Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 60 minutes.

0.05 M Buffer solution— Dissolve 7.0 g of monobasic sodium phosphate in 1 L of water.

Mobile phase— Prepare a mixture of 600 mL of 0.05 M Buffer solution, 1.1 g of sodium 1-octane sulfonate monohydrate and 400 mL of methanol. Adjust with dilute sodium hydroxide to a pH of 6.7 ± 0.05, if necessary, filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm × 15-cm column that contains packing L1 and is heated to 45. The flow rate is about 1 mL per minute. Chromatograph replicate injections of the Standard solution, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%.

Procedure— Inject a volume (about 100 µL) of a filtered portion of the solution under test into the chromatograph, record the chromatogram, and measure the response for the major peak. Calculate the amount of C20H23NO4·HCl dissolved in comparison with a Standard solution having a known concentration of USP Naltrexone RS in the same Medium and similarly chromatographed.

Tolerances— Not less than 80% (Q) of the labeled amount of C20H23NO4·HCl is dissolved in 60 minutes.

(Official January 1, 2007)

Solution A, Solution B, Mobile phase, Resolution solution, Standard preparation, and Chromatographic system— Proceed as directed in the Assay under Naltrexone Hydrochloride.

Assay preparation— Transfer not fewer than 20 Tablets to a tared container, and determine the average Tablet weight. Grind the Tablets to a homogeneous mixture. Transfer an accurately weighed portion, equivalent to about 250 mg of naltrexone hydrochloride, to a 100-mL volumetric flask. Add about 80 mL of 0.1 M phosphoric acid, and shake or sonicate for at least 30 minutes. Dilute with 0.1 M phosphoric acid to volume, mix, and filter.

Procedure— Proceed as directed for Procedure in the Assay under Naltrexone Hydrochloride. Calculate the quantity, in mg, of naltrexone hydrochloride (C20H23NO4·HCl) in the portion of Tablets taken by the formula: in which the terms are defined therein.