Buffer solution— Dissolve 13.8g of monobasic sodium phosphate and 10mLof triethylamine in 800mLof water.Adjust with phosphoric acid to a pHof 5.7,dilute with water to 1000mL,and mix.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solutionand acetonitrile (83:17).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Emedastine Difumarate RSin Mobile phaseto obtain a solution having a known concentration of about 0.057mg of emedastine per mL.

System suitability solution— Add 50µLof 30percent hydrogen peroxide to 2mLof Standard preparation,and heat at 100for 30minutes.Add another 2mLof Standard preparation,mix,and use immediately.

Assay preparation— Transfer an accurately measured volume of Ophthalmic Solution into a suitable volumetric flask to obtain a solution having a known concentration of about 0.057mg of emedastine per mL.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×15-cm column that contains packing L7.The flow rate is about 1.0mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 1.0for emedastine and 1.2for emedastine N-oxide;the resolution,R,between emedastine and emedastine N-oxide is not less than 1.5;the column efficiency determined from the emedastine peak is not less than 1000theoretical plates;and the tailing factor is not more than 2.0.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 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the peak responses for emedastine.Calculate the quantity,in mg,of emedastine (C17H26N4O)in each mLof the Ophthalmic Solution taken by the formula: in which 302.42and 534.57are the molecular weights of emedastine and emedastine difumarate,respectively;Cis the concentration,in mg per mL,of USP Emedastine Difumarate RSin the Standard preparation;V1is the volume,in mL,of the volumetric flask used to prepare the Assay preparation;V2is the volume,in mL,of Ophthalmic Solution taken;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.