title: Chemical modification of gas-phase cluster dynamics in ion mobility spectrometry
Creators: name: Stappert, Florian
Publisher: name: Bergische Universität Wuppertal
Year: 2022

description: In this work, the interactions of ions and neutral species in the gas phase are studied from selected points of view. These interactions are important in most analytical methods oper-ating at atmospheric pressure (AP) because they affect substance-specific properties such as the <i>m/z</i> ratio or ion mobility. Therefore, understanding these processes is necessary for the interpretation of measurement results and for the optimization of an analytical system for a particular problem. In addition to, for example, fragmentation reactions due to ener-getic collisions (collision-induced dissociation, CID), the formation of clusters is a main interaction and is the focus of this work. Cluster dynamics with actively added gas-phase modifiers are investigated with differential mobility spectroscopy (DMS) coupled with mass spectrometry (MS) and high kinetic energy ion mobility spectrometry (HiKE-IMS). In the first part, the impact of the ion structure on the cluster effect in DMS is investigated, and two main effects can be observed. Firstly, an increase in the absolute ion size, respec-tively in the collision cross section (CCS), leads to a decrease in the strength of the cluster effect, and secondly, a sterically more challenging ion at the charged position leads to ef-fects such as charge shielding and thus impaired cluster formation. Furthermore, the ratio of ion and modifier sizes defines the behavior of a cluster. In the case of a significantly larger modifier, the behavior of the ion-modifier cluster depends mainly on the modifier. Additionally, the ion structure of a multiply charged ion, well known for ionization with electrospray ionization (ESI), influences the basic cluster formation. Depending on the dis-tance between two charged positions, the experimental results indicate one superior cluster systems or two almost independent cluster systems.<br> In the second part, the cluster dynamic under constant energetic conditions in a HiKE-IMS is investigated. In addition to a permanent cluster formation with water due to a not water-free gas phase, a steady clustering and declustering process is demonstrated. Therefore, this dynamic must also be adopted for all instruments working at atmospheric pressure so that a signal in an ion mobility spectrum cannot be assigned to a specific ion. Instead, it must be assigned to a chemical system that is therefore highly susceptible to external influences such as variation in background temperature or humidity. The only exceptions to this rule are less reactive ions.<br> Finally, an attempt is made to achieve enantiomeric separation by stereoselective clustering due to the addition of an enantiopure modifier. Independently of the chirality of the ion or the optimized conditions, such experimental separation could not be realized with DMS. The reason could be the formation of highly charged nanodroplets in the ESI process, which pass the DMS cell and even the ion inlet of the MS system. This demonstrates a deviation from the textbook ESI mechanism, for which there is also evidence in the literature. The ion-like behavior of these nanodroplets in DMS is shown, which is why complete separa-tion from bare ions is almost impossible. A cluster-like effect on the nanodroplets can also be observed by the addition of a modifier. Furthermore, even numerical studies with opti-mized conditions have demonstrated only a weak stereoselective cluster formation. In summary, this work provides fundamental findings on the cluster dynamic of ions and neutral species in the gas phase. A transfer to other analytical methods and other chemical systems is very likely permissible. Nevertheless, it must be mentioned that the present stud-ies were performed on specially selected chemical systems of small ions. When real sam-ples and bigger ions are studied, other interactions and effects are to be expected. Thus, further experimental and numerical studies are necessary, for example, in the field of HiKE-IMS or nanodroplet distribution.<br> Accordingly, the findings of this work fit into the overall study of cluster chemistry, but they do not claim to fully elucidate all fundamental mechanisms.