0.1 M, pH 7.4 phosphate buffer— Dissolve 2.62 g of monobasic sodium phosphate and 11.50 g of anhydrous dibasic sodium phosphate in 1000 mL of water, and mix.

Medium: 0.1 M, pH 7.4 phosphate buffer; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Procedure— Determine the amount of C14H14O3 dissolved from UV absorbances at the wavelength of maximum absorbance at about 332 nm of filtered portions of the solution under test, suitably diluted with 0.1 M, pH 7.4 phosphate buffer, in comparison with a Standard solution having a known concentration of USP Naproxen RS in the same medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C14H14O3 is dissolved in 45 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a suitable mixture of acetonitrile, water, and glacial acetic acid (50:49:1). Make adjustments if necessary (see System Suitability under Chromatography 621). Increased resolution may be achieved by increasing the proportion of water in the Mobile phase.

Solvent mixture— Prepare a suitable mixture of acetonitrile and water (90:10).

Internal standard solution— Dilute 5 mL of butyrophenone with acetonitrile to make 100 mL. Dilute 1 mL of the resulting solution with acetonitrile to make 100 mL. Each mL of this solution contains about 0.5 µL of butyrophenone.

Standard preparation— Dissolve an accurately weighed quantity of USP Naproxen RS in Solvent mixture to obtain a solution having a known concentration of about 2.5 mg per mL. Transfer 1.0 mL of the resulting solution and 2.0 mL of Internal standard solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 25 µg of USP Naproxen RS per mL.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed quantity of the powder, equivalent to about 250 mg of naproxen, to a 100-mL volumetric flask. Add 10 mL of water, and sonicate for 10 minutes until the material is completely dispersed. Add about 80 mL of acetonitrile, and sonicate for an additional 5 minutes. Allow the flask to reach room temperature, dilute with acetonitrile to volume, and mix. Allow any insoluble matter to settle, then transfer 1.0 mL of the clear supernatant to a 100-mL volumetric flask, add 2.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L1. The flow rate is about 1.2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the column efficiency, determined from the analyte peak, is not less than 4000 theoretical plates when calculated by the formula: the resolution between the analyte and internal standard peaks is not less than 11.5 when calculated by the formula: and the relative standard deviation of replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.6 for naproxen and 1.0 for the internal standard. Calculate the quantity, in mg, of C14H14O3 in the portion of Tablets taken by the formula: in which C is the concentration, in µg per mL, of USP Naproxen RS in the Standard preparation, and RU and RS are the ratios of the response of the naproxen peak to the response of the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.