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

B: Shake a quantity of finely powdered Tablets, equivalent to about 75 mg of norfloxacin, with 50 mL of a mixture of acidic methanol (prepared by mixing 1000 mL of methanol and 9 mL of hydrochloric acid) and methylene chloride (1:1). Centrifuge a portion of the suspension thus obtained, and use the clear supernatant as the test solution. Apply 50 µL each of the test solution and a standard solution of USP Norfloxacin RS in the same solvent containing 1.5 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Place the plate in a suitable chromatographic chamber that contains and has been equilibrated with a developing system consisting of a mixture of chloroform, methanol, toluene, diethylamine, and water (40:40:20:14:8), and develop the chromatogram until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots on the plate by examination 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.

pH 4.0 buffer— To 900 mL of water in a 1000-mL volumetric flask add 2.86 mL of glacial acetic acid and 1.0 mL of a 50% (w/w) solution of sodium hydroxide, dilute with water to volume, and mix. If necessary, adjust with glacial acetic acid or the sodium hydroxide solution to a pH of 4.0.

Medium: pH 4.0 buffer; 750 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C16H18FN3O3 dissolved from UV absorbances at the wavelength of maximum absorbance at about 278 nm of filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Norfloxacin RS in the same medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C16H18FN3O3 is dissolved in 30 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of phosphoric acid solution (1 in 1000) and acetonitrile (850:150). Make adjustments if necessary (see System Suitability under Chromatography 621).

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

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 100 mg of norfloxacin, to a 200-mL volumetric flask. Add 80 mL of Mobile phase, sonicate for 10 minutes, dilute with phosphoric acid solution (1 in 1000) to volume, and mix. Transfer 10.0 mL of this solution to a 25-mL volumetric flask, dilute with Mobile phase to volume, mix, and filter through a filter having a porosity of 1 µm or less.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 275-nm detector and a 3.9-mm × 30-cm column that contains packing L1, and is operated at 40 ± 1.0.

Procedure— [NOTE—Use peak areas where peak responses are indicated.] Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the peak responses for the major peaks. Calculate the quantity, in mg, of C16H18FN3O3 in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Norfloxacin RS in the Standard preparation, and rU and rS are the norfloxacin peak responses obtained from the Assay preparation and the Standard preparation, respectively.