Characterization of the interactions of gas-phase ions with acoustic fields: fundamental studies and applications

Abstract

Ion-based analytical techniques, such as mass spectrometry (MS) and ion mobility spectrometry(IMS), are key methods in a scientist’s toolbox. Used everywhere from crime scenes to operating rooms, the reliability, efficiency and accessibility of MS and IMS are essential for modern technicians and analysts. At their core, MS and IMS rely on the predictable interactions of ions with external electric and/or magnetic fields, which are provided through components known as ion optics. As both instrument types require precise ion control, MS and IMS use complex, expensive vacuum systems and/or strong field producers (i.e., high voltage electronics or large magnets) to ensure proper function and overcome dampening effects caused by background gases. To meet the plethora of demands from MS and IMS users, it is important to develop means for ion control that do not require such expensive or difficult to manage components. Recent efforts have focused on the design of effective electrodes for atmospheric pressure ion manipulation, though the solutions still require strong fields and complex electronics. In this dissertation, a foundation of the capabilities and practicalities of the newly discovered phenomenon, acoustic ion manipulation (AIM) is established. Standing acoustic fields were used to redirect, gate, focus, and separate ions, analogous to traditional electrostatic and magnetic ion optics. An AIM ion gate was characterized in detail to identify critical parameters for effective acoustic manipulations, such as average linear velocity of the ions and acoustic field strength. The introduction of ions to the acoustic field via electrospray ionization (ESI) enabled the study of nonvolatile species, such as proteins and peptides. Charge-state dependent behavior was identified, demonstrating the importance of electrostatic properties of the ions on AIM susceptibility. This concept was further explored with different drying tube temperatures, which showed that the behaviors of large, multiply charged ions varied more than that of singly charged ions. Further study of these results suggested a relationship between both the mass and electrostatic environment with transmission through an AIM gate, shown by the seeming close relationship of m/z and transmission. Lastly, several possible avenues for the continuation of AIM are presented that include changes to the acoustic field, changes to the surrounding instrumentation, detailed studies of the ions, and complex device development. The combined development of AIM-device applications and characterization of fundamental ion behaviors in acoustic fields lays an effective foundation for the development of AIM into a viable component of new instrumentation and analytical techniques.