(Official January 1, 2007)

pH 3.1 Tetramethylammonium phosphate buffer— Dissolve 1.38 g of monobasic sodium phosphate and 2.0 mL of 25% tetra-methylammonium hydroxide solution in methanol in 800 mL of water, adjust with phosphoric acid to a pH of 3.1 ± 0.1, dilute with water to 1000 mL, and mix.

Mobile phase— Transfer 15 mL of tetrahydrofuran and 75 mL of acetonitrile to a 1000-mL volumetric flask, and dilute with pH 3.1 Tetramethylammonium phosphate buffer to volume.

Standard preparation— Dissolve an accurately weighed quantity of USP Bretylium Tosylate RS in water, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 0.2 mg per mL.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 10 mg of bretylium tosylate, to a 50-mL volumetric flask, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 220-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.7 for tosylate and 1.0 for bretylium; the resolution, R, between the bretylium and tosylate peaks is not less than 3.0; and the relative standard deviation for replicate injections is not more than 1.4%.

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 C18H24BrNO3S in each mL of the Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Bretylium Tosylate RS in the Standard preparation; V is the volume, in mL, of Injection taken; and rU and rS are the bretylium peak responses obtained from the Assay preparation and the Standard preparation, respectively.