A: The retention time of the major peak in the chromatogram of the Assay preparationcorresponds to that of the Standard preparationobtained as directed in the Assay.

B: Shake a quantity of finely powdered Tablets,equivalent to about 75mg of norfloxacin,with 50mLof a mixture of acidic methanol (prepared by mixing 1000mLof methanol and 9mLof 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 50µLeach of the test solution and a standard solution of USP Norfloxacin RSin the same solvent containing 1.5mg per mLto a suitable thin-layer chromatographic plate (see Chromatography á621ñ)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 UVlight:the RFvalue of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

pH4.0buffer— To 900mLof water in a 1000-mLvolumetric flask add 2.86mLof glacial acetic acid and 1.0mLof 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 pHof 4.0.

Medium:pH4.0buffer; 750mL.

Apparatus 2: 50rpm.

Time: 30minutes.

Procedure— Determine the amount of C16H18FN3O3dissolved from UVabsorbances at the wavelength of maximum absorbance at about 278nm of filtered portions of the solution under test,suitably diluted with Dissolution Medium,if necessary,in comparison with a Standard solution having a known concentration of USP Norfloxacin RSin the same medium.

Tolerances— Not less than 80%(Q)of the labeled amount of C16H18FN3O3is dissolved in 30minutes.

Mobile phase— Prepare a filtered and degassed mixture of phosphoric acid solution (1in 1000)and acetonitrile (850:150).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

Chromatographic system (see Chromatography á621ñ)—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 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the peak responses for the major peaks.Calculate the quantity,in mg,of C16H18FN3O3in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Norfloxacin RSin the Standard preparation,and rUand rSare the norfloxacin peak responses obtained from the Assay preparationand the Standard preparation,respectively.