Square-wave voltammetry detection for capillary electrophoresis
Gerhardt, Geoff Charles
While the development and application of capillary electrophoresis (CE) has matured significantly in the last two decades, its application to trace analysis has been limited by The lack of a generally applicable, Sensitive detection method. This Ph.D. project's aim was to develop such a method using electrochemical detection. After an initial review of the work done in electrochemical detection (ECD) for CE, it was determined that of the system that had been report most had significant limitations for practical use. These systems were typically fragile and difficult to construct and were applicable to only electrochemically active analytes. Given this, a simplified and more robust CE-ECD instrumentation system was developed. Using this instrumentation, an ECD system based on square-wave voltammetry was developed (CE-SWV). Detection limits obtained with CE-SWV for neurotransmitters (150 nM, S/N = 3) were comparable to those obtained using more fragile and complex amperometric CE detection systems. While CE-SWV worked well for detection of electroactive analytes (redox processes), one of the reasons for investigating SWV was to determine whether it could detect other electrode processes such as analyte adsorption to develop a more generally applicable CE detection method. Using SWV, changes in the background electrolyte response were observed as organic analytes exited the capillary and interacted with a Pt electrode. Initially, for non-aromatic analytes, these changes appeared to result from physical adsorption of the analyte which "blocked" the hydrogen adsorption/reduction normally observed and produced negative electropherogram peaks. Using this as a detection method, reasonable detection limits (~1[mu]M, SN = 3) were obtained for analytes that did not have UV-chromophores. When this detection method was applied to analytes with [pi]-electron density (i.e. analytes with phenyl or carbonyl functionality), positive SWV responses were observed, indicating an additional chemisorption process was involved. Positive voltage pulses (400 mV for 5 ms) applied before the SWV scan were found to be necessary to elicit this positive response. It appeared that these positive voltage pulses oxidized the Pt electrode, possibly creating a reactive Pt-OH layer which catalyzed oxidation of these organic analytes which were then reduced during the negative-going SWV scan used. These positive voltage pulses also served to maintain a clean electrode surface to prevent electrode fouling--a common problem encountered in ECD techniques. This positive SW response, when plotted versus migration time, resulted in detection limits (5 nM, S/N = 3) over an order of magnitude better than UV detection for analytes, with good UV chromophores. This detection method, termed adsorption-based electrochemical detection (AdsECD), was demonstrated to be applicable to a wide variety of important organic analytes (over 50 organic analytes previously considered to be electrochemically inactive were detected) separated by CE using a variety of separation systems (i.e. separation systems with varied pH, ionic strength, micellar systems, added organic solvents and chiral selectors were investigated). Coupled with the rugged and easy-to-use CE-ECD instrumentation that was developed, AdsECD was shown to be a technique that should prove to be useful for a wide variety of practical chemical analysis problems.