Triple resonant electromagnetic structures for polarization transfer in DNP

Abstract

Despite, its low inert sensitivity, it is irrefutable that nuclear magnetic resonance (NMR) is extremely useful for both analytical spectroscopy and imaging. As the study of magnetic resonance evolved, detection instruments improved and magnets increased yielding modest improvements to sensitivity. Ultimately, it is the polarization of nuclear spins between Zeeman energy levels that dictate the NMR signal intensity. Several hyperpolarization methods exist in aiding to increase nuclear spin polarization but it is dynamic nuclear polarization (DNP) the offers the most versatility in application to an exhaustive range of nuclear spins. The method relies on transferring spin polarization from unpaired electrons to nuclear spins facilitated my irradiation at the electron resonant frequency. In 2003, a technique emerged based on DNP to produce polarized liquid-state solutions. The technique involves cooling a sample to approx. 1 K in a high magnetic field (≥ 3.35 T) where electron spin polarization is very high and rapidly dissolving the sample with a hot solvent to produce the solution. Dissolution dynamic nuclear polarization enabled real time surveillance of metabolic conversions in both spectral and spatial dimensions, finding employment in the study of cancer progression and response to therapy. Polarizer systems have since rapidly evolved to primarily reach higher nuclear polarization levels, but also increase sample throughput, limit dependence on cryogenics and incorporate automation. The latest polarizer design realizes a variable field (up to 10.1 T) cryogen-free polarizer system. This thesis serves to investigate the development of instruments to improve the polarization process in a system of that type. Herein a probe is developed facilitating the ability to perform double resonance solid-state DNP experiments with dissolution capabilities. Moreover, the design is optimized to minimize static heat load, manufacturing complexity and cost. To improve throughput another probe capable of performing cross-polarization is developed, yielding 27% 13C polarization with a 12 min build-up time that is twice the direct 13C polarization and 4.4 times faster. Dissolution compatible coil geometries are explored. Techniques to design single and double resonant detection circuits including methods to evaluate their sensitivity is discussed. In low pressure environments arcing is probable due to high voltages during pulsing. As such, arc detection methods and mitigation strategies are explored and experimentally verified. Microwave power in solid-state sources is increasingly scarce at higher magnetic fields. To combat this limitation and greater transmission losses, two microwave strategies were designed and experimentally verified. A process is described to reduce waveguide attenuation due to conductive loss thereby doubling the delivered power. A chamfer and reflector are designed, fabricated and tested to increases the microwave field density across the sample volume resulting in an equivalent 1.3 dB increase in power. A compact two channel benchtop spectrometer is developed, suitable for use up to 450 MHz. This, in part, aids the deployment of polarizer without the need of a traditional full-rack spectrometer. Sensitivity tests indicate the bench spectrometer achieves 90% and 50% the signal-to-noise ratio value of that from a dedicated full rack spectrometer for 1H and 13C spectra measured at 6.7 T.