Test specimen— Extract a volume of Injection, equivalent to about 2.5 mg of carboprost tromethamine, with 1.5 to 2 times its volume of chloroform. Discard the chloroform layer, and acidify the aqueous layer with 3 to 5 drops of hydrochloric acid. Extract the acidified solution with an equivalent volume of chloroform. Filter the chloroform layer through a pledget of cotton, and concentrate it to a volume of less than 1 mL. Combine the resulting solution with 150 to 180 mg of potassium bromide. Dry the potassium bromide mixture in vacuum overnight, and prepare a pellet from the dried mixture.

Mobile phase and Citrate buffer— Prepare as directed in the Assay under Carboprost Tromethamine.

Internal standard preparation— Using the Mobile phase, prepare a solution containing about 3 mg of guaifenesin per mL.

Standard preparation— Using an accurately weighed quantity of USP Carboprost Tromethamine RS, prepare a Standard solution containing about 0.332 mg of USP Carboprost Tromethamine RS per mL and 9 mg of benzyl alcohol per mL. Transfer 2.0 mL of the solution so obtained into a stoppered centrifuge tube. Add 20.0 mL of methylene chloride and 1.0 mL of Citrate buffer, shake the stoppered tube for about 10 minutes, and centrifuge. Remove and discard the top (aqueous) layer, transfer an 8.0-mL aliquot of the lower (methylene chloride) layer to a suitable vial, and evaporate the solution with the aid of a stream of nitrogen. [note—The residue does not evaporate to dryness because of the presence of benzyl alcohol.] Add 100 µL of a freshly prepared solution of -bromo-2¢-acetonaphthone in acetonitrile (1 in 50), and swirl to wash down the sides of the vial. Add 50 µL of a freshly prepared solution of diisopropylethylamine in acetonitrile (1 in 100). Swirl again, and place the vial in a suitable heating device maintained at a temperature of 30 to 35 for not less than 15 minutes. Evaporate the acetonitrile from the vial with the aid of a stream of nitrogen, add 1.0 mL of Internal standard preparation, mix, and pass the resulting solution through a fine-porosity filter. Protect the filtered solution from light prior to injection to prevent degradation of the naphthacyl ester of carboprost.

Assay preparation— Pipet an accurately measured volume of Injection, equivalent to about 500 µg of carboprost, to a stoppered, 50-mL centrifuge tube. Proceed as directed for Standard preparation, beginning with “Add 20.0 mL of methylene chloride”.

Chromatographic system— Proceed as directed in the Assay under Carboprost Tromethamine. To evaluate the system suitability requirements, use the Standard preparation prepared as directed in the Assay under Carboprost Tromethamine.

Procedure— Proceed as directed in the Assay under Carboprost Tromethamine. Calculate the quantity, in µg, of carboprost (C21H36O5) in each mL of Injection taken by the formula: in which 368.51 and 489.64 are the molecular weights of carboprost and carboprost tromethamine, respectively; C is the concentration, in µg per mL, of USP Carboprost Tromethamine RS in the Standard solution used to prepare the Standard preparation; and the other terms are as defined therein.