Computational Simulation of Redox Reactions within a Metal Electrospray Emitter

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Abstract
A computational simulation of the oxidation of chemical species inside a metal emitter electrospray ion source, in the context of electrospray mass spectrometry (ES-MS), has been developed. The analysis code employs a boundary integral method for the solution of the Laplace equation for the electric potential and current and incorporates standard activation and concentration polarization functions for the redox-active species in the system to define the boundary conditions. This paper provides a demonstration of the capability of this simulation method. Due to the approximate nature of some of the input data, and certain simplifying assumptions, the present results must be considered semiquantitative. The specific system modeled consisted of a 100-μm-i.d., inert metal capillary ES emitter and a spray solution composed of an analyte dissolved in CH3CN/H2O (90/10 v/v). Variable parameters included the concentration (i.e., 5.0, 10, 20, and 50 μM) of the easily oxidized analyte ferrocene (Fc, dicyclopentadienyl iron) in the solution, and solution conductivities of 1.9, 3.8, and 7.6 × 10-7 Ω-1/cm, with an operational flow rate of 5.0 μL/min and ES currents on the order of 0.05 μA. Under these defined conditions, the two most prominent reactions at the emitter metal/solution interface were assumed to be H2O oxidation (2H2O = O2 + 4H+ + 4e-) and ferrocene oxidation (Fc = Fc+ + e-). Using this model, it was possible to predict the interfacial potentials, as well as the current density for each of the reactions, as a function of axial position from the emitter spray tip back upstream, under the various operational conditions. The simulations show that the majority of the current from the redox reactions is generated within a 200−300-μm region near the spray tip. The lower the value of E0 for a specific reaction, the further upstream from the tip the reaction extends.