Chemical shift imaging with continuously flowing hyperpolarized xenon for the characterization of materials

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density

The extraordinary sensitivity of ¹²⁹Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized ¹²⁹Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and ¹²⁹Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of ¹²⁹Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Exploring Local Disorder within CAU-1 Frameworks Using Hyperpolarized 129Xe NMR Spectroscopy

The sorption properties of metal-organic frameworks (MOFs) can be influenced by introducing covalently attached functional side chains, which make this subclass of porous materials promising for applications as diverse as gas storage and separation, catalysis, and drug delivery. The incorporation of side groups usually comes along with disorder, as the synthesis procedures rarely allow for one specific position among a larger group of equivalent sites to be selected. For a series of isoreticular CAU-1 frameworks, chosen as model compounds, one out of four positions at every linker is modified with equal probability. Here, we investigate the influence of this disorder on Ar sorption and ¹²⁹Xe nuclear magnetic resonance spectroscopy using hyperpolarized ¹²⁹Xe gas. Models used for predicting the pore dimensions as well as their distributions were derived from the unfunctionalized framework by replacing one proton at every linker with either an amino, an acetamide, or a methyl urea functionality. The resulting structures were optimized using density functional theory (DFT) calculations. Results from void analyses and Monte Carlo force field simulations suggest that for available Ar nonlocal DFT (NLDFT) kernels, neither the pore dimensions nor the distributions induced by the side-chain disorder are well-reproduced. By contrast, we found the ¹²⁹Xe chemical shift analysis for the shift observed at high temperature to be well-suited to develop a detailed fingerprint of the porosity and side-chain disorder within the isoreticular CAU-1 series. After calibrating the ¹²⁹Xe limiting shift of the amino-functionalized framework with DFT calculations, the downfield shifts for the other two derivatives are an excellent measure for the reduction of the accessible pore space and reveal a strong preference for the side chains toward the octahedral voids for both cases. We expect that the strategy presented here can be commonly applied to disorder phenomena within MOFs in the future.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Para-hydrogen perspectives in hyperpolarized NMR

The first instance of para-hydrogen induced polarization (PHIP) in an NMR experiment was serendipitously observed in the 1980s while investigating a hydrogenation reaction (Seldler et al., 1983; Bowers and Weitekamp, 1986, 1987; Eisenschmid et al., 1987) [1-4]. Remarkably a theoretical investigation of the applicability of para-hydrogen as a hyperpolarization agent was being performed in the 1980's thereby quickly providing a theoretical basis for the PHIP-effect (Bowers and Weitekamp, 1986) [2]. The discovery of signal amplification by a non-hydrogenating interaction with para-hydrogen has recently extended the interest to exploit the PHIP effect, as it enables investigation of compounds without structural alteration while retaining the advantages of spectroscopy with hyperpolarized compounds [5]. In this article we will place more emphasis of the future applications of the method while only briefly discussing the efforts that have been made in the understanding of the phenomenon and the development of the method so far.

Perspectives of hyperpolarized noble gas MRI beyond 3He

Nuclear Magnetic Resonance (NMR) studies with hyperpolarized (hp) noble gases are at an exciting interface between physics, chemistry, materials science and biomedical sciences. This paper intends to provide a brief overview and outlook of magnetic resonance imaging (MRI) with hp noble gases other than hp (3)He. A particular focus are the many intriguing experiments with (129)Xe, some of which have already matured to useful MRI protocols, while others display high potential for future MRI applications. Quite naturally for MRI applications the major usage so far has been for biomedical research but perspectives for engineering and materials science studies are also provided. In addition, the prospects for surface sensitive contrast with hp (83)Kr MRI is discussed.

