A: It meets the requirements under Identification—Organic Nitrogenous Bases 181.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

C: It responds to the tests for Chloride 191.

Mobile phase— Prepare a solution of acetonitrile, water, and triethylamine (50:50:0.5), adjust with glacial acetic acid to a pH of 6.5, filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Diphenhydramine Hydrochloride RS in water to obtain a solution having a known concentration of about 0.5 mg per mL.

Assay preparation— Transfer about 25 mg of Diphenhydramine Hydrochloride, accurately weighed, to a 50-mL volumetric flask, dissolve in and dilute with water to volume, mix, and filter.

System suitability solution— Dissolve about 5 mg of benzophenone in 5 mL of acetonitrile, dilute with water to 100 mL, and mix. Transfer 1.0 mL of this solution and 5 mg of diphenhydramine hydrochloride to a 10-mL volumetric flask, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L10. The flow rate is about 1 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between the benzophenone and diphenhydramine peaks is not less than 2.0. Chromatograph replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%; and the tailing factor for the diphenhydramine hydrochloride peak is not more than 2.0.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C17H21NO·HCl in the portion of Diphenhydramine Hydrochloride taken by the formula: in which C is the concentration, in mg per mL, of USP Diphenhydramine Hydrochloride RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.