Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

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

Tolerances— Not less than 75%(Q)of the labeled amounts of C8H9NO2and C17H21NO·C6H8O7are dissolved in 45minutes.

Mobile phase— Prepare a suitable degassed and filtered mixture of water and methanol (60:40),making adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

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

Chromatographic system(see Chromatography á621ñ)— 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 1mLper 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 1000theoretical 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 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 0.5for acetaminophen and 1.0for guaifenesin.Calculate the quantity,in mg,of acetaminophen (C8H9NO2)in the portion of Tablets taken by the formula: in which WSis the weight,in mg,of USP Acetaminophen RStaken;and RUand RSare the ratios of the peak response of acetaminophen to that of the internal standard obtained from the Assay preparationand the Standard preparation,respectively.

Mobile phase— Prepare a suitable degassed and filtered mixture of methanol,water,and glacial acetic acid (61:38:1)containing 1.0813g of sodium 1-octanesulfonate in each 1000mLof solution,making adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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 8mg per mL.

Standard preparation— Transfer about 38mg of USP Diphenhydramine Citrate RS,accurately weighed,to a 100-mLvolumetric flask containing 500mg of acetaminophen.Add 5.0mLof Internal standard solutionand about 50mLof Solvent mixture,and mix until solution is complete.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 38mg of diphenhydramine citrate,to a 100-mLvolumetric flask,add about 65mLof Solvent mixture,and shake by mechanical means for about 15minutes.Add 5.0mLof Internal standard solution,dilute with Solvent mixtureto volume,and mix.

Chromatographic system(see Chromatography á621ñ)— 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 1.5mLper 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 1000theoretical 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 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the peak responses for the diphenhydramine citrate and xylometazoline hydrochloride peaks.The relative retention times are about 0.3for acetaminophen,0.7for diphenhydramine citrate,and 1.0for xylometazoline hydrochloride,respectively.Calculate the quantity,in mg,of diphenhydramine citrate (C17H21NO·C6H8O7)in the portion of Tablets taken by the formula: in which WSis the weight,in mg,of USP Diphenhydramine Citrate RStaken;and RUand RSare the ratios of the peak response of diphenhydramine citrate to that of the internal standard obtained from the Assay preparationand the Standard preparation,respectively.