A: Infrared Absorption 197K.

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

C: A solution of 10 mg per mL in water meets the requirements of the silver nitrate precipitate test for Chloride 191.

Phosphoric acid solution— Transfer 6.0 mL of phosphoric acid to a 50-mL volumetric flask, and dilute with water to volume.

Buffer solution— Dissolve about 3.5 g of sodium 1-pentanesulfonate in 1000 mL of water, and 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 solution— 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.

Test solution— Transfer about 57 mg of Tizanidine Hydrochloride, accurately weighed, to a 50-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

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 solution, 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— 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 Tizanidine Hydrochloride taken by the formula: in which 253.71 and 290.17 are the molecular weights of tizanidine and tizanidine hydrochloride, respectively; CS and CT are the concentration, in mg per mL, of tizanidine hydrochloride in the Standard solution and the Test solution; F is the relative response factor for each impurity relative to tizanidine 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 obtained from the Standard solution. The limits for the impurities are specified in Table 1.

Buffer solution— Dissolve 6.8 g of monobasic potassium phosphate in 1000 mL of water, and adjust with 5.3 N potassium hydroxide to a pH of 7.5 ± 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).

System suitability preparation— Dissolve suitable quantities of USP Tizanidine Hydrochloride RS and USP Tizanidine Related Compound C RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution containing about 46 µg per mL and 0.12 µg per mL, respectively.

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— Transfer about 23 mg of Tizanidine Hydrochloride, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix. Transfer 10.0 mL of this solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 230-nm detector and a 4.6-mm × 15-cm column that contains packing L7. The flow rate is about 1.0 mL per minute. The column temperature is maintained at 35. Chromatograph the System suitability preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.5 for tizanidine related compound C and 1.0 for tizanidine; the resolution, R, between tizanidine and tizanidine related compound C is not less than 6; and the tailing factor for the tizanidine peak is not more than 2.0. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

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 percentage of C9H8ClN5S·HCl in the portion of Tizanidine Hydrochloride taken by the formula: in which CS and CU are the concentrations of tizanidine hydrochloride, in mg per mL, in the Standard preparation and the Assay preparation, respectively; and rU and rS are the peak areas obtained from the Assay preparation and the Standard preparation, respectively.