Reevaluating the Role of Polarizability in Ion Mobility Spectrometry

Abstract

With the expanding commercial availability of gasphase separation systems that incorporate gas-phase mobility, there is a concurrent rise in efforts to cast the gas-phase mobility coefficient in terms of an ion-neutral collision cross-section (CCS). The motivating factors for this trend are varied, but many aim to complement experimental results with computationally generated CCS values from in silico structural approximations. Unfortunately, the current paradigm for relating experimental mobility results to computationally derived structures relies upon empirical approaches, including a myriad of variables that do not realistically bound the comparison. In this Critical Insight, we advocate for the development of a self-consistent experimental and computational framework that uses laboratory results to constrain the scope of the modeling effort. This paper aims to prompt discussion, challenge assumptions, and promote the development of more efficient, accurate computational techniques within the gas-phase ion measurement community. Specifically, we postulate whether experimental deviations from Langevin's polarization limit (K pol ) are suitable to estimate the relative contributions of hard-sphere collisions and long-range interactions within CCS values. Not surprisingly, different molecule classes exhibit different trends in the K/K pol ratio when normalized for reduced mass, and the most common IMS calibrants (e.g., tune mix, polyalanine, tetraalkylammonium salts) follow different polarizability trends than many of the analytes probed in the literature. Succinctly, if gas-phase ion structure is largely invariant based upon the colliding neutral and newly developed experimental efforts can quantitatively capture ion polarizability, then modeling efforts describing a target analyte must be self-consistent as the collision neutral is changed in silico.

Collision Cross-Section Measurements of Collision-Induced Dissociation Precursor and Product Ions in an FTICR-MS and an IM-MS: A Comparative Study

“…This produces momentum transfer collision integrals, Ω. At these lower collision energies, long-range interactions are important, producing Ω that are fundamentally larger than σ and requiring both careful interpretation and more extensive computational modeling . Generally, it appears that comparing general CRAFTI and IM measurements by ratio is a viable way to compare the results.…”

Propagating Error through Traveling-Wave Ion Mobility Calibration

“…If the exact molar fractions of the two gases are known, then they could be accounted for using Blanc’s law. A recent publication suggests that, with knowledge of mobilities in neat gases, Blanc’s law can be applied retrospectively . However, in the Synapt G2-S, we do not know the exact molar ratio of the two gases.…”

“…A recent publication suggests that, with knowledge of mobilities in neat gases, Blanc's law can be applied retrospectively. 28 However, in the Synapt G2-S, we do not know the exact molar ratio of the two gases. Though we anticipate that the gas mixture adds uncertainty, Gabelica et al suggests that the uncertainty may be minimal.…”

“…Monatomic (e.g., He) and diatomic drift gases (e.g., N 2 ) have differences in polarizability, which affect analyte mobility due to changes in long-rage interactions. − Equation also assumes that the drift gas is not a mixture, which affects μ . If the user chooses to utilize a mixed buffer gas, K can be corrected by incorporating the mole fraction of the gases. , However, for some instruments, such as the Waters Synapt G2-S, unintentional mixing occurs in which the ratio of gases is not known, preventing K from being corrected. While these assumptions are accepted to calculate CCS values with the Mason–Schamp equation, they introduce uncertainty.…”

“…18 If the user chooses to utilize a mixed buffer gas, K can be corrected by incorporating the mole fraction of the gases. 18,28 However, for some instruments, such as the Waters Synapt G2-S, unintentional mixing occurs in which the ratio of gases is not known, preventing K from being corrected. While these assumptions are accepted to calculate CCS values with the Mason−Schamp equation, they introduce uncertainty.…”