NOVEL COMPUTATIONAL SIMULATION OF REDOX REACTIONS WITHIN A METAL ELECTROSPRAY EMITTER

To further both our fundamental understanding implications of the electrolytic nature of the electrospray and our understanding of the analytical ion source, in the context of electrospray mass spectrometry (ES-MS), a computational simulation of the oxidation of chemical species inside a metal emitter 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. The specific system modeled consisted of a 100 {mu}m i .d., inert metal capillary CHICN/H2O (90/10 V/V). ES emitter and a spray solution comprised of an analyte dissolved in Variable parameters included the concentration (i.e., 5, 10, 20, and 50 ~M) of the easily oxidized analyte ferrocene (Fe, dicyclopentadienyl iron) in the solution, and solution conductivities of 1.9, 3.8, and 7.6 x 107 Mho/cm. ES currents were on the order of 0.05 {mu}A and the flow rate was 5 @A_nin. Under these defined conditions, the two most prominent reactions at the emitter metakolution interface were assumed to be H20 oxidation (2H20 = 02 + 4H+ + 4e") and Fe oxidation (Fe = Fe' +e-). Using this model it was possible to predict the inter-facial 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. Computational fluid dynamics (CFD) calculations showed that the imposed flow rate through the emitter was adequate to prevent significant back-diffusion of Fe+ into the emitter against the flow direction. The computational simulations predict the same behavior for the ES ion source as has been observed experimentally and is consistent with the controlled-current electrolytic cell analogy of Van Berkel and Zhou (Anal. Chem. 1995, 67,.2916-2923). Furthermore, the simulations demonstrate that the majority of the current involved in the redox reactions originated within a 200- 300 ~m region near the spray tip