Exploring the Effects of Inelastic Collisions and Ion-Gas Energy Exchange on Electrical Mobility
CARLOS LARRIBA-ANDALUZ, Viraj Gandhi, IUPUI
Abstract Number: 562
Working Group: Aerosol Physics
Abstract
Recent advancements in ion mobility spectrometry (IMS), exemplified by systems like the Structure for Lossless Ion Manipulation (SLIM), now operate with unprecedented precision. This level of precision allows for ion separations that challenge existing theoretical frameworks. Understanding these separations often requires diving into complex algorithms and higher-order approximations to grasp how internal degrees of freedom affect mobility. Specifically, it requires a deeper understanding of how collisions with the gas affect the exchange of angular, linear, and internal momentum. To shed light on these intricate energy exchanges, we have developed the IMoS 2 software. This software closely mirrors experimental conditions and is based on complex kinetic gas theory rooted in statistical mechanics. Within this framework, ions are surrounded by a non-equilibrium (NVT) ensemble of gas molecules, interacting as they move through the medium under external forces. A recent addition to the software enables ion vibration using a custom-integrated MM3 forcefield. This advancement allows for the study of energy exchange among the various internal degrees of freedom of ions and gas. The internal velocity distributions (rotational and vibrational) can be obtained, and more importantly, their change with the electric field probed, which is akin to solving the WUB equation for all-atom systems and hence precise mobility shifts can be explored. Initially, due to collisions with the gas and consequent heating, the ion has a period of relaxation before attaining an internal equilibrium temperature akin to its translational temperature. This equilibrium temperature might never be reached for large ions and fast varying fields suggesting the appearance of a fourth temperature: one for the gas, two for translation (longitudinal and transversal) and one for the internal degrees of freedom. This fourth temperature might be a requirement for the study of larger molecular systems in varying fields to obtain accurate mobilities and correctly calibrate instruments.