Test specimen— Transfer one or more Tablets, equivalent to about 200 mg of metoprolol succinate, to a stoppered centrifuge tube. Add about 40 mL of pH 6.8 phosphate buffer (see Buffer Solutions in the section Reagents, Indicators, and Solutions) and 40 mL of methylene chloride, and shake for 5 minutes. Centrifuge, filter, and use the aqueous phase as the Test solution. Transfer 3 mL of the Test solution to a separator, add 2 mL of ammonium hydroxide, and extract with 20 mL of methylene chloride. Filter the methylene chloride phase. Grind 1 mL of the filtrate with 300 mg of potassium bromide, dry in a current of warm air, and prepare a disk: the IR spectrum of the Test specimen exhibits maxima only at the same wavelengths as that obtained from a similar preparation of USP Metoprolol Succinate RS (presence of metoprolol).

Test specimen— Transfer 5 mL of the Test solution prepared as directed for Identification test A to a glass-stoppered test tube, add 2 mL of 5 N hydrochloric acid, and extract with 5 mL of ether. Filter the ether phase. Grind 2 mL of the filtrate with 300 mg of potassium bromide, dry in a current of warm air, and prepare a disk: the IR spectrum of the Test specimen exhibits maxima only at the same wavelengths as that obtained from a similar preparation of succinic acid (presence of succinate).

Time (hours)	Amount dissolved
1	not more than 25%
4	between 20% and 40%
8	between 40% and 60%
20	not less than 80%

Time (hours)	Amount dissolved
1	not more than 25%
4	between 20% and 40%
8	between 40% and 60%
20	not less than 80%

pH 3.0 Phosphate buffer— Mix 50 mL of 1 M monobasic sodium phosphate and 8.0 mL of 1 M phosphoric acid, and dilute with water to 1000 mL. If necessary, adjust with 1 M monobasic potassium phosphate or 1 M phosphoric acid to a pH of 3.0.

Mobile phase— Prepare a filtered and degassed mixture of pH 3.0 Phosphate buffer and acetonitrile (375:125). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Dissolve a quantity of USP Metoprolol Succinate RS, accurately weighed, in Mobile phase to obtain a solution having a known concentration of about 0.05 mg per mL.

Test stock solution— Transfer 1 Tablet, accurately weighed, to a volumetric flask of suitable capacity to obtain a solution having a concentration of about 1 mg per mL of metoprolol succinate. Add about 5 mL of water, and allow the Tablet to disintegrate. Add a volume of alcohol such that when diluted to volume, the concentration of alcohol is 30%. Shake for 30 minutes. Add a portion of 0.1 N hydrochloric acid equivalent to about one half of the flask volume, and shake for 30 minutes. Dilute with 0.1 N hydrochloric acid to volume, and mix.

Test solution— Filter the Test stock solution, and discard the first 10 mL of the filtrate. Dilute the filtrate quantitatively with Mobile phase to obtain a solution containing about 0.05 mg per mL of metoprolol succinate.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4-mm × 12.5-cm column that contains packing L7. The flow rate is about 1 mL per minute. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 40 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of metoprolol succinate (C15H25NO3)2·C4H6O4 in the Tablet taken by the formula: in which C is the concentration, in mg per mL, of USP Metoprolol Succinate RS in the Standard solution; V is the volume of the Test stock solution used to prepare the Test solution; and rU and rS are the peak responses obtained from the Test solution and the Standard solution, respectively.