Test solution: 20 mg per mL, in alcohol.

Mobile phase— Proceed as directed in the Assay.

Standard stock solution— Prepare as directed for Standard preparation in the Assay.

Standard solution— Transfer 1.0 mL of the Standard stock solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Test solution— Prepare as directed for the Assay preparation.

Chromatographic system— Proceed as directed in the Assay. Chromatograph the Test solution, and record the peak responses as directed for Procedure: the resolution, R, between dinoprost tromethamine and any other adjacent peak is not less than 1.0.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the percentage of each impurity in the portion of Dinoprost Tromethamine taken by the formula: in which C is the concentration, in µg per mL, of USP Dinoprost Tromethamine RS in the Standard solution; W is the weight, in mg, of Dinoprost Tromethamine taken to prepare the Test solution; F is the relative response factor and is equal to 0.25 for any peak at a relative retention time of about 0.30, 1.7 for any peak at a relative retention time of about 1.15, and 1.0 for any other peak; ri is the peak response of each impurity obtained from the Test solution; and rS is the peak response of dinoprost tromethamine obtained from the Standard solution: not more than 2.0% of any impurity having a relative retention time of about 0.94 is found; not more than 1.5% of any impurity having a relative retention time of about 0.84 is found; not more than 0.5% of any other impurity is found; and not more than 2.0% of all other impurities is found.

Mobile phase— Prepare a filtered and degassed mixture of water, acetonitrile, and phosphoric acid (750:250:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Dinoprost Tromethamine RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 1.0 mg per mL.

Assay preparation— Transfer about 25.0 mg of Dinoprost Tromethamine, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 200-nm detector and a 3.9-mm × 15-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the column efficiency is not less than 6000 theoretical plates, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 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 C20H34O5·C4H11NO3 in the portion of Dinoprost Tromethamine taken by the formula: in which C is the concentration, in mg per mL, of USP Dinoprost Tromethamine RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.