Insight into Desorption Mechanisms in a Helium Low-Temperature Plasma Ionization Source Using Computational Simulations

Understanding desorption mechanisms is essential for the optimization of analytical techniques that enable the direct sampling and ionization of condensed-phase samples without preparation. The low-temperature plasma (LTP) ionization source, first described by Harper (Harper; et al. Anal. Chem. 2008, 80, 9097-9104) and based on the dielectric barrier discharge principle, is among the more representative and replicated plasma-based ambient desorption/ionization tools for mass spectrometry (MS), although there are a wide array of designs and configurations. However, the fundamental desorption mechanisms directly associated with LTP and other related plasma-based sources remain unclear. In this study, we utilized plasma simulations using COMSOL Multiphysics to understand analyte release from solid samples placed on glass and exposed to the plasma of a simplified helium LTP configuration. Our simulations revealed that the accumulation of surface charge on the sample-substrate that is caused by the plasma results in localized electric fields strong enough to likely aid in disruption of analyte-substrate interactions and facilitate desorption. Importantly, our model estimates that electrons in plasma have energies of approximately 2.5 eV, suggesting this simulated energy level is an indicator for desorption efficiency. Our findings provide new insight into the complex interplay between plasma-induced phenomena and desorption processes.