A First Principle Model of Differential Ion Mobility: the Effect of Ion-Solvent Clustering
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J. Am. Soc. Mass Spectrom. (2019) DOI: 10.1007/s13361-019-02340-1
RESEARCH ARTICLE
A First Principle Model of Differential Ion Mobility: the Effect of Ion-Solvent Clustering Alexander Haack,1 Jeff Crouse,2 Femke-Jutta Schlüter,1 Thorsten Benter,1 W. Scott Hopkins2 1
Department of Physical and Theoretical Chemistry, University of Wuppertal, Gauss Str. 20, 42119, Wuppertal, Germany Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
2
Abstract. The use of differential mobility spectrometry (DMS) as a separation tool prior to mass analysis has increased in popularity over the years. However, the fundamental principles behind the difference between high- and low-field mobility is still a matter of debate—especially regarding the strong impact of solvent molecules added to the gas phase in chemically modified DMS environments. In this contribution, we aim to present a thorough model for the determination of the ion mobility over a wide range of field strengths and subsequent calculation of DMS dispersion plots. Our model relies on first principle calculations only, incorporating the modeling of the “hard-sphere” mobility, the change in CCS with field strength, and the degree of clustering of solvent molecules to the ion. We show that all three factors have to be taken into account to qualitatively predict dispersion plots. In particular, type A behavior (i.e., strong clustering) in DMS can only be explained by a significant change of the mean cluster size with field strengths. The fact that our model correctly predicts trends between differently strong binding solvents, as well as the solvent concentration and the background gas temperature, highlights the importance of clustering for differential mobility. Keywords: DMS, Dispersion plot, Ion mobility, Collision cross section, Population distribution Received: 28 June 2019/Revised: 10 September 2019/Accepted: 10 September 2019
Introduction Mobility at Low Field Strength
T
he movement of ions through a collision gas at elevated pressures induced by an electrical field is of great interest in mass spectrometry and related areas: Not only is ion mobility important in modern atmospheric pressure ionization sources [1–3], but in ion mobility spectrometry (IMS), it is also used to separate ions according to their
Electronic supplementary material The online version of this article (https:// doi.org/10.1007/s13361-019-02340-1) contains supplementary material, which is available to authorized users. Correspondence to: Thorsten Benter; e-mail: tbenter@uni–wuppertal.de, W. Hopkins; e-mail: [email protected]
size prior to mass analysis [4–6]. The mobility of ions has also been subject to theoretical investigations [7–10] since it offers insights into fundamental physics of collisions and ion-neutral interactions, e.g., kinetic theory in the context of the Boltzmann transport equation [11]. The acceleration of the ion ensemble through the electrical field is countered by collisions with the background gas, lea
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