A: Infrared Absorption á197Kñ.

B: Ultraviolet Absorption á197Uñ—

Mobile phase andSystem suitability solution— Proceed as directed in the Assay.

Test preparation— Use the Assay preparation.

Chromatographic system— Proceed as directed in the Assayexcept to use a 225-nm detector.

Procedure— Inject a volume (about 10µL)of the Test preparationinto the chromatograph,record the chromatogram,and measure the peak responses.Calculate the percentage of each impurity in the portion of Mephenytoin taken by the formula: in which Fis the relative response factor and is equal to 1.16for any peak with a relative retention time of 0.66,0.37for propiophenone,and 1.0for all other peaks;riis the peak response for each impurity;and rsis the sum of the responses of all of the peaks,adjusted for the relative response factor:not more than 1.0%of any individual impurity is found,and not more than 1.5%of total impurities is found.

Solvent: dimethyl sulfoxide.

Mobile phase— Prepare a filtered and degassed mixture of water,methanol,and acetonitrile (52:38:10).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

System suitability solution— Transfer about 7.5mg of propiophenone,accurately weighed,to a 50-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.Transfer 1.0mLof this solution to a 10-mLvolumetric flask,add about 15mg of Mephenytoin,dissolve in Mobile phase,sonicate,dilute with Mobile phaseto volume,and mix.

Standard preparation— Dissolve an accurately weighed quantity of USP Mephenytoin RSin Mobile phase,sonicate,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 5.0mg per mL.

Assay preparation— Transfer about 125.0mg of Mephenytoin,accurately weighed,to a 25-mLvolumetric flask,dissolve in Mobile phase,sonicate,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 257-nm detector and a 3.9-mm ×15-cm column that contains packing L7.The flow rate is about 1mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 1.0for mephenytoin and 1.5for propiophenone;the column efficiency is not less than 4000theoretical plates for the mephenytoin peak;and the relative standard deviation for replicate injections for the mephenytoin peak 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.Calculate the quantity,in mg,of C12H14N2O2in the portion of Mephenytoin taken by the formula: in which Cis the concentration,in mg per mL,of USP Mephenytoin RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.