test 1—

Medium: 0.1 N hydrochloric acid; 500 mL, degassed.

test 2— If the product complies with this test, the labeling indicates that the product meets USP Dissolution Test 2.

Phosphoric acid solution, Buffer solution, Mobile phase, Tizanidine related compound A solution, Tizanidine related compound B solution, Tizanidine related compound C solution, Resolution solution, and Chromatographic system— Proceed as directed in the Assay.

Standard solution— Prepare as directed for the Standard preparation in the Assay.

Test solution— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 20 mg of tizanidine, to a 100-mL volumetric flask, add about 50 mL of Buffer solution, sonicate for about 15 minutes with occasional shaking, and shake by mechanical means for 15 minutes. Add 20 mL of acetonitrile, and mix. Allow to cool, dilute with Buffer solution to volume, and mix. Centrifuge a portion of this solution at 2000 rpm or higher for 10 minutes. Pass a portion of this solution through a filter having a 45-µm or finer porosity, and use the filtrate.

Procedure— Inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major analyte peaks, disregarding the peaks due to the solvent. Calculate the percentage of each impurity in the portion of Tablets taken by the formula: in which 253.71 and 290.17 are the molecular weights of tizanidine and tizanidine hydrochloride, respectively; C is the concentration, in mg per mL, of USP Tizanidine Hydrochloride RS in the Standard solution; A is the average weight, in mg, of each Tablet; D is the labeled dose, in mg, of tizanidine per Tablet; W is the weight, in mg, of sample taken to prepare the Test solution; F is the relative response factor and is given in Table 1; ri is the peak area for each impurity obtained from the Test solution; and rS is the peak area of tizanidine in the Standard solution. The limits for the impurities are specified in Table 1.

Phosphoric acid solution— Dilute 6 mL of phosphoric acid with water to make 50 mL of solution.

Buffer solution— Dissolve about 3.5 g of sodium 1-pentanesulfonate in 1000 mL of water. Adjust with Phosphoric acid solution or 1 N sodium hydroxide to a pH of 3.0 ± 0.05.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and acetonitrile (80:20). Make adjustments if necessary (see System Suitability under Chromatography 621).

Tizanidine related compound A solution— Dissolve an accurately weighed quantity of USP Tizanidine Related Compound A RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.1 mg per mL.

Tizanidine related compound B solution— Dissolve an accurately weighed quantity of USP Tizanidine Related Compound B RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.1 mg per mL.

Tizanidine related compound C solution— Dissolve an accurately weighed quantity of USP Tizanidine Related Compound C RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.1 mg per mL.

Resolution solution— Transfer about 23 mg of USP Tizanidine Hydrochloride RS to a 100-mL volumetric flask, add 20 mL of Mobile phase and 10 mL each of Tizanidine related compound A solution, Tizanidine related compound B solution, and Tizanidine related compound C solution. Sonicate to dissolve the USP Tizanidine Hydrochloride RS, and dilute with Mobile phase to volume.

Standard preparation— Dissolve an accurately weighed quantity of USP Tizanidine Hydrochloride RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.046 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 20 mg of tizanidine, to a 500-mL volumetric flask, add about 250 mL of Buffer solution, sonicate for about 15 minutes with occasional shaking, and shake by mechanical means for 15 minutes. Add 100 mL of acetonitrile, and mix. Allow to cool, dilute with Buffer solution to volume, and mix. Centrifuge a portion of this solution at 2000 rpm or higher for 10 minutes. Pass a portion of this solution through a filter having a 45-µm or finer porosity, and use the filtrate.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 230-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1.0 mL per minute. The column temperature is maintained at 50. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the relative retention times are given in Table 1; the resolution, R, between tizanidine and tizanidine related compound C is not less than 4.0, and the resolution, R, between tizanidine and tizanidine related compound B is not less than 4.0. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the column efficiency is not less than 5000 theoretical plates; the tailing factor is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 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 tizanidine (C9H8ClN5S) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Tizanidine Hydrochloride RS in the Standard preparation; and rU and rS are the peak areas obtained from the Assay preparation and the Standard preparation, respectively.