A sensitive gas chromatographic/mass spectrometric method for the resolution and quantification of ethosuximide enantiomers in biological fluids

A modified specific, sensitive and reproducible chiral gas chromatographic (GC) method for the resolution and quantification of ethosuximide enantiomers in urine and plasma was developed. The samples were extracted by liquid–liquid extraction, using diethylether and the enantiomers were separated and quantified on a chiral gas chromatographic column (25QC2/CYDEX-β 0.25). The method involved the use of GC/MS instrumentation for the acquisition of data in the electron impact selective-ion monitoring mode, collecting ions characteristic of both ethosuximide and α,α-dimethyl-β-methylsuccinimide, the internal standard and of mass-to-charge ratio (m/z) exactly equal to 55 and 70 units. The limit of quantitation of the method was 2.5 μg/ml for both urine and plasma with both enantiomers. The method proved to be linear, precise and reproducible in the 5–300 μg/ml concentration range for urine samples and in the 10–250 μg/ml concentration range for plasma samples. Future research work envisaged the application of this method in pharmacokinetic and pharmacodynamic studies.