A: A 1 in 2000 solution in dilute sulfuric acid (1 in 350) exhibits a vivid blue fluorescence. On the addition of a few drops of hydrochloric acid, the fluorescence disappears.

B: In the test for Chromatographic purity, the RF value of the principal spot obtained from the Test preparation corresponds to that from the Standard preparation.

C: A 1 in 50 solution made with the aid of a few drops of hydrochloric acid responds to the tests for Sulfate 191.

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 filtered and degassed mixture of water, acetonitrile, Methanesulfonic acid solution, and Diethylamine solution (860:100:20:20). Adjust with Diethylamine to a pH of 2.6.

System suitability solution— Transfer about 10 mg each of quinidine sulfate and dihydroquinidine hydrochloride to a 50-mL volumetric flask. Dissolve in about 5 mL of methanol, dilute with Mobile phase to volume, and mix.

Test solution— Transfer about 20 mg of Quinidine Sulfate to a 100-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 235-nm detector and a 3.9-mm × 30-cm column that contains packing L1. Chromatograph the System suitability solution, and proceed as directed for Procedure: the relative retention times for quinidine and dihydroquinidine are 1 and 1.5, respectively, the resolution, R, between the quinidine and dihydroquinidine is not less than 2.5, and the relative standard deviation for the peak response is not more than 2.0%.

Procedure— Inject about 50 µL of the Test solution into the chromatograph, record the chromatogram, and measure the peak responses. The response of the dihydroquinidine peak is not greater than 0.25 that of the quinidine peak (20.0%).

Standard preparation— Prepare a solution of USP Quinidine Sulfate RS in diluted alcohol to contain 6 mg per mL.

Diluted standard preparation— Dilute a portion of the Standard preparation with diluted alcohol to a concentration of 0.06 mg per mL.

Related substances preparation— Prepare a solution in diluted alcohol containing in each mL 0.05 mg of USP Quininone RS (corresponding to 0.06 mg of the sulfate), and 0.10 mg of cinchonine (corresponding to 0.12 mg of the sulfate).

Test preparation— Prepare a solution of Quinidine Sulfate in diluted alcohol to contain 6 mg per mL.

Procedure— Apply 10-µL portions of the Test preparation, the Standard preparation, the Diluted standard preparation, and the Related substances preparation to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel. Allow to dry, and develop the chromatogram in a solvent system consisting of a mixture of chloroform, acetone, and diethylamine (5:4:1), the solvent chamber being used without previous equilibration. When the solvent front has moved about 15 cm, remove the plate from the chamber, mark the solvent front, and allow the solvent to evaporate. Spray the chromatogram with glacial acetic acid. Locate the spots on the plate by examination under long-wavelength UV light. Any spot produced by the Test preparation at the RF value of a spot produced by the Related substances preparation is not greater in size or intensity than that corresponding spot. Apart from these spots and from the spots appearing at the RF value of Quinidine Sulfate and dihydroquinidine sulfate (the two spots most evident from the Standard preparation), any additional fluorescent spot is not greater in size or intensity than the principal spot of the Diluted standard preparation. Spray the plate with potassium iodoplatinate TS. Any spot produced by the Test preparation is not greater in size or intensity than a corresponding spot from the Related substances preparation.