|M.Sc Student||Katz Itai|
|Subject||Earth Field NMR with Chemical Shift Spectral Resolution:|
Theory and Proof of Concept
|Department||Department of Chemistry||Supervisor||Professor Aharon Blank|
|Full Thesis text|
This work presents a new method to obtain a NMR signal in the earth magnetic field (EF). Our method enables to obtain NMR data with spectral resolution sufficient for retrieving information on chemical shift (CS) - an achievement not considered possible in EF NMR until now. The underlying concept is to have the magnetization evolve under a strong and homogeneous field (B1), so that CS induced phase differences between magnetizations of different chemical species can quickly form. The detection is subsequently carried out under the EF.
The method relies on applying various DC magnetic fields rather than using radio frequency fields. According to our method, a sample is first polarized in a direction parallel to the EF. The polarizing field then moderately decays allowing the magnetization that has formed to adiabaticaly follow the decaying field as it rotates from its initial misaligned direction relative to the EF to the perfectly aligned direction at the end of the decay. Next, a field of moderate strength and high homogeneity, perpendicular to the EF and termed B1, is rapidly switched on. The magnetization cannot follow the total field as its 90 degrees flip is almost instantaneous. Thus, because the transient is fast, the magnetization begins to precess about the B1 field. The frequency of precession under B1 is high enough for a phase difference to form due to differences between different chemical species in a relatively short time of several ms. Following that, B1 is moderately quenched and thus, the total field (a sum of B1 and EF) slowly rotates to the EF direction. Because the transient is slow now, the magnetization maintains its precession about the total field and ends up, at the end of the B1 decay, precessing about the EF. The phase accumulated during the application of B1 is maintained throughout this process. Following that, detection takes place as the magnetization precesses about the EF, and the phase accumulated during B1 evolution is measured. Aside from obtaining spectroscopic NMR, this method is also useful to easily acquire EF NMR signals when applying only DC fields.
This pulse sequence was implemented experimentally by construction of a dedicated EF NMR system and a unique probe with a polarizing coil, a homogenous B1 coil and sensitive noise immune reception coils. The experimental results demonstrated the capability to differentiate between three types of samples made of common fluorine compounds, based on their CS data.