Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 30minutes.

Procedure— Determine the amount of C7H8N4O2dissolved from UVabsorbances at the wavelength of maximum absorbance at about 270nm of filtered portions of the solution under test,suitably diluted with water,if necessary,in comparison with a Standard solution having a known concentration of USP Oxtriphylline RSin the same medium.

Tolerances— Not less than 80%(Q)of the labeled amount of C7H8N4O2is dissolved in 30minutes.

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).

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.

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 solution,equivalent to about 20mg of Oxtriphylline,to a 200-mLvolumetric flask,dilute with water to volume,and mix.

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 System suitability preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.7for theobromine and 1.0for theophylline,and the resolution,R,between the theobromine and theophylline peaks is not less than 3.0.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure: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.[NOTE—The major peaks recorded in the chromatograms represent the theophylline moiety of oxtriphylline.]Calculate the quantity,in mg,of C7H8N4O2in the portion of Tablets 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 solution taken for the Assay preparation;and rUand rSare the theophylline peak responses obtained from the Assay preparationand the Standard preparation,respectively.