A: Dissolve about 20 mg in 2 mL of buffer solution, prepared by adjusting 50 mL of 1 M monobasic potassium phosphate with 1 N sodium hydroxide to a pH of 7.0, diluting with water to 100 mL, and mixing. Add 0.2 mL of ninhydrin solution (1 in 200) and boil: a purple color is produced.

B: It meets the requirements of the tests for Sulfate 191.

C: Liquid Chromatographic Identification Test—

Mobile phase— Prepare a mixture of 0.1 M tribasic sodium phosphate and acetonitrile (77:23), and adjust with phosphoric acid to a pH of 3.0. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Prepare a solution of USP Colistin Sulfate RS in Mobile phase having a concentration of about 6 mg per mL. Protect this solution from light.

Test solution— Prepare a solution of Colistin Sulfate in Mobile phase having a concentration of about 6 mg per mL. Protect this solution from light.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 212-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.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, and record the chromatograms. The chromatogram obtained from the Test solution corresponds qualitatively with that obtained from the Standard solution, exhibiting a major peak corresponding to colistin A and minor peaks at relative retention times of about 0.55 (colistin B) and 0.8.