A: The retention time exhibited by theophylline in the chromatogram of the Assay preparationcorresponds to that of theophylline in the chromatogram of the Standard preparation,as obtained in the Assay.

B: Transfer a quantity of finely ground Tablets,equivalent to about 100mg of oxtriphylline,to a suitable test tube,add 10mLof methanol,shake on a vortex mixer for several minutes,and filter to obtain the test solution.Apply 10µLof the test solution and 10µLof a Standard solution of USP Oxtriphylline RSin methanol containing 10mg per mLto a suitable thin-layer chromatographic plate (see Chromatography á621ñ)coated with a 0.25-mm layer of chromatographic silica gel mixture.Allow the applications to dry,and develop the chromatogram in a solvent system consisting of a mixture of chloroform,alcohol,and formic acid (88:10:2)until the solvent front has moved about three-fourths of the length of the plate.Remove the plate from the developing chamber,mark the solvent front,and allow the solvent to evaporate.Observe the plate under short-wavelength UVlight:the principal spot obtained from the test solution corresponds in color,size,and RFvalue to that obtained from the Standard solution.

Mobile phase— Dissolve 6.8g of monobasic potassium phosphate in water to make 1000mL,and adjust with 0.1Npotassium hydroxide to a pHof 5.8±0.1.Prepare a filtered and degassed mixture of this solution and methanol (4:1).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Oxtriphylline RSin water,and dilute quantitatively,and stepwise if necessary,with water to obtain a solution having a known concentration of about 0.1mg per mL.

Assay preparation— Place 10Tablets in a 1000-mLvolumetric flask,and add about 700mLof water.Heat on a steam bath,with occasional shaking,until the Tablets have disintegrated.Cool to room temperature,dilute with water to volume,mix,and filter.Transfer an accurately measured volume of this specimen solution,equivalent to about 20mg of Oxtriphylline,to a 200-mLvolumetric flask,dilute with water to volume,and mix.

System suitability preparation— Dissolve suitable quantities of USP Oxtriphylline RSand theobromine in water to obtain a solution containing about 0.6mg and 0.3mg per mL,respectively.Dilute this solution quantitatively,and stepwise if necessary,with water to obtain a solution containing about 60µg of USP Oxtriphylline RSper mLand about 30µg of theobromine per mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 275-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparationand the System suitability preparation,and record the peak responses as directed for Procedure:the resolution,R,between the theobromine and theophylline peaks is not less than 3.0,and the relative standard deviation for replicate injections of the Standard preparationis not more than 2.0%.The relative retention times are about 0.7for theobromine and 1.0for theophylline.

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.[NOTE—The major peaks recorded in the chromatograms represent the theophylline moiety of oxtriphylline.]Calculate the quantity,in mg,of C7H8N4O2per Tablet taken by the formula: in which 180.17and 283.33are the molecular weights of anhydrous theophylline and oxtriphylline,respectively,Cis the concentration,in µg per mL,of USP Oxtriphylline RSin the Standard preparation,Vis the volume,in mL,of specimen solution taken for the Assay preparation,and rUand rSare the theophylline peak responses obtained from the Assay preparationand the Standard preparation,respectively.