Description
- Abstract:
- This thesis presents the development and physical characterization of a nanopore ion source which emits biomolecular ions directly from aqueous solution into vacuum. The primary motivations for this work are to overcome fundamental limitations in mass spectrometry-based proteomics workflows and to develop a single molecule protein sequencing technique. Traditional electrospray ionization (ESI) methods are widely used to generate vapor phase ions for mass spectrometry, but they suffer from significant sample loss which is intrinsically linked to the ionization mechanism. ESI generates a spray of charged droplets which evaporate due to collisions with background gas molecules, and this same background gas scatters the generated ions in a wide plume and prevents most ions from entering the mass spectrometer. This sample loss limits the sensitivity of proteomic analyses which rely on ESI, and precludes comprehensive single cell or single molecule analyses. In this work, we show that by reducing the emitter size to the nanoscale, the nanopore ion source can sustain a stable water/vacuum interface which generates ions directly by ion evaporation, bypassing the droplet formation stage, eliminating the need for a background gas and minimizing sample loss. We demonstrate that the nanopore ion source can emit amino acid and small peptide ions directly into vacuum from aqueous solution. The emitted current is shown to be composed of ions rather than charged droplets as in conventional ESI, and more than 90\% of the current can be recovered in a distant collector. Theoretical arguments support the hypothesis that for sufficiently small emitters, charge emission from aqueous solution transitions to the purely ionic regime, which has previously only been observed in liquid metals, ionic liquids, and electrolyte solutions in low volatility solvents like glycerol and formamide. We further explore the thermal and electrical properties of the nanopore ion source. A thermal model which describes the competing processes of evaporative cooling and joule heating near the tip is developed and used to show that evaporative cooling in a conical emitter scales inversely with the tip radius, explaining why nanopore ion sources can maintain stable water/vacuum interfaces without freezing. We also develop a model of the current-voltage characteristic of a nanopore ion source that generates ions by direct ion evaporation, and demonstrate that it matches the source's measured behavior. Finally, we investigate the use of ultraviolet (UV) light to fragment peptides in aqueous solution as a potential precursor to single-molecule peptide sequencing. We monitor peptide bond scission through changes in the absorption spectra following irradiation with 193~nm and 222~nm light, using a combination of real-time in situ transmission measurements and UV-vis spectroscopy. We measure the quantum yields of peptide bond scission and discuss how UV-induced fragmentation in solution could be effectively coupled with the nanopore ion source to facilitate new types of single-molecule analyses.
- Notes:
- Thesis (Ph. D.)--Brown University, 2024
Citation
Drachman, Nicholas Ari,
"The Nanopore Ion Source: Mechanisms of Direct Ion Emission from Aqueous Solutions into Vacuum"
(2024).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://repository.library.brown.edu/studio/item/bdr:gas8ns8n/
