Analysis of Six Anticonvulsant Drugs using Solid-Phase Extraction, Deuterated Internal Standards, and Gas Chromatography-Mass Spectrometry

A rapid method for simultaneously determining the anticonvulsant drugs carbamazepine, ethosuximide, phenobarbitone, phenytoin, primidone, and valproic acid is described. Blank plus single-point calibration gives reliable quantitation from therapeutic to high fatal concentrations, except for ethosuximide, for which it gives semiquantitative results. Whole blood and liver tissue samples containing deuterated internal standards were extracted using Bond Elut Certify columns. Butyl derivatives were formed using n-iodobutane and TMAH under mild conditions and were extracted into ethyl acetate as a cleanup step. Recoveries were greater than 50%, except for valproic acid (42%). Sample preparation time was less than 2 h, and the GC run time was less than 20 min per injection. At least two ion pairs formed by electron impact ionization were monitored for each drug. Intraday CVs were less than 6.28% (4.20%) and interday CVs less than 14.1% (for midtherapeutic concentrations in blood [liver], except for ethosuximide). Linearity was observed from subtherapeutic to high fatal levels for all drugs. This method has been applied to forensic cases and has significantly reduced analytical time while improving case-work quality. Results of a case study involving anticonvulsant drugs are given.

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