A: Digest a quantity of powdered Tablets, equivalent to about 100 mg of chlorthalidone, with 10 mL of acetone on a steam bath for about 5 minutes. Filter the solution into a 50-mL beaker, add 20 mL of water, and boil on the steam bath for about 5 minutes, passing a gentle current of air above the solution to remove the acetone. Cool in an ice bath, filter, and dry the crystals at 105 for 4 hours: the crystals so obtained respond to Identification test A under Chlorthalidone.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that of the Standard preparation, both relative to the internal standard, as obtained in the Assay.

Medium: water; 900 mL.

Apparatus 2: 75 rpm.

Time: 60 minutes.

Standard preparation— Dissolve an accurately weighed quantity of USP Chlorthalidone RS in methanol to obtain a solution having a known concentration of about 5 mg per mL.

Procedure— Determine the amount of C14H11ClN2O4S dissolved from UV absorbances at the wavelength of maximum absorbance at about 275 nm of filtered portions of the solution under test, suitably diluted with water, in comparison with a quantitative dilution in water of the Standard preparation having a known concentration of USP Chlorthalidone RS comparable to the concentration of the solution under test.

Tolerances— Not less than 70% (Q) of the labeled amount of C14H11ClN2O4S is dissolved in 60 minutes.

(Official January 1, 2007)

Mobile phase and Internal standard solution—Prepare as directed in the Assay under Chlorthalidone.

Standard preparation— Prepare as directed in the Assay under Chlorthalidone, except to substitute 10.0 mL of methanol for the CCA solution.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 100 mg of Chlorthalidone, to a 100-mL volumetric flask. Dissolve in about 50 mL of methanol, shake for 30 minutes, dilute with methanol to volume, and mix. Transfer about 30 mL of this solution to a 50-mL centrifuge tube, and centrifuge for 10 minutes. Pipet 5 mL of the clear supernatant into a 50-mL volumetric flask containing 5.0 mL of Internal standard solution and 10.0 mL of methanol. Dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L7. The flow rate is about 1.0 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 resolution factor between chlorthalidone and the internal standard is not less than 1.5. The tailing factors for the chlorthalidone and internal standard peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 25 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.8 for chlorthalidone and 1.0 for the internal standard. Calculate the quantity, in mg, of C14H11ClN2O4S in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Chlorthalidone RS in the Standard preparation; and RU and RS are the peak response ratios of chlorthalidone and the internal standard obtained from the Assay preparation and the Standard preparation, respectively.