Development and applications of a low-field portable NMR system : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Manawatu, New Zealand
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Date
2011
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Massey University
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Abstract
Nuclear magnetic resonance (NMR) is a phenomenon similar to MRI
in which radio frequency signals are used to excite and manipulate atomic
nuclei within a static magnetic field. Following excitation, the nuclei return
to equilibrium, all the while offering valuable molecular level information
pertaining to the sample.
Within the last decade, the development of small and inexpensive NMR
spectrometers and permanent magnet NMR sensors has been a significant
focus within the NMR community. More recently, application scientists have
sought practical applications for the new technologies.
In this thesis, a prototype NMR apparatus consisting of a spectrometer
and 3.2MHz permanent magnet sensor was extended to enable scientifc
measurements. This involved developing radio frequency electronic circuitry
for the spectrometer front-end, and electromagnetic noise shielding and temperature
regulation for the magnetic sensor. Experimental results confirmed
that repeatable measurements using the modified apparatus were indeed
possible.
The NMR apparatus was thereafter successfully used to study flow,
diffusion and kiwifruit using several different experimental techniques. A
significantly larger effort was then expended upon the study of T2 relaxation in
pectin model systems using pH as the adjustable parameter. The fascinating
experimental results were successfully interpreted and modeled across three
pH zones in terms of a proton chemical exchange model and molecular
conformational changes. In addition, it was found that pectin carboxyl
de-protonation was significantly less than expected. Further experiments
performed upon galacturonic acid monomers, dimers and trimers appeared
to further illuminate the pectin results. Future experiments are planned.
Also while studying pectin solutions, an unexpected pH-dependent water
transverse relaxation behavior was observed at both 3.2MHz and 400MHz.
The only references found in the literature were from a small publication
almost 50 years ago, and a 2011 publication.
Altogether, this thesis contributed to original knowledge in several ways:
it showed how a low- eld apparatus and single-sided sensor could be improved
and utilized for a variety of scientific measurements; it showed both
experimentally and theoretically how T2 for pectin solutions change with
pH; it revealed an unexpected de-protonation limit for pectin molecules; it
revealed a T2 pH dependence for water.
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Keywords
Nuclear magnetic resonance spectroscopy, Magnetic spectrometer, NMR, Diffusion, Dimer, Galacturonic acid, Heptamer, Kiwifruit, Mole probe, Momomer, Nuclear magnetic resonance, Octamer, Pectin, Polysaccharides, Pre-amp duplexer, Radio frequency, Sensors, Signal processing, Spectrometer, Titration, Trimer, pH