Investigation of Surface-Induced Dissociation Processes via Molecular Dynamics Simulations of Wall Collisions of Large Droplets Produced by Electrospray Ionization.

Electrospray ionization is one of the most utilized ionization techniques in atmospheric pressure mass spectrometry. Recent experimentally reported results are in disagreement with fundamentals revolving around ESI droplet sizes and their lifetimes. Specifically, much larger droplet sizes and longer lifetimes have been experimentally observed to exist in typical ESI ion sources. Experiments involving a custom scan mode on a triple quadrupole system have shown that high-mass fragments of large ESI droplets can be observed in mass spectra. Initial hypotheses rationalizing these results were focused on the creation of droplet fragments by collision-induced dissociation (CID). The collision energy accumulated by CID is most likely too small to lead to the observed mass spectra. In response, surface-induced dissociation (SID) was proposed as an additional mechanism to provide large amounts of collision energy to the droplets. The present work thus investigates the possible fragmentation pathways and dynamics of droplet fragments resulting from aspirated ESI droplets upon surface collisions through classical molecular dynamics simulations. Different types of collisions are simulated, where the impact of the simulated droplet fragments is either frontal or angled. The resulting fragmentation dynamics are thoroughly analyzed, showing the possibility for charged fragments to be liberated through SID events. A second, much larger droplet fragment is employed to illustrate the altered collision dynamics found for such larger aggregates, where no charged clusters are released through the surface collision. Since approximated force fields have to be used to model the interactions between the particles observed in the simulation, a sensitivity study is carried out regarding the critical parameters governing such processes. Further modifications of the MD system have to be carried out, including more realistic walls and much larger ESI droplets, to clarify the possibility of charged fragment releases from larger droplet fragments through SID.