Configuration and Performance of a Mobile (129)Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of (129)Xe by spin exchange optical pumping with Rb is described. This mobile polarizer may be operated in batch or continuous flow modes with medium amounts of hyperpolarized (129)Xe for spectroscopic or small animal applications. A key element is an online nuclear magnetic resonance module which facilitates continuous monitoring of polarization generation in the pumping cell as well as the calculation of the absolute (129)Xe polarization. The performance of the polarizer with respect to the crucial parameters temperature, xenon and nitrogen partial pressures, and the total gas flow is discussed. In batch mode the highest (129)Xe polarization of P(Xe) = 40 % was achieved using 0.1 mbar xenon partial pressure. For a xenon flow of 6.5 and 26 mln/min, P(Xe) = 25 % and P(Xe) = 13 % were reached, respectively. The mobile polarizer may be a practical and efficient means to make the applicability of hyperpolarized (129)Xe more widespread.

Porosity of pillared clays studied by hyperpolarized 129Xe NMR spectroscopy and Xe adsorption isotherms

The influence of the layer charge on the microstructure was studied for a series of three hybrid pillared interlayered clays based on the organic dication Me(2)DABCO(2+) and charge reduced synthetic fluorohectorites. To get a detailed picture of the local arrangements within the interlayer space, multinuclear solid-state NMR spectroscopy was performed in conjunction with high-resolution (129)Xe MAS NMR, temperature-dependent wide-line 1D and 2D (129)Xe NMR, and Ar/Ar(l) and Xe/Xe(l) physisorption measurements. The resulting layer charge (x) for the three samples are 0.48, 0.44, and 0.39 per formula unit (pfu). The samples exhibit BET equivalent surfaces between 150 and 220 m(2)/g and pore volumes which increase from 0.06 to 0.11 cm(3)/g while the layer charge reduces. 1D and 2D (1)H, (13)C, (19)F, and (29)Si MAS data reveal that the postsynthetic charge reduction induces regions with higher defect concentrations within the silicate layers. Although the pillars tend to avoid these defect-rich regions, a homogeneous and regular spacing of the Me(2)DABCO(2+) pillars is established. Both the Ar/Ar(l) physisorption and (129)Xe NMR measurements reveal comparable pore dimensions. The trend of the temperature-dependent wide-line (129)Xe spectra as well as the exchange in the EXSY spectra is typical for a narrow 2D pore system. (129)Xe high-resolution experiments allow for a detailed description of the microstructure. For x = 0.48 a bimodal distribution with pore diameters between 5.9 and 6.4 Å is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 Å (x = 0.39). The adsorption enthalpies ΔH(ads) determined from the temperature-dependent (129)Xe chemical shift data fit well to the ones derived from the Xe/Xe(l) physisorption measurements in the high-pressure limit while the magnitude of ΔH(ads) in the low-pressure limit is significantly larger. Thus, the (129)Xe data are influenced by adsorbate-adsorbent as well as adsorbate-adsorbate interactions.

Theoretical predictions of the spectroscopic parameters in noble-gas molecules: HXeOH and its complex with water

We employ state-of-the-art methods and basis sets to study the effect of inserting the Xe atom into the water molecule and the water dimer on their NMR parameters. Our aim is to obtain predictions for the future experimental investigation of novel xenon complexes by NMR spectroscopy. Properties such as molecular structure and energetics have been studied by supermolecular approaches using HF, MP2, CCSD, CCSD(T) and MP4 methods. The bonding in HXeOH···H(2)O complexes has been analyzed by Symmetry-Adapted Perturbation Theory to provide the intricate insight into the nature of the interaction. We focus on vibrational spectra, NMR shielding and spin-spin coupling constants-experimental signals that reflect the electronic structures of the compounds. The parameters have been calculated at electron-correlated and Dirac-Hartree-Fock relativistic levels. This study has elucidated that the insertion of the Xe atom greatly modifies the NMR properties, including both the electron correlation and relativistic effects, the (129)Xe shielding constants decrease in HXeOH and HXeOH···H(2)O in comparison to Xe atom; the (17)O, as a neighbour of Xe, is deshielded too. The HXeOH···H(2)O complex in its most stable form is stabilized mainly by induction and dispersion energies.

