Buffer solution— Dissolve 4.9 g of phosphoric acid, accurately weighed, in water. Dilute with water to 1000 mL. Adjust with triethylamine to a pH of 3.5.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and acetonitrile (83.5:16.5). Make adjustments, if necessary (see System Suitability under Chromatography 621).

Solution A— Transfer 9 g of sodium chloride, accurately weighed, to a 1000-mL flask, and dissolve in and dilute with water to volume. Adjust with hydrochloric acid to a pH between 3.5 and 5.0.

Solution B— Transfer 9 g of sodium chloride, accurately weighed, to a 1000-mL flask, dissolve in water, and add 5 g of chlorobutanol. Dilute with water to volume, and adjust with hydrochloric acid to a pH between 3.5 and 5.0.

Standard preparation— Dissolve an accurately weighed quantity of USP Desmopressin Acetate RS in water to obtain a solution having a known concentration of about 1 mg per mL. Dilute this solution with Solution A or Solution B, as directed for the Assay preparation, to obtain a solution with a concentration of desmopressin equivalent to that in the Assay preparation.

Assay preparation— For injections at concentrations of desmopressin between 4 µg per mL and 0.1 mg per mL, use undiluted Desmopressin Acetate Injection. For injections at concentrations exceeding 0.1 mg per mL and without preservatives, dilute 1000 µL of Desmopressin Acetate Injection, accurately measured, with 10 mL of Solution A. For injections at concentrations exceeding 0.1 mg per mL and containing preservatives, dilute 1000 µL of Desmopressin Acetate Injection, accurately measured, with 10 mL of Solution B.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 1.5 mL per minute. Chromatograph about 50 µL of the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 1.4; and the relative standard deviation for replicate injections is not more than 5.0%.

Procedure— Separately inject equal volumes (about 100 µL) of the Standard preparation and the Assay preparation, both freshly prepared, into the chromatograph, and record the chromatograms for a total time of not less than 2.5 times the retention time of the desmopressin peak. Calculate the quantity of desmopressin (C46H64N14O12S2), in mg, in the volume of Injection taken by the formula: in which C is the concentration of desmopressin, in mg per mL, in the Standard preparation; D is the dilution factor used to prepare the Assay preparation; and rU and rS are the peak responses for desmopressin obtained from the Assay preparation and the Standard preparation, respectively.