title: Investigation of ion-solvent interactions in electrospray ionization mass spectrometry
Creators: name: Polaczek, Christine
Publisher: name: Bergische Universität Wuppertal
Year: 2021
Subjects: mass spectrometry
electrospray
ionization
proton-bound cluster
cluster chemistry

description: Techniques causing the increase of the average charge state of multiply charged ions in electrospray ionization mass spectrometry (ESI-MS) are of particular relevance, especially in the field of life sciences. Chemical modification of the gas as well as the solution phase can significantly alter the observed charge state distribution. <br> In this work, the impact of protic and aprotic polar modifiers on the ion distribu- tion, recorded with a nanoESI setup, is studied. In experiments with Substance P, depletion of the higher charge state (3+) in favor of the lower charge state (2+) is observed with polar protic gas phase modifiers. Retention of the 3+ charge state is induced by the addition of polar aprotic compounds to the ESI solution (solution phase modification) or as vapors to the gas phase (gas phase modification). Charge retention is in both cases accompanied by observation of ion-modifier clusters. The crucial role of the formation of proton-bound ion-modifier clusters is elucidated in a charge retention/charge depletion model: Addition of a chemical modifier results in preferential solvation of the analyte ion by the modifier. Formation of these ion- modifier clusters facilitates intramolecular proton transfer to modifier clusters, when the gas-phase basicity (GB) of the modifier cluster solvating the charge site exceeds the GB of the charge site. It is hypothesized that due to their ability to form hydrogen-bond networks, the GB of an associated protic modifier cluster can increase until proton transfer becomes thermodynamically favorable. In contrast, the number of aprotic modifiers solvating a charge site is limited because aprotic ligands can only form one hydrogen bond. Furthermore, proton transfer to modifier clusters is driven by collisional activation of the ions during their passage through the transfer stages of the mass spectrometer. <br> This model is validated by a systematic experimental study on the impact of gas phase modifiers on the ion signal distribution of terminal alkyldiamines, which represent a model system for adjacent charge sites in multiply protonated macromolecules. This study addresses the influence of the chemical structure, particularly the alkyl chain length, of the diamines, the location of the gas phase modifier addition, and ion activation processes on external solvation of the ions and charge retention/depletion processes, respectively. By chemical modification of the collision gas in a quadrupole ion trap instrument, cluster reactions between doubly protonated diamines and neutral acetonitrile occurring in the gas phase are investigated and the implications of chemical modification for mass analysis are characterized.<br> In addition to the experimental studies, theoretical methods are applied to further elucidate ion-solvent interactions on a molecular level. Ab initio calculations of singly and doubly protonated ethylenediamine ions clustered with water (H<sub>2</sub>O) and acetonitrile (ACN) provide comparative analysis of the cluster structures and their stability as well as determination of thermodynamically favored reaction pathways. Calculations of collision cross sections and equilibrated cluster distributions enable comparison between modeled and measured ion mobilities of the dynamic cluster reaction systems. The theoretical results support the notion that intracluster proton transfer to aprotic solvent clusters is unlikely to occur, whereas proton transfer to protic solvent clusters is feasible. <br> In this work, a connection between liquid and gas phase based charge retention and depletion mechanisms is established. A distinction is drawn between protic and aprotic polar compounds and their impact on the observed charge state distribution of multiply protonated ions. The proposed model is based on multi-phase cluster chemistry and represents an extension to common charge partition mechanisms.