A: The retention time of the major peak in the chromatogram of the Assay preparationcorresponds to that in the chromatogram of the Standard preparation,as obtained in the Assay.

B: It meets the requirements of the test for Citrate á191ñ.

C: Transfer about 4g of potassium iodide to a 100-mLvolumetric flask.Add 10mLof water,and shake until the potassium iodide is dissolved.Transfer 2g of iodine to the volumetric flask,and shake until dissolved.Dilute with water to volume,and mix.Transfer 5drops of the solution so obtained to a 25-mLcentrifuge tube containing 5.0mLof the Oral Solution,and mix.Add 0.5mLof 2.0Mhydrochloric acid solution,and mix:a brown precipitate is produced that dissolves on neutralization with 0.5mLof sodium hydroxide TS.

Mobile phase andTheophylline solution— Proceed as directed in the Assay.

Standard solution— Use the Standard preparation,prepared as directed in the Assay.

System sensitivity solution— Transfer 2.5mLof the Standard solutionto a 100-mLvolumetric flask,dilute with water to volume,and mix.

Test solution— Use the Assay preparation,prepared as directed in the Assay.

Chromatographic system(seeChromatography á621ñ)— Proceed as directed in the Assay.Chromatograph the System sensitivity solution,and record the peak responses as directed for Procedure:the theophylline peak produces a discernible peak response at its retention time.

Procedure— Separately inject equal volumes (about 10µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the percentage of any related compound in the portion of Oral Solution taken by the formula: in which Fis the relative response factor and is equal to 0.878for theobromine at a relative retention time of about 0.4,equal to 1.10for paraxanthine at a relative retention time of about 0.6,equal to 0.905for theophylline at a relative retention time of about 0.7,and equal to 1.0for any other related compound;CSis the concentration,in mg per mL,of USP Caffeine RSin the Standard solution;CWis the caffeine concentration,in mg per mL,in the Test solution,as obtained in the Assay;riis the individual peak response for each related compound obtained from the Test solution;and rSis the caffeine peak response obtained from the Standard solution:not more than 0.10%of any individual related compound is found;and not more than 0.1%of total impurities is found.

Mobile phase— Prepare a mixture of 0.01Msodium acetate,acetonitrile,and tetrahydrofuran (191:5:4).Adjust with glacial acetic acid to a pHof 4.5,filter,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Theophylline solution— Dissolve an accurately weighed quantity of theophylline in water,and dilute quantitatively,and stepwise if necessary,with water,to obtain a solution having a concentration of about 0.02mg per mL.

Standard preparation— Transfer about 5mg of USP Caffeine RS,accurately weighed,to a 25-mLvolumetric flask.Add 5mLof the Theophylline solution,dissolve in and dilute with water to volume,and mix.

Assay preparation— Transfer an accurately measured volume of Oral Solution,equivalent to about 50mg of caffeine,to a 250-mLvolumetric flask.Dilute with water to volume,mix,and filter through a polyvinylidene difluoride or equivalent membrane having a porosity of 0.45µm.

Chromatographic system (seeChromatography á621ñ)— The liquid chromatograph is equipped with a 275-nm detector and a 4.6-mm ×150-cm column that contains 5-µm packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.7for theophylline and 1.0for caffeine;the resolution,R,between theophylline and caffeine is not less than 6.0;the tailing factor,determined from the theophylline and caffeine peaks,is not more than 2.0;and the relative standard deviation for replicate injections,determined from the caffeine peaks,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 caffeine peak responses.Calculate the quantity,in mg,of caffeine (C8H10N4O2)in the volume of Oral Solution taken by the formula: in which Cis the concentration,in mg per mL,of USP Caffeine RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.USP28