Voltammetry in supercritical CO(2) and in the rat striatum using microelectrode devices

The use of voltammetric microelectrodes in highly-resistive CO{dollar}sb2{dollar} and in the striatum of anesthetized rat brain is described. Chapter I characterizes the electro-enzymatic reduction of hydrogen peroxide in subcritical CO{dollar}sb2{dollar}. This work shows for the first time that CO{dollar}sb2{dollar} is able to support redox-mediated enzymatic catalysis on an electrode surface. The results indicate a higher apparent rate constant versus conventional aqueous solvents, presumably because of the high diffusivity of CO{dollar}sb2{dollar}. Chapter 2 describes the voltammetry of anthracene in CO{dollar}sb2{dollar} at subcritical and supercritical pressures. Carboxylation of the anion radical of anthracene by CO{dollar}sb2{dollar} is observed at a reaction rate at least as fast as in organic solvents. Taken together, Chapters 1 and 2 demonstrate the potential for developing electrosynthetic procedures in CO{dollar}sb2{dollar}.; Chapters 3 and 4 describe the design, development, and application of an electrochemical detector for supercritical fluid chromatography (SFC). The detector consists of platinum working and counter electrodes coated with a thin film of a conductive poly(ethylene oxide)-lithium triflate mixture. The combination of microelectrodes and poly(ethylene oxide)-based films enables the effects of the high solution resistance of CO{dollar}sb2{dollar} to be overcome. The detector is used in the analysis of EPA standard mixtures of phenols and polyaromatic hydrocarbons. The detector exhibits compatibility with unmodified and modified CO{dollar}sb2{dollar} mobile phases and detection limits in the low nanogram range for packed column separations. These results demonstrate that the detector is a valuable new tool for users of SFC.; Chapter 5 describes in vivo, voltammetric studies of dopamine using multi-electrode devices. Independently-addressable, two- and four-electrode devices have been made to have individual tip diameters of 1-2 {dollar}mu{dollar}m and tip-to-tip spacings of 20 {dollar}mu{dollar}m or less. These devices have been used to test the hypothesis that extracellular dopamine concentrations during stimulated release are heterogeneous over a spatial scale of micrometers. Data obtained with two-electrode devices support this hypothesis. In 18% of the locations monitored with two-electrode devices, clear differences in either the amplitudes or the temporal shapes of the two responses have been observed. These results suggest strongly that micro-scale heterogeneities of the dopamine fields exist.