Electrohydrodynamics of Gas-Assisted Electrospray Ionization Mass Spectrometry.

Gas-flow assistance is commonly used in ESI-MS for improved transport and desolvation, and fundamental understanding of the underlying phenomena is essential for improvement of aerodynamic interfaces that couple ESI sources and MS. For this purpose, an electrohydrodynamic model is developed for simulation of charged droplet dynamics under the combined effects of gas flow and electric fields with consideration of space charge interactions within the charged aerosol plume. The model is implemented in COMSOL by exploiting a formalism for establishing the droplet trajectories as a sequence of successive droplets ejected at a frequency defined by the electrospray current. The model is used to assess the effect of two distinct flow configurations and compared to the baseline care of electrospray without assist gas. The simulated flows are jet flows oriented coaxially with the ESI spray, with and without imposed vorticity (swirling). Droplet trajectory simulations of a bimodal droplet population consisting of large primary droplets and small progeny droplets reveal a unique capability for vortical assist jet flow to selectively transmit smaller droplets into the MS due to inertial separation. ESI-MS analysis of fluorinated phosphazines subjected to the different gas flow conditions supports the model predictions. The electrohydrodynamic model developed in this work provides a versatile tool to analyze and design aerodynamic ESI interfaces with rigorous incorporation of drag, inertia, and space-charge repulsion and can be used as a powerful simulation methodology for optimizing charged droplet transmission and ultimately improved analytical performance of gas-assisted ESI-MS workflows.