Assessment of coulometric array electrochemical detection coupled with HPLC-UV for the absolute quantitation of pharmaceuticals
The use of a coulometric array detector in tandem with HPLC-UV was evaluated for the absolute quantitation of pharmaceutical compounds without standards, an important capability gap in contemporary pharmaceutical research and development. The high-efficiency LC flow-through electrochemical detector system allows for the rapid evaluation of up to 16 different potentials, aiding in the identification and quantitation of electrochemically reactive species. By quantifying the number of electrons added or removed from an analyte during its passage through the detector, the number of moles of the analyte can be established. Herein we demonstrate that molecules containing common electroactive functional groups (e.g. anilines, phenols, parabens and tertiary alkyl amines) can in some cases be reliably quantified in HPLC-EC-UV without the need for authentic standards. Furthermore, the multichannel nature of the CoulArray detector makes it well suited for optimizing the conditions for electrochemical reaction, allowing the impact of changes in potential, flow rate, temperature and pH to be conveniently studied. The electrochemical oxidation of albacivir, zomepirac, diclofenac, rosiglitazone and several other marketed drugs resulted in large linear ranges, predictable recoveries and excellent quantitation using the total moles of electrons and back-calculating using Faraday's law. Importantly, we observed several instances where subtle structural changes within a given class of molecules (e.g. aromatic ring isomers) led to unanticipated changes in electrochemical behavior. Consequently, some care should be taken when applying the technique to the routine quantitation of compound libraries where standards are not available.