A study of irreversible electrode processes under steady-state flowing solution conditions with a microelectrode based scanning electrochemical detector

A study of the irreversible oxidation of l-tyrosine and a series of catecholamines has been undertaken at a microelectrode under both flow analysis and reversed-phase liquid chromatography (LC) conditions. Experimental parameters (electrode radius 5 μm, flow rate 1.0 ml min−1, scan rate in the range 1–2 V s−1) were chosen so that the complete current-voltage curve could be obtained via the use of fast potential scan rates, but still under steady-state or near steady-state conditions. This feature means that unlike peak currents associated with transient techniques, the limiting current achieves the ideal analytical characteristic of being relatively insensitive to kinetic terms. The individual voltammograms obtained from flow analysis or LC experiments exhibit the characteristics of the non-reversible response expected for l-tyrosine and the catecholamines with the expected changes to both the half-wave potential and the shape of the voltammograms being observed as the eluant pH was adjusted from 2.0 to 7.0. Interestingly, it is shown that the irreversibly oxidised catecholamines, l-adrenaline and 6-hydroxy-l-dopa which are poorly resolved chromatographically may be resolved in the electrochemical domain under fast scan rate, but near steady-state conditions achievable at microelectrodes in flowing solution.