for acetaminophen,salicylic acid (if present),aspirin,and benzoic acid,respectively.Calculate the quantity,in mg,of acetaminophen (C8H9NO2)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Acetaminophen RSin the Standard preparation;and RUand RSare the ratios of the peak responses of acetaminophen and benzoic acid obtained from the Assay preparationand the Standard preparation,respectively.Calculate the quantity,in mg,of aspirin (C9H8O4)in the portion of Tablets taken by the same formula,except to read “USP Aspirin RS”where “USP Acetaminophen RS”is specified,and “aspirin”where “acetaminophen”is specified.