A novel dual mode X-band EPR resonator for rapid in situ microwave heating

Andrea Folli; Heungjae Choi; Michael Barter; Jaafar Harari; Emma Richards; Daniel Slocombe; Adrian Porch; Damien M Murphy

doi:10.1016/j.jmr.2019.106644

A novel dual mode X-band EPR resonator for rapid in situ microwave heating

J Magn Reson. 2020 Jan:310:106644. doi: 10.1016/j.jmr.2019.106644. Epub 2019 Nov 13.

Authors

Andrea Folli¹, Heungjae Choi², Michael Barter², Jaafar Harari², Emma Richards³, Daniel Slocombe², Adrian Porch², Damien M Murphy³

Affiliations

¹ School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK. Electronic address: follia@cardiff.ac.uk.
² School of Engineering, Cardiff University, The Parade, Cardiff CF24 3AA, UK.
³ School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK.

PMID: 31812887
DOI: 10.1016/j.jmr.2019.106644

Abstract

A unique dual mode X-band Continuous Wave (CW) EPR resonator designed for simultaneous EPR measurement and rapid microwave (MW) induced sample heating is described. Chemical reactions subjected to a flow of energy and matter can be perturbed away from the thermodynamic equilibrium by imposing a rapid shock or physical change to the system. Depending on the magnitude of the perturbation, these changes can dictate the subsequent evolution of the entire system, allowing for instance to populate non-equilibrium reactive intermediate states. Temperature jump (T-jump) experiments are a common method to achieve such perturbations. Most T-jump experiments are based on Joule Heating methods or IR lasers. Here we demonstrate the principle of rapid sample heating based on microwaves. The benefits of MW heating include (i) rapid and efficient heating (i.e. using a tuned resonant cavity, >99% efficient power transfer to the sample can be achieved), and (ii) volumetric heating (i.e. the entire sample volume rises in temperature at once, since heat is generated in the sample instead of being transferred to it). Accordingly, the key concept of the design is the use of a cavity resonator allowing EPR detection (at 9.5 GHz) and simultaneous sample heating (at 6.1 GHz). Temperature increments of 50 °C within a few seconds are possible. This is evidenced and illustrated here by probing the temperature-induced variation of the rotational dynamics of 16-doxyl stearic acid methyl ester (16-DSE) spin probe grafted on the surface of sodium dodecyl sulphate (SDS) micelles in water, as well as copper (II) acetylacetonate in chloroform. Rapid changes in the rotational dynamics of the paramagnetic centres provide direct evidence for the in situ and simultaneous EPR measurement-heating capabilities of the resonator. Improvements afforded by the use of pulsed MW sources will enable faster heating time scales to be achieved. In the longer term, this current study demonstrates the simple and direct possibilities for using MW heating as a means of performing T-jump experiments.

Keywords: Copper; Dielectric heating; Electron Paramagnetic Resonance (EPR); Kinetics; Perturbation methods; Resonator design; Spin probes; T-jump.