Atomistic Insights into the Operation of an Electrospray Ionization Emitter: Dielectric Effects and Electrophoretic Charge Separation

Many operating principles of electrospray ionization (ESI) emitters have been uncovered, but some aspects are yet to be fully elucidated. One of these is the electrophoretic migration of dissolved ions in the solution near the emitter outlet. Also, it remains unclear to what extent the electric field generated by the ESI voltage penetrates the solution (keeping in mind that dielectric effects tend to suppress external fields). We performed molecular dynamics (MD) simulations of ESI emitters containing aqueous NaCl to tackle these topics. ESI in positive ion mode requires the accumulation of excess positive charge in the solution at the emitter outlet. The electric field between the emitter and counter electrode acts on this charge, producing a Taylor cone. We show that this excess charge has two contributions: (i) a polarization charge resulting from H 2 O dipole alignment and (ii) cation accumulation resulting from electrophoretic charge separation. Factors (i) and (ii) weaken the electric field in the solution, but a residual component remains that drives cation migration in the direction of the counter electrode. The residual field is strongest in the jet that forms at the Taylor cone apex. Electrophoretic movement of solvated cations in the jet promotes elongation and subsequent rupture of the jet, thereby producing ESI droplets. This work marks the first time that basic operating principles of an ESI emitter have been captured in MD simulations, using a model that employs mass spectrometry-relevant dimensions and realistic electrostatic properties.