Methanesulfonic acid solution— Add 35.0 mL of methanesulfonic acid to 20.0 mL of glacial acetic acid, dilute with water to 500 mL, and mix.

Diethylamine solution— Dissolve 10.0 mL of diethylamine in water to obtain 100 mL of solution.

Mobile phase— Prepare a suitable filtered and degassed solution of water, acetonitrile, Methanesulfonic acid solution, and Diethylamine solution (80:20:2:2). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve USP Quinidine Sulfate RS in Mobile phase to obtain a solution having a known concentration of about 100 µg per mL.

Assay preparation— Agitate the container of Oral Suspension for 30 minutes on a rotating mixer, remove a 5-mL sample, and store in a clear glass vial at –70 until analyzed. At the time of analysis, remove the sample from the freezer, allow it to reach room temperature, and mix on a vortex mixer for 30 seconds. Pipet 1.0 mL of the sample solution into a 100-mL volumetric flask, and dilute with Mobile phase to volume.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 235-nm detector and a 4.6-mm × 25-cm analytical column that contains 5-µm packing L1. The flow rate is about 1.0 mL per minute. Chromatograph the 100 µg per mL Standard preparation, and record the peak responses as directed for Procedure: the retention time for quinidine sulfate is about 8.5 minutes, and the relative standard deviation for replicate injections is not more than 1.0%.

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 quinidine sulfate [(C20 H24 N2 O2)2·H2SO4·2H2O] in the volume of Oral Suspension taken by the formula: in which C is the concentration, in µg per mL, of USP Quinidine Sulfate RS in the Standard preparation; V is the volume, in mL, of Oral Suspension taken; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.