A:Infrared Absorption 197K—

B: The retention time of the major peak in the chromatogram of the Test solution corresponds to that of emedastine in the chromatogram of the System suitability solution, as obtained from the Chromatographic purity test.

C: Dissolve about 23 mg of Emedastine Difumarate in 25 mL of water. Add 1 mL of a solution prepared by mixing 20 mL of cupric sulfate solution (1 in 5) and 8 mL of pyridine: a precipitate is formed in the blue solution within one minute.

Buffer solution— Dissolve 3.9 g of monobasic sodium phosphate and 2.5 g of sodium dodecyl sulfate in 1.0 L of water. Adjust with phosphoric acid to a pH of 2.4.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and acetonitrile (1:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

System suitability solution— Prepare a solution in Mobile phase containing 0.10 mg of USP Emedastine Difumarate RS and 0.04 mg of 4-methylbenzophenone per mL.

Standard solution— Dissolve an accurately weighed quantity of USP Emedastine Difumarate RS in Mobile phase, and dilute, stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.003 mg per mL.

Test solution— Prepare a solution of Emedastine Difumarate in Mobile phase containing about 1.0 mg per mL.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L1. 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 0.2 for fumaric acid, 1.0 for emedastine, and 2.0 for 4-methylbenzophenone; the resolution, R, is not less than 2.0; the column efficiency determined from the emedastine peak is not less than 1500 theoretical plates; the tailing factor is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Mobile phase, Standard solution, and Test solution into the chromatograph, and record the chromatograms, allowing the elution to continue for a period of not less than twice the retention time of emedastine. Measure the areas for all of the peaks, disregarding the fumaric acid peak and peaks corresponding to those obtained from the Mobile phase. Calculate the percentage of each impurity in the portion of Emedastine Difumarate taken by the formula: in which ri is the peak response for each impurity obtained from the Test solution; and rS is the peak response for emedastine in the Standard solution: not more than 0.3% of any individual impurity is found; and not more than 1.0% of total impurities is found.