Development and characterisation of gyrotron-based Dynamic Nuclear Polarisation and Electron Paramagnetic Resonance spectroscopy at 9 Tesla

The Dynamic nuclear polarisation (DNP) enhanced nuclear magnetic resonance (NMR) results presented in this thesis demonstrate the functionality of a tunable and switchable gyrotron operating at 260 GHz with a 9 T spectrometer. This gyrotron was designed at the Swiss Plasma Center. Millimeter wave passive components were built by the startup company SwissTo12. Monochromatic as well as frequency swept cross effect DNP experiments validate the basic functioning and frequency tunability of the gyrotron. High power operation is tested on systems exhibiting the Overhauser and solid effects with the aid of a novel planar probe designed to mitigate dielectric heating. Microwave polarisation control using a Matching Optics Unit is verified using cross effect DNP. Continuous wave electron paramagnetic resonance (EPR) measurements performed on a home built quasi-optical spectrometer operating at 260 GHz on the same sample space as the NMR spectrometer are also presented. EPR spectra of hyperpolarising agents measured on this spectrometer are used to prepare for and subsequently analyse the DNP experiments. Preliminary studies on surface modified titanium dioxide nanopowders using EPR at 9 GHz, displaying significant g-factor shifts are reported.

Dynamic nuclear polarization techniques for magnetic resonance imaging and particles targets experiments

Sami Jannin

Within a collaboration with the Sample Environment and Polarised Targets (SEPT) group at Paul Scherrer Institut (PSI), we developed two Dynamic Nuclear Polarization (DNP) machines operating at ~ 1 K. The first one is working at an electron spin resonance frequency of 94 GHz and the other is an upgrade to 140 GHz, with an improvement of about 50% in polarization. The DNP mechanism we take advantage of relies on the transfer of polarization from the highly thermally polarized electron spin of TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl) to the surrounding nuclear spins. The equipment we developed allows to bring the dynamically polarized frozen sample to room temperature and to inject it into a living rodent or in a standard tube standing in the nuclear magnetic resonance (NMR) magnet within ~ 4 – 6 seconds. NMR signal enhancement factors on the order of 10000 have been routinely observed. The solid state DNP data were treated with a set of equations that (semi-quantitatively) describe the stationary state of the DNP spin system under microwave irradiation. The situation we were confronted with had not been discussed before in the literature since the existing theories were restricted to high temperature approximation or strong saturation limit. Taking advantage of the high electron spin polarization of the photo-excited triplet state of pentacene, we developed an X-band "Triplet DNP" polarizer (operating in a field of 0.3 T and at a temperature of ~ 100 K), and performed DNP in pentacene doped naphthalene samples. In the late eighties, W.Th. Wenckebach et.al invented the technique using single crystals, and more recently, M. Takeda et.al showed it is also quite effective on powder ground from a crystal. We show that this approach also works well in samples obtained by rapidly cooling a melt (a "glass") and we describe some features of our apparatus. Glassy samples could eventually host molecules of interest. We thus show that the scope of application of the method can be broadened and that "Triplet DNP" has the potential to become an interesting tool in solid state NMR and chemistry.

EPFL2009

Chemical Developments in DNP-NMR

Pascal Miéville

The Nuclear Magnetic Resonance (NMR) is an extraordinary tool to understand molecular structures. Its main limitation resides in its relatively low sensitivity when compared with other physico-chemical methods such as Mass Spectrometry (MS) or Optical Spectroscopy. Dynamic Nuclear Polarization (DNP), invented in the 50's but only widely applied in the last ten years, allows, by transferring the high polarization of paramagnetic centers to surrounding nuclei at low temperature, to enhance NMR signals by several orders of magnitude. This technique can be applied to solids at 100 K using Magic Angle Spinning (DNP-MAS) or in liquid phase through Dissolution-DNP as described by Ardenkjaer-Larsen in 2003. The efficiency of the polarization transfer is strongly dependent on the nature of the paramagnetic centers and on the way they are mixed with the target molecules. Part of this work will consist in the study of new ways to integrate paramagnetic centers into the matrix and also to limit their negative effects on NMR signals in liquid phase after dissolution. Because DNP-NMR has reached a stage where it should become a standard chemical method, a second objective of this thesis is to propose new chemical applications such as kinetic measurements that may be of interest in other domain of chemistry.

EPFL2011

Novel Sample Formulations for Pure and Persistent Hyperpolarized Solutions via Dissolution Dynamic Nuclear Polarization

Basile Vuichoud

Nuclear Magnetic Resonance (NMR) spectroscopy allows one to study and analyze the structure, motions and interactions of a broad variety of molecules. However, this technique has a major inconvenience: its low sensitivity, which therefore often results in the use of highly concentrated samples in order to observe tiny signals. As recently as 2003, a new technique known as dissolution-DNP or d-DNP was invented by Ardenkjaer-Larsen et al. to overcome this drawback and to get more intense NMR signals in solution with enhancements factors larger than 10'000. This method consists essentially in mixing paramagnetic species (radicals) with samples containing the metabolites to be analyzed. These mixtures are then rapidly frozen at very low temperatures (T = 1.2 â 4.2 K) in liquid helium and, by applying a proper microwave irradiation, the polarization of the electrons can be transferred to nuclei such as 1H or 13C. This allows one to build up and store the enhanced magnetization of these nuclei and then to dissolve the samples by injecting a superheated solvent via a dissolution stick. The resulting hyperpolarized solution can be finally transferred to an NMR spectrometer where the signals can be recorded. In the first two chapters of this thesis, the principles of NMR are introduced and the theory of DNP is explained in some detail. The dissolution equipment used in our laboratory and its different parts are shown, in particular the DNP polarizer where the transfer of polarization from electrons to nuclei occurs, the microwave source that is connected to the polarizer, and the dissolution system itself, which comprises the dissolution stick and the dissolution transfer line. Another chapter is dedicated to the optimization of our DNP setup in order to achieve the highest possible polarization before the dissolution process. Several radicals are tested under carefully controlled conditions to identify the best suited for our DNP system. Furthermore, the modulation of the microwave frequency has been optimized in order to enhance the polarization transfer. Finally, a number of dissolution experiments are presented that relate to different projects that have been carried out in the course of this thesis. The extent of the polarization can be determined accurately by looking directly at the hyperpolarized NMR spectrum. Filterable polymers containing suitable radical moieties have been synthesized in order to obtain pure hyperpolarized solutions. A final chapter outlines future applications and projects that can benefit from dissolution Dynamic Nuclear Polarization.