A:Infrared Absorption á197Kñ.

B: 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.

C: It meets the requirements of the tests for Sodium á191ñ.

Buffer solution,Mobile phase,andChromatographic system— Proceed as directed in the Assay.

Standard solution— Dissolve accurately weighed quantities of USP Phenytoin Related Compound A RS,USP Phenytoin Related Compound B RS,and USP Phenytoin RSin Mobile phase;and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having known concentrations of about 3.0µg per mL,3.0µg per mL,and 1.5µg per mL,respectively.

Test solution— Transfer about 150mg of Fosphenytoin Sodium,accurately weighed,to a 50-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms for not less than six times the retention time of the major peak,and measure all the peak responses.Calculate the percentage of phenytoin,phenytoin related compound B,and phenytoin related compound A,if present,in the portion of Fosphenytoin Sodium taken by the formula: in which CSis the concentration,in mg per mL,of the USP Reference Standard of the respective impurity in the Standard solution;CUis the concentration,in mg per mL,of Fosphenytoin Sodium in the Test solution;and riand rSare the peak responses for each impurity obtained from the Test solutionand the Standard solution,respectively:not more than 0.1%of phenytoin is found;not more than 0.1%of any other impurity is found;and not more than 0.5%of total impurities is found.[NOTE—Use the peak area and concentration of USP Phenytoin RSin the Standard solutionas rSand CS,respectively,to calculate the percentage of the unknown impurities.]

Buffer solution— Dissolve 6.80g of monobasic potassium phosphate and 30mLof 0.5Mdodecyltriethylammonium phosphate in 900mLof water,adjust with 1.5Mphosphoric acid to a pHof about 5.0,dilute with water to 1000mL,mix,and filter.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solutionand acetonitrile (13:7).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Fosphenytoin Sodium RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.15mg per mL.

Assay preparation— Transfer about 150mg of Fosphenytoin Sodium,accurately weighed,to a 50-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 214-nm detector and a 3.9-mm ×15-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 0.5for phenytoin and 1.0for fosphenytoin sodium;the column efficiency determined from fosphenytoin sodium is not less than 5000theoretical plates;the tailing factor for the fosphenytoin sodium peak is not more than 1.6;and the relative standard deviation for replicate injections is not more than 0.5%.

Procedure— Separately inject equal volumes (about 20µL)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 C16H13N2Na2O6Pin the portion of Fosphenytoin Sodium taken by the formula: in which Cis the concentration,in mg per mL,of USP Fosphenytoin Sodium RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.