Measurement of anticonvulsants in serum by reversed-phase ion-pair liquid chromatography

We describe a method for the simultaneous liquid-chromatographic determination of ethosuximide, ethylphenacemide, primidone, phenobarbital, carbamazepine, and phenytoin in serum. The drugs, together with an internal standard, are extracted into ethyl acetate at pH 7.0. The extract is analyzed isocratically at ambient temperature on a reversed-phase column of "SAS Hypersil" with a mobile phase of acetonitrile/tetrabutyl ammonium phosphate solution (2/8 by volume). The eluted drugs are detected by their absorption at 200 nm, and quantitated from their peak heights as compared with those of extracted standards. The day-to-day CV of the method varied between 5.1 and 9.6% for concentrations ranging from less than therapeutic to toxic. The results, when compared with those by gas chromatography, gave correlation coefficients of 0.936 for phenytoin, 0.977 for phenobarbital, and 0.939 for primidone. No drug interference was found except that amobarbital and ethylphenacemide co-chromatographed.