A: Infrared Absorption 197K—

Test specimen— Dry in vacuum over phosphorus pentoxide for 4 hours.

B: Ultraviolet Absorption 197U—

C: To 5 mL of a solution (1 in 200) add 5 drops of 2 N nitric acid and 2 mL of silver nitrate TS: a yellowish white precipitate is formed.

Test solution: 18 mg per mL (warm it, if necessary, to dissolve). Its specific rotation, determined photoelectrically at 325 nm, does not differ from that of the similarly prepared solution of USP Dextromethorphan Hydrobromide RS by more than 1.0%.

Mobile phase— Prepare a filtered and degassed solution containing 0.007 M docusate sodium and 0.007 M ammonium nitrate in a mixture of acetonitrile and water (70:30), and adjust the solution with glacial acetic acid to a pH of 3.4. [note—Dissolve the docusate sodium in the acetonitrile and water mixture before adding the ammonium nitrate.]

Standard preparation— Dissolve an accurately weighed quantity of USP Dextromethorphan Hydrobromide RS in water to obtain a stock solution having a known concentration of about 1 mg per mL. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer about 100 mg of Dextromethorphan Hydrobromide, accurately weighed, to a 100-mL volumetric flask, add water to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor for the major peak is not more than 2.5; and the relative standard deviation 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 responses for the major peaks. Calculate the quantity, in mg, of C18H25NO· HBr in the portion of Dextromethorphan Hydrobromide taken by the formula: in which C is the concentration, in mg per mL, of USP Dextromethorphan Hydrobromide RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.