Towards the Development of a Prototype Ion Mobility Spectrometer

Abstract

Ion mobility spectrometry (IMS) is an analytical technique that separates ions in the gas-phase based on differences in their mobility under the influence of an electric field. In traditional drift-tube IMS systems, an axial DC electric field pushes ions through a static bath gas, and the different ion subpopulations achieve averaged steady-state drift velocities that are dependent on their shape and size. Mobility separation is achieved due to multiple collisions between the molecular ions and gas molecules. Smaller, more compact ions will collide less frequently with the buffer gas in comparison to larger, more elongated ions. Consequently, the smaller ions have higher mobilities and therefore reach the detector faster. One of the most recent developments in IMS, which is referred to as trapped ion mobility spectrometry (TIMS) was reported first in 2011 by Park and coworkers. In contrast to the conventional drift tube approach described above, in the TIMS configuration, ions are trapped axially by balancing an applied DC electric field gradient against a parallel flow of neutral carrier gas flowing towards the detector. A radiofrequency induced quadrupolar field radially confines ions in the center of the TIMS separation region. Trapped ions are focused first and then eluted towards the mass analyzer from the separation region based on differences in their mobility by gradually reducing the electric field strength.

The research described in this thesis covers the development and characterization of a prototype ion mobility spectrometer designed to improve upon the existing TIMS platform. This instrument, referred to as a variable flow trapped ion mobility spectrometer (vfTIMS), has a segmented mobility separation region comprised of four sectors with decreasing inner diameter. A gas flow velocity gradient that is generated through the decreasing sectors of the mobility region can be harnessed for high-resolution separations. Another improvement compared to the conventional TIMS is instead of a quadrupolar radially confining field, the vfTIMS employs a hexapolar field for improved ion focusing and increased ion capacity. Additionally, the DC electric field gradient that traps the ions is completely customizable by utilizing individually addressable electrodes, so the profile is not limited to a simple linear field gradient.

The main research objectives of this thesis are as follows:

1. Construct a prototype ion mobility spectrometer, combining multiple commercial and in-house components.

2. Implement an improved electrical layout to drive the instrument, with the added ability to define all the electric fields within the analyzer.

3. Fully characterize the performance of the prototype instrument for ion trapping and ion mobility separation.

In summary, this thesis aims to address the above objectives described in the chapters that make up its content. Chapter 2 and Chapter 3 focus on the design and construction of the vfTIMS. More specifically, Chapter 2 outlines the various hardware components of the vfTIMS that were either designed and built in-house or purchased commercially and modified. Chapter 3 details the design and testing of the electronics that power the system, along with the development of the user interface to control the instrument. Chapter 4 presents the extensive experimental optimization that was completed to get the instrument operational. Additionally, Chapter 4 also covers the experimental results that establish a baseline set of instrumental conditions that should be used for more complex experiments. Chapter 5 describes the first two proof of concept studies that demonstrate the feasibility of ion mobility separation experiments on the vfTIMS, building on the knowledge gleaned from the experiments in Chapter 4. Lastly, Chapter 6 serves as a roadmap for what must be done next to advance the project, in both the short term as well as the longer term.