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Mobile phase— Prepare a filtered and degassed mixture of water,dimethylformamide,glacial acetic acid,and 11.7Npotassium hydroxide (1834:160:4:2).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).Increase the proportion of dimethylformamide to decrease the retention time of cephapirin.

Resolution solution— Prepare a solution of Cephapirin Sodium in pH2.0hydrochloric acid buffer (see Buffer Solutionsin the section Reagents,Indicators,and Solutions)containing about 1mg per mL.Place 10mLof this solution in a test tube,and heat at 95for 10minutes,accurately timed.Promptly cool the tube in an ice water bath.Dilute 5mLof the cooled solution with Mobile phaseto obtain 50mLof Resolution solution.

Standard preparation— Transfer about 21mg of USP Cephapirin Sodium RS,accurately weighed,to a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.This solution contains about 0.2mg of cephapirin per mL.USP28

Assay preparation 1(where it is packaged for dispensing and is represented as being in a single-dose container)— Constitute Cephapirin for Injection in a volume of water,accurately measured,corresponding to the volume of solvent specified in the labeling.Withdraw all of the withdrawable contents,using a suitable hypodermic needle and syringe,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution containing the equivalent of about 0.2mg of cephapirin per mL.

Assay preparation 2(where the label states the quantity of cephapirin in a given volume of constituted solution)— Constitute Cephapirin for Injection in a volume of water,accurately measured,corresponding to the volume of solvent specified in the labeling.Dilute an accurately measured volume of the constituted solution quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution containing the equivalent of about 0.2mg of cephapirin per mL.[NOTE—Use the Standard preparation and the Assay preparation within 1hour.]

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 4-mm ×30-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the resolution,R,between the cephapirin peak and the peak having a retention time of about 0.9relative to that of cephapirin is not less than 0.9.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.9for cephapirin lactone and 1.0for cephapirin;the column efficiency determined from the cephapirin peak is not less than 1200theoretical plates;the tailing factor for the cephapirin peak is not more than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.USP28

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the appropriate Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of cephapirin (C17H17N3O6S2)withdrawn from the container,or in the portion of constituted solution taken by the formula: in which Lis the labeled quantity,in mg,of cephapirin in the single-dose container,or in the volume of constituted solution taken;Dis the concentration,in mg per mL,of cephapirin in Assay preparation 1or in Assay preparation 2,on the basis of the labeled quantity in the container,or in the portion of constituted solution taken,respectively,and the extent of dilution;Cis the concentration,in mg per mL,of USP Cephapirin Sodium RSin the Standard preparation;Pis the potency,in µg of cephapirin per mg,of USP Cephapirin Sodium RS;and rUand rSare the cephapirin peak responses obtained from the Assay preparationand the Standard preparation,respectively.