The dependence of reduced mobility, ion-neutral collisional cross sections, and alpha values on reduced electric field strengths in ion mobility

Naylor, Cameron N.; Schaefer, Christoph; Zimmermann, Stefan

doi:10.1039/d3an00493g

Cited by 2 publications

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“… 1 Because the change of mobility with field strength strongly depends on the structure and charge distribution of the ion, 25 , 78 it is difficult to make a general statement about the direction or magnitude of this nonlinearity. Consequently, measuring 79 or computing the α-function of a target analyte can help in choosing suitable calibrants (with known α functions), decreasing uncertainties in calibration procedures.…”

Section: Resultsmentioning

confidence: 99%

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

Haack,

Schaefer,

Zimmermann

2024

Anal. Chem.

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Ion mobility spectrometry (IMS) is a widely used gas-phase separation technique, particularly when coupled with mass spectrometry (MS). Modern IMS instruments often apply elevated reduced field strengths for improved ion separation and ion focusing. These alter the collision dynamics and further drive ion reaction processes that can change the analyte's structure. As a result, the measured arrival time distribution (ATD) can change with the applied reduced field strengths. In this work, we systematically study how the ion collision dynamics and the ion reaction dynamics, as a function of the reduced field strength, can alter the ATD. To this end, we investigate 2,6-di-tert-butylpyridine, methanol, and ethyl acetate using a home-built drift tube IMS coupled to a homebuilt MS and extensive first-principles Monte Carlo modeling. We show how elevated reduced field strengths can actually lower resolving power through increased ion diffusion and how the field dependency of the ion mobility can introduce uncertainties to collision cross sections (CCS) calculated from the measured mobilities. On top of the collision dynamics, we show how chemical transformation processes that alter the analyte's CCS, e.g., dynamic clustering or fragmentation, can lead to broadened, shifted, or non-Gaussian ATDs and how sensitive these processes are to the applied field strengths. We highlight how first-principles ion dynamics simulations can help to understand and even harness the mentioned effects.

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Section: Resultsmentioning

confidence: 99%

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

Haack,

Schaefer,

Zimmermann

2024

Anal. Chem.

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Eliminating the Deadwood: A Machine Learning Model for CCS Knowledge-Based Conformational Focusing for Lipids

Keng,

Merz

2024

J. Chem. Inf. Model.

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Accurate elucidation of gas-phase chemical structures using collision cross section (CCS) values obtained from ion-mobility mass spectrometry benefits from a synergism between experimental and in silico results. We have shown in recent work that for a molecule of modest size with a proscribed conformational space we can successfully capture a conformation(s) that can match experimental CCS values. However, for flexible systems such as fatty acids that have many rotatable bonds and multiple intramolecular London dispersion interactions, it becomes necessary to sample a much greater conformational space. Sampling more conformers, however, accrues significant computational cost downstream in optimization steps involving quantum mechanics. To reduce this computational expense for lipids, we have developed a novel machine learning (ML) model to facilitate conformer filtering according to the estimated gas-phase CCS values. Herein we report that the implementation of our CCS knowledge-based approach for conformational sampling resulted in improved structure prediction agreement with experiment by achieving favorable average CCS prediction errors of ∼2% for lipid systems in both the validation set and the test set. Moreover, most of the gas-phase candidate conformations obtained by using CCS focusing achieved lower energy-minimum geometries than the candidate conformations without focusing. Altogether, the implementation of this ML model into our modeling workflow has proven to be beneficial for both the quality of the results and the turnaround time. Finally, while our approach is limited to lipids, it can be readily extended to other molecules of interest.

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The dependence of reduced mobility, ion-neutral collisional cross sections, and alpha values on reduced electric field strengths in ion mobility

Abstract: A homologous series of molecules are measured under low to high field conditions on a drift tube ion mobility spectrometer and the underpinning assumptions made for calculating ionneutral collisional cross sections are compared.

Cited by 2 publications

References 73 publications

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

Eliminating the Deadwood: A Machine Learning Model for CCS Knowledge-Based Conformational Focusing for Lipids

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