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

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and acetonitrile (83:17). Make adjustments if necessary (see System Suitability under Chromatography 621).

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

System suitability solution— Add 50 µL of 30 percent hydrogen peroxide to 2 mL of Standard preparation, and heat at 100 for 30 minutes. Add another 2 mL of 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 0.057 mg of emedastine per mL.

Chromatographic system (see Chromatography 621)— 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 1.0 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 1.0 for emedastine and 1.2 for 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 1000 theoretical 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 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the peak responses for emedastine. Calculate the quantity, in mg, of emedastine (C17H26N4O) in each mL of the Ophthalmic Solution taken by the formula: in which 302.42 and 534.57 are the molecular weights of emedastine and emedastine difumarate, respectively; C is the concentration, in mg per mL, of USP Emedastine Difumarate RS in the Standard preparation; V1 is the volume, in mL, of the volumetric flask used to prepare the Assay preparation; V2 is the volume, in mL, of Ophthalmic Solution taken; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.