Propagation of dynamic nuclear polarization across the xenon cluster boundaries: Elucidation of the spin-diffusion bottleneck

Abstract

Earlier dynamic nuclear polarization (DNP) experiments with frozen xenon/1-propanol/trityl mixtures have demonstrated spontaneous formation of pure xenon clusters above 120 K, enabling spectrally-resolved real-time measurements of 129Xe nuclear magnetization in the clusters and in the surrounding radical-rich matrix. A spin-diffusion bottleneck was postulated to explain the peculiar time evolution of 129Xe signals in the clusters as well as the apparent discontinuity of 129Xe polarization across the cluster boundaries. A self-contained ab initio model of nuclear spin diffusion in heterogeneous systems is developed here, incorporating the intrinsic T1 relaxation towards the temperature-dependent equilibrium along with the spin-diffusion coefficients based on the measured NMR line widths and the known atomic densities in each compartment. This simple model provides the physical basis for the observed spin-diffusion bottleneck and is in a good quantitative agreement with the earlier measurements. A simultaneous fit of the model to the time-dependent NMR data at two different DNP frequencies provides excellent estimates of the cluster size, the intrinsic sample temperature, and 129Xe T1 constants. The model was also applied to the NMR data acquired during relaxation towards thermal equilibrium after microwaves were turned off to estimate T1 relaxation time constants inside and outside the clusters. Fitting the model to data during and after DNP provides estimates of cluster size that are in complete agreement.

Low‐temperature dynamic nuclear polarization of gases in Frozen mixtures

“…In contrast to our older system , the new manifold directly measures gas volume and pressure, making the regulation of sample composition far easier. This resulted in homogeneously mixed samples, as evidenced by the single‐peak lineshape of the 129 Xe mixture shown in Figure A; in our past studies , samples that were not well mixed generated spectra with two distinct peaks separately reflecting the pure 129 Xe and the glassy matrix.…”

“…This indicates a much smaller pure 129 Xe domain for which interaction with the radical (either direct or by spin diffusion) is sufficient for substantial polarization of the interior atoms. Based on our previous study (4,18), annealing does not increase the overall polarization of xenon during DNP. However, the remarkable increase in T 1 relaxation time constant as demonstrated in Figure 6 can facilitate transport of polarized samples to MRI sites for imaging.…”

Heterogeneous Parahydrogen Pairwise Addition to Cyclopropane

“…Hyperpolarization of nuclear spins is a highly efficient way to boost the sensitivity of NMR and MRI through the increase of P by several orders of magnitude . Hyperpolarization techniques such as dynamic nuclear polarization (DNP), spin‐exchange optical pumping (SEOP) of noble gases,,, and parahydrogen‐induced polarization (PHIP) can produce hyperpolarized (HP) gases, which can be potentially inhaled. The inhaled HP gas can be imaged on a single breath hold resulting in 2D and 3D functional MRI images of gas ventilation, diffusion and perfusion ,,.…”

NMR Hyperpolarization Techniques of Gases

“…The matrix is then rapidly warmed, sublimating the HP xenon as a pure gas‐hence the name “sublimation DNP” given for this approach . Using DNP, polarization values of ∼30 % have been achieved in ∼1.5 h, despite identified issues with a spin‐diffusion bottleneck between electron and 129 Xe spins . Although the amounts and polarization values achieved thus far with DNP are not as high as corresponding values achieved with SEOP, the advantages are the increasing availability and general applicability of dissolution DNP polarizers in biomedical facilities‐including Oxford's HyperSense and more recently, GE Healthcare's SPINlab …”