Electrical Equivalence of Electrospray Ionization with Conducting and Nonconducting Needles

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Abstract
An electrical equivalent circuit is derived for the electrospray process. It is a series circuit which consists of the power supply, the electrochemical contact to the solution, the solution resistance (Rs), a constant-current regulator which represents the processes of charge separation and charge transport in the gap between the spray needle aperture and the counter electrode, and charge neutralization at the counter electrode. A current i, established by the constant-current regulator flows throughout the entire circuit. Current−voltage curves are developed for each element in the circuit. From these it is shown that in the case where Rs is negligible (the power supply is connected directly to a conducting needle) the shape of the current−voltage curve is dictated by the constant-current regulator established by the charge separation process, the gap, and the counter electrode. The solution resistance may be significant if a nonconducting needle is used so that the electrochemical contact to the solution is remote from the tip. Experiments with a nonconducting spray needle quantify the effect of the solution resistance on the current−voltage curve. Subtracting the iRs voltage from Vapp (power supply voltage) yields the current−voltage curve for the constant-current regulator. When iRs drop is a significant fraction of Vapp, the current−voltage curve of the constant-current regulator is changed substantially from the case when the solution resistance is negligible.