October 2020 Virtual Meeting

Speaker: Carlos Larriba-Andaluz, Indiana University–Purdue University Indianapolis

Topic: Understanding Ion Mobility Separation in High-Resolution Instruments. Caveats of and deviations from the Mason-Schamp Equation for small molecules

Date: Monday, October 19th, 2020

Time: 1:00 pm Presentation

Location: Webinar – see emails on Oct. 8 and 15 for invite link. Join the mailing list

Abstract: Ion mobility Spectrometry (IMS) is an analytical tool that has recently carried a great deal of interest in the field of Analytical Chemistry. As such, IMS is now ubiquitously present as an integrated part of many MS systems. This has resulted in major improvements in experimental setups, with very recent impressive achievements in terms of separation and peak capacity, alongside instruments like the Structure for Lossless Ion Manipulation (SLIM) or Field Asymmetric Waveform IMS (FAIMS) that have shown the capability of separating even isotopomers. Its progress is so remarkable that the theoretical ground commonly used to describe IMS has become insufficient to explain some experimentally observed separations; with some interpretations of this high-resolution separations remaining merely speculative at this point. It is therefore necessary to carefully analyze our common theoretical tools and describe the simplifications employed to arrive at equations such as the Mason-Schamp approximation so that a clear picture is provided on when these estimations may be employed. An effort is made to provide a concise and simple explanation of the simplifications that result on the Mason-Schamp equation. This is done from a method of moments perspective (up to the two-temperature theory) with an estimate of the error for different types of approximations. Based on this knowledge, a numerical tool, IMoS 2, that models an ion in a physical gas with the ability to stochastically calculate the drift velocity for a rotating ion under an arbitrary field, is proposed that may explain some of these separations recently observed.
Three main issues have been addressed in this work: 1) how the Collision Cross Section (CCS) is calculated and when its validity may be compromised due to rotation, moment of inertia or dipole alignment and other influences, 2) the effect of the electric field; when is it safe to ignore and when it should be included, and 3) the influence of higher order corrections even for low fields and small masses. The main idea of these theoretical study and simulations is to ascertain the resolution required in an IMS instrument for any of these effects to be noticeable, which will establish whether or not an empirically observed separation is due to some of these influences, and if it is, to perhaps improve instrument separation by underlining the cause. Preliminary analysis of the equations show that the Mason-Schamp approximation can have up to 4-10% deviations from the exact mobility values at low fields of spherical ions with particular emphasis on very small ions. Part of the deviations may be halved by choosing suitable ion-potential parameters, but this solution is dubiously the most promising way, as the authentic influence of the potentials and higher order effects is obscured. When increasing the field, a shift in mobility starts to be visible even below 8-10 Townsends and can represent up to a deviation of 15-30% for small ions in systems with high field to density ratios. The effect becomes larger for ions with existing dipole moments. The isotopomer separation effect is shown to be tied to differences in the moment of inertia producing shifts in mobility through the speed of rotation of the ion in the gas field. This explanation is supported by experimental results.