A: Mix a portion of the contents of Capsules, equivalent to about 200 mg of methsuximide, with 25 mL of water in a separator, extract with 50 mL of ether, and discard the aqueous layer. Wash the ether extract with 25 mL of water, and discard the water. Filter the extract, evaporate with the aid of a current of warm air to dryness, and dry the methsuximide over phosphorus pentoxide for 16 hours: the residue so obtained responds to Identification test A under Methsuximide.

B: The retention time exhibited by methsuximide in the chromatogram of the Assay preparation corresponds to that of methsuximide in the chromatogram of the Standard preparation as obtained in the Assay.

Medium: water; 900 mL.

Apparatus 1: 100 rpm.

Time: 120 minutes.

Procedure— Determine the amount of C12H13NO2 dissolved, employing the procedure set forth in the Assay, making any necessary modifications.

Tolerances— Not less than 75% (Q) of the labeled amount of C12H13NO2 is dissolved in 120 minutes.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of water and acetonitrile (55:45). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Methsuximide RS in Mobile phase to obtain a solution having a known concentration of about 0.6 mg per mL.

Assay preparation— Place 10 Capsules in a 500-mL volumetric flask, and add 280 mL of water. Sonicate in a water bath at 40 to 50, with occasional shaking, until the Capsules have broken, and cool to room temperature. Dilute with acetonitrile to volume, mix, and filter. Transfer an accurately measured volume of this specimen solution, equivalent to about 30 mg of methsuximide, to a 50-mL volumetric flask, 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 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1 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 2100 theoretical plates, and the relative standard deviation for 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. Calculate the quantity, in mg, of methsuximide (C12H13NO2) per Capsule taken by the formula: in which C is the concentration, in mg per mL, of USP Methsuximide RS in the Standard preparation; V is the volume, in mL, of specimen solution taken for the Assay preparation; and rU and rS are the methsuximide peak responses obtained from the Assay preparation and the Standard preparation, respectively.