Date of Award

Summer 8-23-2025

Document Type

Masters Thesis

Degree Name

M.S. in Chemistry and Biochemistry

Organizational Unit

College of Natural Science and Mathematics, Chemistry and Biochemistry

First Advisor

Gareth R. Eaton

Second Advisor

Sandra S. Eaton

Third Advisor

Sunil Kumar

Fifth Advisor

Brian Majestic

Sixth Advisor

Barry Zink

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Keywords

Background signal, Dosimetry, EPR, L-band, Rapid scan, Surface coils

Abstract

Electron Paramagnetic Resonance (EPR) analysis requires resonators optimized for specific experimental challenges. Volume resonators enclose samples, while surface coil resonators allow imaging of larger subjects. For in vivo studies, L-band frequencies are commonly used because they offer a good balance between signal-to-noise ratio (S/N) and the ability to penetrate tissue. Rapid Scan (RS) EPR at L-band frequencies was chosen for my studies of resonators because of this balance but presents a challenge for sensitivity.

The spatial position dependence of signal intensity was evaluated for selected paramagnetic samples using 10 mm and 30 mm surface coils. The 10 mm coil, with its higher efficiency, enabled superior signal-to-noise ratio (S/N) at lower power levels. In contrast, the 30 mm coil required higher power to achieve comparable signal, which introduced additional source noise and adversely affected overall S/N, although it permits observation of signal at greater depth.

Background signals affecting detection of the nitroxide signal were characterized for both surface coils and volume resonators. A 180° phase shift method was applied to suppress background intensity. The effectiveness of this approach was evaluated across various scan frequencies, showing minimal impact on S/N but a significant improvement in the ratio of actual signal to background. 3D EPR imaging was demonstrated for a 30 mm surface coil and a 25 mm loop gap resonator using nitroxide samples and analysed using MATLAB.

Relaxation times for SO2⁻ and SO3⁻ radicals were measured at X-band (~9.6 GHz) in Na2S2O4, Na2S2O5, and K2S2O5. Thermal dissociation of Na2S2O4 produces SO2⁻ radicals, while Na2S2O5, and K2S2O5 generate both SO2⁻ and SO3⁻. CW EPR results confirmed that SO2⁻ is dominant in Na2S2O4, SO3⁻ is dominant in Na2S2O5, and both radicals coexist in K2S2O5 in different crystal environments.

The T1 relaxation behaviour of SO2⁻ and SO3⁻ radicals at 293 K revealed a strong dependence on both the radical type and the host lattice environment. Notably, SO2⁻ exhibited consistently longer T1 values in K2S2O5 compared to Na2S2O4 suggesting reduced spin-lattice interactions. Conversely, SO3⁻ radicals show significantly longer relaxation times in Na2S2O5 relative to their potassium counterparts.

Analysis using stretched exponential fitting shows that the distribution of T1 values broadens in the order Na2S2O4 < Na2S2O5 < K2S2O5, with the widest distribution observed in K2S2O5. This broadening, supported by Uniform Penalty (UPEN) probability distributions, is attributed to the overlapping contributions of both SO2⁻ and SO3⁻ radicals, reflecting a more heterogeneous local magnetic environment in potassium lattices.

Overall, the results show that both the identity of the radical (SO2⁻ vs. SO3⁻) and the host lattice (Na⁺ vs. K⁺) critically influence spin relaxation behaviour. SO2⁻ radicals generally relax more quickly and exhibit shorter phase memory times (Tₘ), whereas SO3⁻ radicals—particularly in K⁺ lattices—demonstrate longer T1 values and weaker temperature dependence at low temperatures. These findings are essential for understanding spin dynamics.

Copyright Date

8-2025

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Georgina Naa Odey Amassah

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

96 pgs

File Size

1.9 MB

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