Electrical and Thermal Properties of Double-Barreled Ultra Microelectrodes

An attempt is made to provide a deeper understanding, based on physical principles, of the factors that govern (and may limit) use of double-barreled ultra microelectrodes. Equations are developed which relate the geometry and material properties inside and outside the electrodes to their resistance and capacitance parameters in appropriate equivalent circuits. These equations show what to expect of the microelectrode and the direction and amount of change required to improve its performance in a particular application. It is shown that glass leakage resistance between barrels is of the order of 1013 ohms, but this value creates an error of one percent or less in a measurement of coupling resistance in conventional applications. Temperature changes due to heating by the current inside and just outside the tip become important at current levels above one microampere, and combinations of current and medium resistivity are shown which would produce boiling. The time for the fluid around the tip opening to reach steady-state temperatures is calculated to be significantly less than a microsecond.