129Xe*

Fig. 1.26 I maging of 129Xe using (a) regular imaging and (b) chemical shift selective imaging sequence along the cross section of three NMR tubes containing toluene, water...

Fig. 2.6.6 Remotely reconstructed high field NMR images of laser-polarized 129Xe gas in the hollow CAL pores. Owing to the flow pattern where the spins have to flow around two corners [see the probe design in...

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

X. H. Ren, M. Bertmer, H. Kuhn, S. Stapf, D. E. Demco, B. Blumich, C. Kem, A. Jess 2002, ( H, 13C and 129Xe NMR study of changing pore size and tortuosity during deactivation and decoking of a naphtha reforming catalyst), NATO Sci. Ser. ITMath., Phys. Chem. 7b, 603. 

R. W. Mair, R. L. Walsworth 2005, (Study of gas-fluidization dynamics with laser-polarized 129Xe), Magn. Reson. /mag. 23, 203-207 (b) R. Wang 2005, (Study of gas flow dynamics in porous and granular media with laser-polarized 129Xe NMR), Ph.D. Thesis, Department of Nuclear Engineering, Massachusetts Institute of Technology, February 2005. 

Hyperpolarized 129Xe NMR Spectroscopy, MRI and Dynamic NMR Microscopy for the In Situ Monitoring of Gas Dynamics in Opaque Media Including Combustion Processes... 

Hyperpolarized 129Xe not only allows for sufficient signal intensity for chemical shift selective gas phase MRI, it also provides the means for a unique type of contrast. The imaging contrast derived from the transport of hyperpolarized gases into the material can be utilized to obtain snapshots of the gas flow and diffusion into porous samples. In this context, it is important to appreciate that Figures... 

Fig. 5.3.2 (A) NMR spectrum of hyperpolar- abundance of approximately 25% of the, 29Xe ized 129Xe from a sample that contains bulk gas isotope. (B) 2D slice of 3D chemical shift phase (0.3 ppm) and xenon occluded within selective MRI of the bulk gas phase. (C-E) 2D aerogel fragments (25 ppm). The gas mixture slices of 3D chemical shift selective MRI of the used for the experiment contained 100 kPa of 25 ppm region for various recycle times T.

Fig. 5.3.3 (A) NMR spectrum of hyperpolarized 129Xe in NaX zeolites. (B) 2D slice in the flow direction of a 3D chemical shift selective MRI of gas in the zeolite pellets. (C) 2D slice perpendicular to the flow direction of the same 3D chemical shift selective MRI as in (A). Adapted from Ref. [14].

One of the resulting 129Xe NMR spectra is shown in Figure 5.3.8 (solid line 2) in comparison with the spectrum of the same initial mixture without combustion (dashed line 1). Referenced with 0 ppm is the gas phase peak at room temperature. 

Fig. 5.3.8 Photograph of the detection region of the NMR probe with radiofrequency coil. A methane—air mixture was ignited above the zeolite pellets. The mixture also contained xenon for NMR detection. Hp-129Xe NMR spectra with 30% xenon (from high-density xenon optical pumping) in 70% methane is depicted. (1) The spectrum in the absence of combustion and (2) the spectrum during combustion. Adapted from Ref. [2],...

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