A: Infrared Absorption á197Kñ.

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

C: Prepare a test solution of it in a mixture of methanol and water (1:1)containing 50mg per mL.Similarly prepare a Standard solution,using USP Zalcitabine RS.Separately apply 10µLportions of the test solution and the Standard solution to 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 paper-lined chromatographic chamber saturated with a solvent system consisting of the clear lower layer of a mixture of alcohol,dichloromethane,and water (3:2:2),and develop the chromatogram.When 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 to dry.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.

Test solution: 7mg per mL,in water.

Phosphate buffer ,Mobile phase,Resolution solution,Standard preparation,and Assay preparation—Prepare as directed in the Assay.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 270-nm detector and a 4.6-mm ×15-cm column that contains 5-µm packing L1.The flow rate is about 1mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the resolution,R,between zalcitabine and zalcitabine related compound Ais not less than 2.0,and the tailing factor for zalcitabine is not greater than 1.5.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation is not more than 2%.

Procedure— Inject a volume (about 20µL)of the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses of the major peaks.Inject a volume of acetonitrile in water (3in 100)as a chromatographic blank.Calculate the percentage of each impurity in the portion of zalcitabine taken by the formula: in which riis the peak response for each impurity,and rsis the sum of the responses of all of the peaks:not more than 0.3%of any individual impurity is found,and the sum of all impurities is not more than 2.0%.

Test solution: 50mg per mL,in a mixture of methanol and water (1:1).

Standard solution: a mixture of methanol and water (1:1).

Eluant: the lower layer of a mixture of alcohol,dichloromethane,and water (3:2:2).

Visualization: 1.

Phosphate buffer— Dissolve 6.8g of monobasic potassium phosphate and 8.7g of dibasic potassium phosphate in 2000mLof water.Adjust,if necessary,with dilute phosphoric acid or potassium hydroxide solution (1in 10)to a pHof 6.8±0.05.

Mobile phase— Prepare a filtered and degassed mixture of Phosphate bufferand acetonitrile (97:3).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Resolution solution— Dissolve USP Zalcitabine RSand USP Zalcitabine Related Compound A RSin a mixture of acetonitrile in water (3in 100),and dilute quantitatively,and stepwise if necessary,with the same solvent to obtain a solution containing about 0.024mg of each per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Zalcitabine RSin a mixture of acetonitrile and water (3in 100)to obtain a solution having a known concentration of about 0.5mg per mL.

Assay preparation— Transfer about 100mg of Zalcitabine,accurately weighed,to a 200-mLvolumetric flask.Dissolve in a mixture of acetonitrile and water (3in 100),dilute with the same solvent to volume,and mix.

Chromatographic system— The liquid chromatograph is equipped with a 270-nm detector and a 4.6-mm ×15-cm column that contains packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor for the zalcitabine peak is not greater than 1.5,and the relative standard deviation is not more than 2.0%.Chromatograph the Resolution solution:the resolution,R,between zalcitabine and zalcitabine related compound Ais not less than 2.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C9H13N3O3in the portion of Zalcitabine taken by the formula: in which Cis the concentration,in mg per mL,of USP Zalcitabine RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.