Flowing solutions voltammetry and current measurements

Because the current that flows at a UME is extremely small (10 pA-100 nA), we can keep the effect of the iT-drop very small and can measure voltammograms even in solutions of high resistance, i.e. in solutions without supporting electrolyte [69] and in solutions of nonpolar solvents like benzene, toluene and hexane [70]. UMEs have also been used for voltammetry in the gaseous phase [71] and in supercritical fluids [72]. 

This section will explore the quantitative behavior of current-potential curves (voltammetry) and steady-state current measurements in flowing solution prior to considering liquid chromatographic assays. A similar examination of quiet solutions is reserved for the section on in vivo measurements. (In this and all subsequent voltammetry discussions, only oxidation reactions are treated, not because reductions are unimportant, but because there are to date few, if any, neurochemical applications.) 

Cyclic voltammetry is one such electrochemical technique which has found considerable favour amongst coordination chemists. It allows the study of the solution electron-transfer chemistry of a compound on the sub-millisecond to second timescale it has a well developed theoretical basis and is relatively simple and inexpensive. Cyclic voltammetry is a controlled potential technique it is performed at a stationary microelectrode which is in contact with an electrolyte solution containing the species of interest. The potential, E, at the microelectrode is varied linearly with time, t, and at some pre-determined value of E the scan direction is reversed. The current which flows through the cell is measured continuously during the forward and reverse scans and it is the analysis of the resulting i—E response, and its dependence on the scan rate dE/dt, which provides a considerable amount of information. Consider, for example, the idealized behaviour of a compound, M, in an inert electrolyte at an inert microelectrode (Scheme 1). 

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