A: Apply a measured volume of Injection containing 0.025 mg of metronidazole and a measured volume of a solution of USP Metronidazole RS containing 0.025 mg of metronidazole to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Allow the spots to dry, and develop the chromatogram in a solvent system consisting of a mixture of chloroform, methanol, water, and ammonium hydroxide (70:28:4:2) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots on the plate by viewing under short-wavelength UV light: the RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

B: The retention time of the major peak obtained from the Assay preparation corresponds to that of the Standard preparation obtained as directed in the Assay.

(Official January 1, 2007)

Mobile phase— Prepare a suitable filtered and degassed mixture of monobasic potassium phosphate, prepared by dissolving 0.68 g of monobasic potassium phosphate in 930 mL of water, and methanol (930:70), and adjust with 1 M phosphoric acid to a pH of 4.0 ± 0.5.

Standard preparation— Transfer about 25 mg of USP Metronidazole RS, accurately weighed, to a 25-mL volumetric flask, dissolve in methanol, dilute with methanol to volume, and mix. Pipet 2 mL of this solution into a 10-mL volumetric flask containing 2 mL of water, dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 25 mg of metronidazole, to a 25-mL volumetric flask, dilute with water to volume, and mix. Pipet 2 mL of this solution into a 10-mL volumetric flask containing 2 mL of methanol, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 320-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 2.0 mL per minute. Chromatograph five replicate injections of the Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation is not more than 2.0%, and the tailing factor is not greater 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 metronidazole (C6H9N3O3) in each mL of the Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Metronidazole RS in the Standard preparation; V is the volume, in mL, of Injection taken; and rU and rS are the peak responses for metronidazole obtained from the Assay preparation and the Standard preparation, respectively.