Identification—
Place a volume of Injection, equivalent to about 40 mg of metoprolol tartrate, in a separator, add 4 mL of dilute ammonium hydroxide (1 in 3), and extract with 20 mL of chloroform, filtering the chloroform extract through chloroform-pre-rinsed 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.
Assay—
Mobile phase—
Prepare a 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.
Sodium chloride solution—
Dissolve 9.0 g of sodium chloride in water to make 1000 mL.
Standard preparation—
Dissolve an accurately weighed quantity of
USP Metoprolol Tartrate RS in
Sodium chloride solution to obtain a stock solution having a known concentration of about 1000 µg per mL. Mix equal volumes, accurately measured, of this stock solution and of
Internal standard solution.
Assay preparation—
Dilute an accurately measured volume of Injection, if necessary, quantitatively with Sodium chloride solution to obtain a stock solution having a concentration of about 1000 µg per mL. Mix equal volumes, accurately measured, of this stock solution and of Internal standard 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 1 mL per minute. Chromatograph three replicate injections of the
Standard preparation, and record the peak responses as directed under
Procedure: the relative standard deviation is not more than 2.0%, and the resolution factor between metoprolol tartrate and oxprenolol hydrochloride is not less 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 responses for the major peaks. The relative retention times are about 0.8 for metoprolol tartrate and 1.0 for oxprenolol hydrochloride. Calculate the quantity, in mg, of metoprolol tartrate [(C
15H
25NO
3)
2·C
4H
6O
6] in each mL of the Injection taken by the formula:
(L / D)(C)(RU / RS),
in which
L is the labeled quantity, in mg, of metoprolol tartrate in the Injection;
D is the concentration, in µg per mL, of metoprolol tartrate in the
Assay preparation, on the basis of the labeled quantity in each mL of Injection taken and the extent of dilution;
C is the concentration, in µg per mL, of
USP Metoprolol Tartrate RS in the
Standard preparation; and
RU and
RS are the peak response ratios of metoprolol tartrate to oxprenolol hydrochloride obtained from the
Assay preparation and the
Standard preparation, respectively.
