Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 20minutes.

Phosphate buffer— Transfer 8.7g of dibasic potassium phosphate to a 1Lvolumetric flask,dilute with water to volume,and adjust with phosphoric acid to a pHof 6.8.

Mobile phase— Prepare a filtered and degassed mixture of Phosphate buffer,methanol,and acetonitrile (96:4:3).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Transfer about 10mg of USP Zalcitabine RS,accurately weighed,to a 250-mLvolumetric flask.Add about 200mLof Medium,and sonicate until complete solution is effected.Dilute with Mediumto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 270-nm detector and a 3.9-mm ×15-cm column that contains 10-µm packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as described for Procedure:the retention time is about 5minutes;the tailing factor is not more than 2;and the relative standard deviation is not more than 2%.

Procedure— Separately inject equal volumes (about 150µL)of the Standard preparationand a filtered portion of the solution under test into the chromatograph,record the chromatograms,and measure the responses of the major peaks.Calculate the quantity,in mg,of zalcitabine in the Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Zalcitabine RSin the Standard preparation;and rUand rSare the zalcitabine peak responses obtained from the solution under test and the Standard preparation,respectively.

Tolerances— Not less than 80%(Q)of the labeled amount of C9H13N3O3is dissolved in 20minutes.

Buffer solution— Dissolve 3.4g of monobasic potassium phosphate in sufficient water to make 1L.Using a suitable pHmeter,adjust with phosphoric acid to a pHof 2.2.[NOTE—If too much phosphoric acid is added,yielding a pHbelow 2.2,the pHmay be adjusted to 2.2with 0.025Mmonobasic potassium phosphate.]Add 1.08g of sodium 1-octanesulfonic acid,and mix.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solutionand acetonitrile (85:15).

Diluent— Prepare a mixture of water and acetonitrile (17:3).

Resolution solution— Dissolve weighed quantities of USP Zalcitabine RSand USP Zalcitabine Related Compound A RSin Diluent,and dilute quantitatively,and stepwise if necessary,with the same solvent to obtain a solution containing about 0.02mg per mLof zalcitabine and 0.002mg per mLof zalcitabine related compound A.

Standard preparation— Dissolve an accurately weighed quantity of USP Zalcitabine RSin Diluent,and dilute quantitatively,and stepwise if necessary,with the same solvent to obtain a solution having a known concentration of about 0.008mg per mL.

Assay preparation— Transfer 5Tablets to a volumetric flask suitable to obtain a solution containing about 0.008mg zalcitabine per mL.Add a volume of Diluentthat is about six-tenths of the volume of the flask,sonicate for 15minutes,and shake by mechanical means for 10minutes.Dilute with Diluentto volume,and mix.Filter before use.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 280-nm detector,a precolumn cartridge that contains packing L1,and a 4.6-mm ×25-cm analytical column that contains 5-µm packing L1.The flow rate is about 1.5mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the resolution,R,between the zalcitabine and zalcitabine related compound Apeaks is not less than 1.1,and the tailing factor for the zalcitabine peak is not more than 1.5.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%.

Procedure— Separately inject equal volumes (about 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses of the major peaks.Calculate the quantity,in mg,of C9H13N3O3in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Zalcitabine RSin the Standard preparation;Vis the volume,in mL,of the volumetric flask used to prepare the Assay preparation;and rUand rSare the zalcitabine peak responses obtained from the Assay preparationand the Standard preparation,respectively.