Use of a microplate scintillation counter as a radioactivity detector for miniaturized separation techniques in drug metabolism

In miniaturized separation techniques, such as capillary electrophoresis (C E) or capillary liquid chromatography (LC), conventional on-line radioactiv ity detection of labeled compounds is restricted, because of insufficient s ensitivity. It will be shown that a microplate scintillation counter for 96 -well plates (TopCount) can be used as a sensitive and easy-to-handle radio activity detector for capillary Lc and CE. The attractive combination of ca pillary LC, eluent fractionation, and subsequent off-line counting is descr ibed. The new method is applied for rapid and sensitive separation and dete ction of H-3-labeled parent drug and its metabolites at levels between 25 a nd 700 cpm in rat urine. The advantages of capillary LC coupled to the TopC ount, and combined with LC-MS data, can be of benefit in many analytical ar eas, including the characterization of metabolites at low concentration wit hin complex biological fluids, With the same setup, the fractionation with subsequent off-line counting is equally applicable to CE, This is demonstra ted with electrophoretically separated C-14-labeled impurities, nicely reso lved from a negatively charged main compound, at low levels.