A: Infrared Absorption á197Kñ.

B: Ultraviolet Absorption á197Uñ—

Perchloric acid solution and Mobile phase— Prepare as directed in the Assay.

System suitability solution— Dissolve accurately weighed quantities of caffeine and USP Pentoxifylline RSin Mobile phaseto obtain a solution containing about 0.0007mg per mLand 0.35mg per mL,respectively.

Standard solution— Dissolve an accurately weighed quantity of USP Pentoxifylline RSin Mobile phaseto obtain a solution having a known concentration of about 0.0007mg per mL.

Test solution— Dissolve an accurately weighed quantity of Pentoxifylline in Mobile phaseto obtain a solution having a concentration of about 0.35mg per mL.

Chromatographic system (see Chromatography á621ñ)— Proceed as directed in the Assay.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the resolution,R,between caffeine and pentoxifylline is not less than 10.0.Chromatograph the Standard solution,and record the peak responses for pentoxifylline as directed for Procedure:the relative standard deviation for replicate injections is not more than 5.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,and allow the chromatogram to run five times longer than the retention time for the pentoxifylline.Measure the areas of all the peaks in the Test solution,except that for pentoxifylline.Calculate the percentage of each impurity in the portion of Pentoxifylline taken by the formula: in which Cis the concentration,in mg per mL,of USP Pentoxifylline RSin the Standard solution;riis the peak area response for each impurity obtained from the Test solution;and rSis the peak area response for pentoxifylline obtained from the Standard solution:not more than 0.2%of any individual impurity is found,and not more than 0.5%of total impurities is found.

Perchloric acid solution— Dissolve 1.0g of perchloric acid in 1000mLof water,and mix.

Mobile phase— Prepare a filtered and degassed mixture of Perchloric acid solution,acetonitrile,tetrahydrofuran,and methanol (80:15:2.5:2).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

System suitability solution— Dissolve suitable quantities of caffeine and USP Pentoxifylline RSin Mobile phaseto obtain a solution containing 0.024mg per mLand 0.048mg per mL,respectively.

Standard preparation— Dissolve an accurately weighed quantity of USP Pentoxifylline RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.05mg per mL.

Assay preparation— Transfer about 25mg of Pentoxifylline,accurately weighed,to a 100-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.Pipet 5.0mLof the solution so obtained to a 25-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 273-nm detector and a 4.6-mm ×25-cm column that contains 5-µm packing L1.The flow rate is about 0.7mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the resolution,R,between caffeine and pentoxifylline is not less than 10.0.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 pentoxifylline peaks.Calculate the quantity,in mg,of C13H18N4O3in the portion of Pentoxifylline taken by the formula: in which Cis the concentration,in mg per mL,of USP Pentoxifylline RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.