Solution: 10 µg per mL, filtered.

Medium: 0.1 N hydrochloric acid.

Mobile phase— To 4.5 g of sodium 1-hexanesulfonate add 1500 mL of water, 400 mL of methanol, and 100 mL of acetonitrile, mix, and allow to cool to room temperature. Adjust with phosphoric acid to a pH of 5.1, and pass through a filter having a 0.5-µm or finer porosity. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Quantitatively dissolve an accurately weighed quantity of USP Amprolium RS in water to obtain a solution having a known concentration of about 0.5 mg per mL.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 960 mg of amprolium, to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of this stock solution to a second 100-mL volumetric flask, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 268-nm detector and a 3.9-mm × 30-cm column that contains packing L11. The column is maintained at a constant temperature of about 45. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 1.5; 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 peak areas for amprolium. Calculate the quantity, in mg, of amprolium (C14H19ClN4·HCl) in each mL of the Oral Solution taken by the formula: in which C is the concentration, in mg per mL, of USP Amprolium RS in the Standard preparation; V is the volume, in mL, of Oral Solution taken to prepare the Assay preparation; and rU and rS are the amprolium peak areas obtained from the Assay preparation and the Standard preparation, respectively.