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

B: Transfer a portion of finely powdered Tablets, equivalent to about 50 mg of isoniazid, to a 500-mL volumetric flask. Add water to volume, mix, and filter a portion of the mixture. Proceed as directed in Identification test B under Isoniazid, beginning with “Transfer 10.0 mL of the resulting solution to a 100-mL volumetric flask.”

Medium: 0.01 N hydrochloric acid; 900 mL.

Apparatus 1: 100 rpm.

Time: 45 minutes.

Procedure— Determine the amount of C6H7N3O dissolved by employing UV absorption at the wavelength of maximum absorbance at about 263 nm on filtered portions of the solution under test, suitably diluted with Dissolution Medium, in comparison with a Standard solution having a known concentration of USP Isoniazid RS in the same Medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C6H7N3O is dissolved in 45 minutes.

Procedure for content uniformity— Transfer 1 finely powdered Tablet to a 500-mL volumetric flask with the aid of 200 mL of water. Shake by mechanical means for 30 minutes, add water to volume, and mix. Filter, and discard the first 20 mL of the filtrate. Dilute a portion of the filtrate quantitatively and stepwise, if necessary, with a 3 in 100 mixture of 0.1 N hydrochloric acid and water to obtain a solution containing about 10 µg per mL. Dissolve an accurately weighed quantity of USP Isoniazid RS in a volume of water corresponding to that used to dissolve a similar amount of isoniazid from the Tablet, and dilute quantitatively and stepwise, if necessary, with a 3 in 100 mixture of 0.1 N hydrochloric acid and water to obtain a Standard solution having a known concentration of about 10 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 263 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of isoniazid (C6H7N3O) in the Tablet taken by the formula: in which T is the labeled quantity, in mg, of isoniazid in the Tablet; C is the concentration, in µg per mL, of USP Isoniazid RS in the Standard solution; D is the concentration, in µg per mL, of isoniazid in the solution from the Tablet, based on the labeled quantity per Tablet and the extent of dilution; and AU and AS are the absorbances of the solution from the Tablet and the Standard solution, respectively.

Buffer solution— Prepare a 0.1 M monobasic potassium phosphate solution, adjust with 10 N sodium hydroxide to a pH of 6.9, add sufficient triethanolamine to obtain a solution having a known concentration of 0.2 mM of triethanolamine, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and methanol (95:5). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Isoniazid RS in Mobile phase, and dilute quantitatively, and stepwise, if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.32 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to 32 mg of isoniazid, to a 100-mL volumetric flask, add 40 mL of Mobile phase, and sonicate for 10 minutes. Cool to room temperature, dilute with Mobile phase to volume, and centrifuge for 5 minutes.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the capacity factor, k¢, is not less than 2.35; the column efficiency is not less than 1800 theoretical plates; the tailing factor is not more than 1.5; and the relative standard deviation for replicate injections is not more than 1.0%.

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 isoniazid (C6H7N3O) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Isoniazid RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.