Assay—
pH 7.1 buffer—
Dissolve 1.36 g of monobasic potassium phosphate and 5.2 mL of a 1 in 4 solution of tetrabutylammonium hydroxide in methanol in water to make 1000 mL, and adjust with phosphoric acid or with the tetrabutylammonium hydroxide solution to a pH of 7.1.
Mobile phase—
Prepare a suitable filtered mixture of
pH 7.1 buffer and 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 phase to obtain a solution having a known concentration of about 0.05 mg per mL.
Assay preparation 1
(where the label states only the total contents of the container)—Transfer the contents of 1 container of Sodium Nitroprusside for Injection to a 100-mL volumetric flask with the aid of Mobile phase, dilute with Mobile phase to volume, and mix. Dilute an accurately measured volume of this solution quantitatively with Mobile phase to obtain a solution containing about 0.05 mg of Na2[Fe(CN)5NO]·2H2O per mL.
Assay preparation 2
(where the label states the quantity of Na2[Fe(CN5)NO]·2H2O in 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 phase to obtain a solution containing about 0.05 mg of Na2[Fe(CN)5NO]·2H2O per mL.
Chromatographic system
(see
Chromatography 
621
)—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 2 mL per 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 25 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 Na
2[Fe(CN)
5NO]·2H
2O in the container or in the portion of constituted solution taken by the formula:
L(C / D)(rU / rS),
in which
L is the labeled quantity, in mg of Na
2[Fe(CN)
5NO]·2H
2O in the container, or in the volume of constituted solution taken;
C is the concentration, in mg per mL, of
USP Sodium Nitroprusside RS in the
Standard preparation; D is the concentration, in mg of Na
2[Fe(CN)
5NO]·2H
2O per mL, of
Assay preparation 1 or 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
rU and
rS are the peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
