for oral suspension packaged in single-unit containers: meets the requirements.

for oral suspension packaged in multiple-unit containers: meets the requirements.

Mobile phase— Prepare a mixture of 500 mL of methanol, 500 mL of water, and 2.46 g of anhydrous sodium acetate, and mix until dissolved. Adjust with glacial acetic acid to a pH of 5.8. Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of ethylparaben in methanol containing about 1.1 mg per mL.

Standard preparation— Transfer about 62.5 mg of USP Naproxen RS, accurately weighed, to a 50-mL volumetric flask, add about 30 mL of methanol, and sonicate to dissolve. Add 5.0 mL of Internal standard solution, dilute with methanol to volume, and mix. Transfer 2.0 mL of this solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 50 µg of USP Naproxen RS and 4.4 µg of ethylparaben per mL.

Assay preparation— Transfer an accurately measured volume of Oral Suspension, previously well-mixed and free from air bubbles, equivalent to about 125 mg of naproxen, to a 100-mL volumetric flask, using a “to contain” pipet. Rinse the pipet several times with methanol, and add the rinsings to the volumetric flask. Add 10.0 mL of Internal standard solution, dilute with methanol to volume, and mix. Transfer 2.0 mL of this solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. Filter, if necessary, to obtain a clear solution.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-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 relative retention times are about 0.6 for ethylparaben and 1.0 for naproxen; the resolution, R, between ethylparaben and naproxen is not less than 3.0; the tailing factor for the naproxen 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 35 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 naproxen (C14H14O3) in each mL of the Oral Suspension taken by the formula: in which C is the concentration, in µg per mL, of USP Naproxen RS in the Standard preparation; V is the volume, in mL, of Oral Suspension taken to prepare the Assay preparation; and RU and RS are the ratios of the response of the naproxen peak to the response of the ethylparaben peak obtained from the Assay preparation and the Standard preparation, respectively.