Identification—
A:
Infrared Absorption 
197S
—
Spectral range:
3000 cm–1 to 1650 cm–1.
Test solution—
Transfer a portion of Oral Solution, equivalent to 150 mg of ethosuximide, to a 125-mL separatory funnel, add 50 mL of ether, and shake well. Allow the layers to separate, and retain the ether layer. Wash the ether layer with three 10-mL portions of water. Transfer the ether layer to a suitable beaker, add 5 g of anhydrous sodium sulfate, and swirl. Filter the mixture into a 50-mL volumetric flask through a small pledget of cotton that has been previously washed with ether. Evaporate to dryness. Dissolve the residue in 5 mL of chloroform.
B:
The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.
Limit of 2-ethyl-2-methylsuccinic acid—
Mobile phase, System suitability solution, and Chromatographic system—
Proceed as directed in the
Assay under
Ethosuximide Capsules.
Standard solution—
Dissolve an accurately weighed quantity of 2-ethyl-2-methylsuccinic acid in Mobile phase, and dilute quantitatively, and stepwise if necessary, to obtain a solution having a known concentration of about 0.05 mg per mL.
Test solution—
Using a “to contain” pipet, transfer a volume of Oral Solution, equivalent to 250 mg of ethosuximide, to a 100-mL volumetric flask. Rinse the pipet several times with Mobile phase, dilute with Mobile phase to volume, and mix.
Procedure—
Separately inject equal volumes (about 10 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 percentage of 2-ethyl-2-methylsuccinic acid, in relation to the quantity of ethosuximide, in the volume of Oral Solution taken by the formula:
40C(rU / rS),
in which
C is the concentration, in mg per mL, of 2-ethyl-2-methylsuccinic acid in the
Standard solution; and
rU and
rS are the peak responses for 2-ethyl-2-methylsuccinic acid obtained from the
Test solution and the
Standard solution, respectively: not more than 2.0% is found.
Assay—
Mobile phase, System suitability solution, Standard preparation, and Chromatographic system—
Proceed as directed in the
Assay under
Ethosuximide Capsules.
Assay preparation—
Transfer an accurately measured volume of Oral Solution, equivalent to about 250 mg of ethosuximide, to a 100-mL volumetric flask. Dilute with Mobile phase to volume, and mix. Transfer 5.0 mL of this solution to a 200-mL volumetric flask, dilute with Mobile phase to volume, and mix.
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. Calculate the quantity, in mg, of ethosuximide (C
7H
11NO
2) in the volume of Oral Solution taken by the formula:
4000C(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Ethosuximide RS in the
Standard preparation; and
rU and
rS are the ethosuximide peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
