A: Place a quantity of finely powdered Tablets, equivalent to about 40 mg of metoprolol tartrate, in a separator, add 25 mL of water and 4 mL of dilute ammonium hydroxide (1 in 3), and extract with 20 mL of chloroform, filtering the chloroform extract through chloroform-prerinsed anhydrous sodium sulfate. Evaporate the chloroform to dryness, and place in a freezer to congeal the residue: the IR absorption spectrum of a potassium bromide dispersion of the residue so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Metoprolol Tartrate RS.

B: Transfer a portion of finely powdered Tablets, equivalent to about 50 mg of metoprolol tartrate, to a 500-mL volumetric flask, dilute with water to volume, and mix. Pass a portion of this solution through a filter of 1 µm or finer porosity: the UV spectrum of the filtrate exhibits maxima and minima at the same wavelengths as that of a solution of USP Metoprolol Tartrate RS in water having a concentration of about 0.1 mg per mL.

C: The retention time of the metoprolol peak in the chromatogram obtained from the Assay preparation corresponds to that in the chromatogram of the Standard preparation as obtained in the Assay.

Medium: simulated gastric fluid TS (without enzyme); 900 mL.

Apparatus 1: 100 rpm.

Time: 30 minutes.

Procedure— Determine the amount of (C15H25NO3)2·C4H6O6 dissolved in filtered portions of the solution under test from UV absorbances at the wavelength of maximum absorbance at about 275 nm in comparison with a Standard solution having a known concentration of USP Metoprolol Tartrate RS in the same medium.

Tolerances— Not less than 75% (Q) of the labeled amount of (C15H25NO3)2·C4H6O6 is dissolved in 30 minutes.

(Official January 1, 2007)

Solvent mixture— Prepare a mixture of methanol and 0.1 N hydrochloric acid (1:1).

Mobile phase— Prepare a suitable and degassed solution by dissolving 961 mg of 1-pentanesulfonic acid sodium salt (monohydrate) and 82 mg of anhydrous sodium acetate in a mixture of 550 mL of methanol and 470 mL of water and adding 0.57 mL of glacial acetic acid.

Standard preparation— Dissolve an accurately weighed quantity of USP Metoprolol Tartrate RS in Solvent mixture to obtain a stock solution having a known concentration of about 1000 µg per mL. Transfer 25.0 mL of this solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Resolution solution— Prepare a solution of oxprenolol hydrochloride in Solvent mixture to obtain a solution containing about 720 µg per mL. Prepare a 1:1 mixture of this solution and the stock solution used to prepare the Standard preparation.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 50 mg of metoprolol tartrate, to a 50-mL volumetric flask, add 30 mL of Solvent mixture, shake by mechanical means for 30 minutes, sonicate for 15 minutes, and heat on a steam bath for 10 minutes. Allow the solution to cool to room temperature, dilute with Solvent mixture to volume, and mix. Centrifuge a portion of the solution, and transfer 25.0 mL of the supernatant to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. Pass a portion of this solution through a filter of 0.5 µm or finer porosity, discarding the first few mL of the filtrate.

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 1 mL per minute. Chromatograph the Resolution solution, and record the peak responses as directed under Procedure: the relative retention times are about 0.8 for metoprolol and 1.0 for oxprenolol, and the resolution, R, between the metoprolol and oxprenolol peaks is not less than 2.0. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 30 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 metoprolol tartrate [(C15H25NO3)2·C4H6O6] in the portion of Tablets taken by the formula: in which C is the concentration, in µg per mL, of USP Metoprolol Tartrate RS in the Standard preparation; and rU and rS are the metoprolol peak responses obtained from the Assay preparation and the Standard preparation, respectively.