A: The retention time of the major peak in the chromatogram of theAssay preparation corresponds to that in the chromatogram of theStandard preparation,as obtained in theAssay.

B: Afew drops of Injection respond to the test for Citrate á191ñ.

Change to read:

0.05M Ammonium phosphate buffer,Mobile phase,System sensitivity solution,and Chromatographic system— Prepare as directed in theAssay.

Standard solution— Use theStandard preparation,prepared as directed in theAssay.

Test solution— Use theAssay preparation,prepared as directed in theAssay.

Procedure— Separately inject equal volumes (about 20µL)of theTest solution and theStandard solution into the chromatograph,record the chromatogram for at least 2.5times the retention time of orphenadrine citrate,and measure all of the peak areas.Calculate the percentage of each impurity in the portion of Injection taken by the formula: in which Fis the relative response factor as found in the table below;Cis the concentration,in mg per mL,of USP Orphenadrine Citrate RSin the Standard solution;Vis the volume,in mL,of Injection taken to prepare the Test solution;Dis the labeled dose of Injection;riis the peak area for each impurity in the Test solution;and rSis the peak area of Orphenadrine Citrate in the Standard solution:not more than 4.0%of total impurities is found. USP28

0.05M Ammonium phosphate buffer— Dissolve 5.8g of monobasic ammonium phosphate in 1000mLof water,and adjust with ammonium hydroxide or phosphoric acid to a pHof 7.9±0.05.

Mobile phase— Prepare a filtered and degassed mixture of methanol,0.05M Ammonium phosphate buffer,and acetonitrile (9:8:3).Make adjustments if necessary (seeSystem Suitability underChromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Orphenadrine Citrate RSinMobile phase,and dilute quantitatively,and stepwise if necessary,withMobile phase to obtain a solution having a known concentration of about 0.9mg per mL.

System sensitivity solution— Dilute a volume of theStandard preparation quantitatively,and stepwise if necessary,withMobile phaseto obtain a solution having a known concentration of about 0.00045mg per mL.

Assay preparation— Transfer an accurately measured volume of Injection,equivalent to about 90mg of orphenadrine citrate,to a 100-mLvolumetric flask,dilute withMobile phase to volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm ×15-cm column that contains 5-µm packing L1.The flow rate is about 1.5mLper minute.The column temperature is maintained at 40.Chromatograph theStandard preparation,and record the peak areas as directed forProcedure:the column efficiency is not less than 4500theoretical plates;the tailing factor is not more than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.Chromatograph theSystem sensitivity solution,and record the peak areas as directed forProcedure:the signal-to-noise ration is not less than 10.

Procedure— Separately inject equal volumes (about 20µL)of theStandard preparation and theAssay preparation into the chromatograph,record the chromatograms,and measure the peak areas for orphenadrine citrate.Calculate the quantity,in mg,of orphenadrine citrate (C18H23NO·C6H8O7)in each mLof the Injection taken by the formula: in whichCis the concentration,in mg per mL,of USP Orphenadrine Citrate RSin theStandard preparation;Vis the volume,in mL,of Injection taken;andrUandrSare the peak responses obtained from theAssay preparation and theStandard preparation,respectively.