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Development of electrochemical detection for capillary electrophoreses



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The main purpose of this work was to develop reliable and sensitive electrochemical (EC) detection for capillary electrophoresis. To achieve this goal, electrochemical behavior of the electrolyte and analytes and the factors affecting the behavior were studied by on-line cyclic voltammetry (CV) to understand electrode response. The parameters investigated included adsorption of organic electrolytes, deposition of metal ions, and H$\sp+$ and O2 reactions. The results showed that the adsorption of organic compounds and deposition of some metals changed the electrode surface, which caused changes of electrochemical processing. The first EC approach studied was pulsed amperometric detection (PAD) with the potential frequency of 5 to 8 Hz at both Au and Pt 25 μm disk electrodes. To optimize PAD detection, pulse duration and applied potential were examined for both cathodic deposition and anodic stripping detection. Under the optimal conditions PAD can offer reliable detection for spiked and snow samples, with detection limits in the range of $2.010\sp{-7}$ to $2.010\sp{-5}$ mol/L, and the response factors were constant to within ±5% over the range of 50 μmol/L to 1000 μmol/L. To improve PAD detectability in CE, a number of experimental parameters were evaluated. These parameters included sample stacking, potential waveform shape and frequency, and signal analysis technique. Of these parameters, the use of a multiple-step pulse waveform gave a maximum S/N enhancement up to 10 fold, and the second harmonic response by means of Fourier transformation corrected baseline shifts. The next EC technique investigated was a fast-scan CV detection over sweep rates of 20 to 1,000 V/s at Au and Pt disk electrodes (25 and 10 μm). The results showed that maximum response was obtained at sweep rates of 100 to 200 V/s; however sweep rates of $>$400 V/s caused peak tailing. With this approach, co-migrating analytes could be identified and quantified. Response factors for metal ions over the range $1.010\sp{-7}$ to $1.010\sp{-5}$ mol/L were 15%, and detection limits were in the range of $510\sp{-9}$ mol/L to $410\sp{-8}$ mol/L, which are one to two order of magnitude better than results obtained previously with pulsed amperometric detection.





Doctor of Philosophy (Ph.D.)







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