pH7.1buffer— Dissolve 1.36g of monobasic potassium phosphate and 5.2mLof a 1in 4solution of tetrabutylammonium hydroxide in methanol in water to make 1000mL,and adjust with phosphoric acid or with the tetrabutylammonium hydroxide solution to a pHof 7.1.

Mobile phase— Prepare a suitable filtered mixture of pH7.1bufferand acetonitrile (about 70:30).

NOTE—Use low-actinic glassware throughout the following sections.

Standard preparation— Dissolve an accurately weighed quantity of USP Sodium Nitroprusside RS,in Mobile phaseto obtain a solution having a known concentration of about 0.05mg per mL.

Assay preparation 1 (where the label states only the total contents of the container)—Transfer the contents of 1container of Sodium Nitroprusside for Injection to a 100-mLvolumetric flask with the aid of Mobile phase,dilute with Mobile phaseto volume,and mix.Dilute an accurately measured volume of this solution quantitatively with Mobile phaseto obtain a solution containing about 0.05mg of Na2[Fe(CN)5NO]·2H2Oper mL.

Assay preparation 2 (where the label states the quantity of Na2[Fe(CN5)NO]·2H2Oin a given volume of constituted solution)—Constitute Sodium Nitroprusside for Injection as directed in the labeling.Dilute an accurately measured volume of the constituted solution thus obtained quantitatively and stepwise with Mobile phaseto obtain a solution containing about 0.05mg of Na2[Fe(CN)5NO]·2H2Oper mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 210-nm detector and a 3.9-mm ×30-cm column that contains 10-µm packing L11.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor for the analyte peak is not more than 2.0;and the relative standard deviation for replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 25µL)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 Na2[Fe(CN)5NO]·2H2Oin the container or in the portion of constituted solution taken by the formula: in which Lis the labeled quantity,in mg of Na2[Fe(CN)5NO]·2H2Oin the container,or in the volume of constituted solution taken;Cis the concentration,in mg per mL,of USP Sodium Nitroprusside RSin the Standard preparation;Dis the concentration,in mg of Na2[Fe(CN)5NO]·2H2Oper mL,of Assay preparation 1or of Assay preparation 2,on the basis of the labeled quantity in the container,or in the volume of constituted solution taken,respectively,and the extent of dilution;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.