Cluster formation restricts dynamic nuclear polarization of xenon in solid mixtures

Abstract

During dynamic nuclear polarization (DNP) at 1.5 K and 5 T, 129 Xe nuclear magnetic resonance (NMR) spectra of a homogeneous xenon/1-propanol/trityl-radical solid mixture exhibit a single peak, broadened by 1 H neighbors. A second peak appears upon annealing for several hours at 125 K. Its characteristic width and chemical shift indicate the presence of spontaneously formed pure Xe clusters. Microwave irradiation at the appropriate frequencies can bring both peaks to either positive or negative polarization. The peculiar time evolution of 129 Xe polarization in pure Xe clusters during DNP can be modelled as an interplay of spin diffusion and T 1 relaxation. Our simple spherical-cluster model offers a sensitive tool to evaluate major DNP parameters in situ, revealing a severe spindiffusion bottleneck at the cluster boundaries and a significant sample overheating due to microwave irradiation. Subsequent DNP system modifications designed to reduce the overheating resulted in four-fold increase of 129 Xe polarization, from 5.3% to 21%.

NMR Hyperpolarization Techniques of Gases

“…In addition to SEOP hyperpolarization for production of HP 3 He and 129 Xe, (i) the SEOP HP technique has been expanded to several other noble gases, (ii) the heterogeneous PHIP [28a] technique has enabled production of HP hydrocarbons in pure form (free from contamination by the catalyst), and (iii) the dissolution DNP technique was demonstrated for production of HP 129 Xe [26, 54a] and 15 N 2 O gases. [238] Thus, many other gases could be potentially amenable to DNP hyperpolarization.…”

“…The selected applications described here include functional lung imaging, [22] metabolic brown fat imaging, [23] and biosensors [24] (including those enabled by genetic encoding [25] ). A recent demonstration of the DNP process to hyperpolarize 129 Xe [26] and hydrocarbon gases [27] is also described.…”

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

“…A slightly modified version, known as ‘AH111501’ or simply ‘EPA’ (for Electron Paramagnetic Agent), is used in human studies due to the ease with which it can be removed at acidic pH by simple filtration [55]. Other substrate species, including those with hydrophobic properties, are best polarized by other trityl derivatives with less polar functional groups, such as the acidified Finland species successfully used for DNP of frozen Xenon [56]. …”

Optimal Glass-Forming Solvent Brings Sublimation Dynamic Nuclear Polarization to 129Xe Hyperpolarization Biomedical Imaging Standards

“…However, the homogeneity of the solid-state xenon preparations is a critical feature required for DNP and the achievable polarization level can be dramatically limited in inhomogeneous samples, questioning the practical application of this technique. 26 In this study, it is shown that the maximum xenon volume and achievable 129 Xe polarization strongly depends on the physicochemical properties of the glass-forming agent in which xenon is embedded, and that sublimation DNP could compare favorably with SEOP when sample preparation is optimized. The necessity to form an amorphous (or glassy) phase, which not only insures an homogeneous distribution of radicals but also promotes spectral diffusion by forcing nearest neighbor molecules to be randomly oriented with respect to one another and thus strong interactions between electron spins with large resonance frequency difference, is undoubtedly the most critical feature to efficiently perform solid-state DNP.…”