Xenon NMR with spectroscopic, spatial, and temporal resolution

Abstract

129Xe NMR has found many applications in material sciences and medicine because of two useful properties of Xenon atoms for NMR: the sensitivity to their environment due to their highly polarizable electron cloud, which results in a wide range of chemical shifts, and the ability of being hyperpolarized, which overcomes the problem of the low signal-to-noise ratio of thermally polarized Xenon. In this work a variety of different experiments were performed that combine NMR measurements with spectroscopic, spatial, and temporal resolution thereby exploiting the large non-equilibrium magnetization of hyperpolarized Xenon to monitor dynamic processes. In the first part of this work a new analytical model was introduced which allows the quantitative evaluation of the Xenon chemical shift in gas and liquid phases and of Xenon dissolved in organic solvents. Extensive measurements of the chemical shifts of thermally polarized Xenon were performed as a function of temperature (150-295 K) and Xenon density (1- 400 amagat). A simplified phenomenological model was developed, which quantitatively describes the dependence of the Xenon chemical shift on temperature and density of the interacting atoms or molecules. The chemical shift can be divided into a part which depends on the interaction of the solvent molecules and the Xenon atoms and into a part which describes the Xenon-Xenon interaction in the solvent. The second part of this work reports for the first time an application of hyperpolarized 129Xe NMR spectroscopy to analyze polymerization processes in real-time which is a challenge in polymer engineering. It has been successfully demonstrated that the chemical shift of 129Xe dissolved in the reaction bulk monitors quantitatively the mole fraction of the monomer allowing the calculation of the reaction constant. The capability of this method to follow not only the living cationic polymerization of THF but also the free radical polymerization of styrene suggests an exciting potential for the monitoring of various kinds of reactions. In the next chapter the possibility to store large quantities of hyperpolarized Xenon in a liquid was evaluated. Therefore, the dynamics of melting, migration, and dissolution of hyperpolarized Xenon ice into ethanol and an ethanol/water mixture were investigated with the method of time resolved two-dimensional MRI and one-dimensional CSI starting from the initial condition of a Xenon ice layer on top of the frozen solvent. A wealth of different physical phenomena, such as the observation of phase transitions, the position dependent line narrowing of Xenon ice, the creation of pores, and the existence of a dense liquid Xenon layer in ethanol have been observed. It has been shown that the dilution of ethanol with water and the subsequent increase of the melting point lead to a very ineffective incorporation of hyperpolarized Xenon. However, the dense liquid Xenon/ethanol mixture is promising for the injection of Xenon into biochemical or biological systems in vitro. The last chapter of this work was devoted to the multi-dimensional imaging of hyperpolarized Xenon produced in the continuous flow mode of the hyperpolarizer. It has been successfully demonstrated that the monitoring of dynamic processes like the dissolution of Xenon in an organic solvent or the penetration of Xenon through a filter medium is viable even with a very small number of hyperpolarized spins (0.07 bar in the gas phase). The penetration of hyperpolarized Xenon in a filter material was followed by time-resolved imaging indicating the possibility to model the dynamics of important gas/solid reactions or the efficiency of filtration processes with hyperpolarized Xenon. It has been also shown that hyperpolarized 129Xe NMR can be used to determine non-invasively the pore size distribution and pore connectivity of porous materials by three-dimensional imaging within reasonable experimental time