A high-volume continuous-flow resonator for aqueous electron paramagnetic resonance and dynamic nuclear polarisation

Abstract

This thesis concerns the design, development and testing of a novel X-band resonator, aimed at improving the sensitivity of aqueous Electron Paramagnetic Resonance (EPR) and expanding the applications of liquid-state Dynamic Nuclear Polarisation (DNP). Aqueous EPR and DNP are currently constrained by the very high dielectric loss of water, which limits sample volumes. The continuous-flow resonator described in this thesis is an overmoded coaxial cavity designed to accommodate large aqueous sample volumes with high filling factors and large volume flow rates. The resonator has potential impact in Magnetic Resonance Imaging (MRI), liquid-state Nuclear Magnetic Resonance (NMR) and EPR.

The resonator presented in this thesis was optimised for high volume flow rate continuous-flow DNP by extensive electromagnetic modelling and has undergone several rounds of iterative prototyping. The current prototype continuous-flow cavity has a sample volume of 747 μL and can sustain a stable resonance at volume flow rates of 30 mL/min. This volume flow rate, combined with large predicted DNP enhancements, is sufficiently high to make continuous-flow DNP MRI applicable to humans for the first time. The resonator also allows for practical continuous-flow NMR measurements at fast flow rates. To test these capabilities, an X-band continuous-flow DNP NMR measurement setup was developed. Preliminary MRI trials were also conducted, to aid the design of a future DNP MRI experiment.

Additionally, the prototype cavity has high predicted EPR sensitivity and was compared with commercial alternatives by performing continuous wave (CW) EPR measurements on aqueous nitroxide samples. The low Q-factor of the prototype coaxial cavity is offset by its large filling factor and sample volume, resulting in performance comparable with state of-the-art aqueous cells.