Interdependence of in-cell xenon density and temperature during Rb/129Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

The (129)Xe nuclear spin polarization (P(Xe)) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe](cell)), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. P(Rb)) caused by increased non-spin-conserving Rb-Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/(129)Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed P(Xe) on the in-cell xenon partial pressure (p(Xe)), wherein P(Xe) values were abnormally low at decreased p(Xe), peaked at moderate p(Xe) (~300 torr), and remained surprisingly elevated at relatively high p(Xe) values (>1000 torr). Using in situ low-field (129)Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (T(OPT)) for Rb/(129)Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/(129)Xe SEOP process, and can be exploited to achieve improved P(Xe) with relatively high xenon densities measured at high field (including averaged P(Xe) values of ~52%, ~31%, ~22%, and ~11% at 50, 300, 500, and 2000 torr, respectively).

Theoretical prediction and experimental measurement of the field dependence of the apparent transverse relaxation of hyperpolarized noble gases in lungs

In this work, computer modeling based on a finite element method is used to simulate the T2* relaxation of hyperpolarized noble gases (HNG) in the lungs. A physical model of lung airways consisting of a phantom constructed from micro-capillary fibers of diameters similar to the size of lung airways with semi-permeable walls is also presented. The fibers are surrounded by a liquid medium (water) of magnetic susceptibility similar to lung tissue. Theoretical predictions of the field strength dependence of T2* for 129Xe in the phantom and in vivo rat lung are presented. These predictions are in good agreement with experimental T2* values obtained from the phantoms and in vivo rat lungs (160, 19 and 8 ms) at three different field strengths (0.074, 1.89 and 3T, respectively) using hyperpolarized 129Xe. The strong dependence of T2* on field strength is consistent with the theoretical prediction that low fields may be optimal for HNG MR imaging of the lungs as the decreased T2* at high fields necessitates an increase in bandwidth for conventional MR imaging.

Mechanical gas capture and release in a network solid via multiple single-crystalline transformations

Metal-organic frameworks have demonstrated functionality stemming from both robustness and pliancy and as such, offer promise for a broad range of new materials. The flexible aspect of some of these solids is intriguing for so-called 'smart' materials in that they could structurally respond to an external stimulus. Herein, we present an open-channel metal-organic framework that, on dehydration, shifts structure to form closed pores in the solid. This occurs through multiple single-crystal-to-single-crystal transformations such that snapshots of the mechanism of solid-state conversion can be obtained. Notably, the gas composing the atmosphere during dehydration becomes trapped in the closed pores. On rehydration, the pores open to release the trapped gas. Thus, this new material represents a thermally robust and porous material that is also capable of dynamically capturing and releasing gas in a controlled manner.

Measurement of xenon diffusing capacity in the rat lung by hyperpolarized 129Xe MRI and dynamic spectroscopy in a single breath-hold

We used the dual capability of hyperpolarized 129Xe for spectroscopy and imaging to develop new measures of xenon diffusing capacity in the rat lung that (analogously to the diffusing capacity of carbon monoxide or DLCO) are calculated as a product of total lung volume and gas transfer rate constants divided by the pressure gradient. Under conditions of known constant pressure breath-hold, the volume is measured by hyperpolarized 129Xe MRI, and the transfer rate is measured by dynamic spectroscopy. The new quantities (xenon diffusing capacity in lung parenchyma (DLXeLP)), xenon diffusing capacity in RBCs (DLXeRBC), and total lung xenon diffusing capacity (DLXe)) were measured in six normal rats and six rats with lung inflammation induced by instillation of fungal spores of Stachybotrys chartarum. DLXeLP, DLXeRBC, and DLXe were 56 +/- 10 ml/min/mmHg, 64 +/- 35 ml/min/mmHg, and 29 +/- 9 ml/min/mmHg, respectively, for normal rats, and 27 +/- 9 ml/min/mmHg, 42 +/- 27 ml/min/mmHg, and 16 +/- 7 ml/min/mmHg, respectively, for diseased rats. Lung volumes and gas transfer times for LP (TtrLP) were 16 +/- 2 ml and 22 +/- 3 ms, respectively, for normal rats and 12 +/- 2 ml and 35 +/- 8 ms, respectively, for diseased rats. Xenon diffusing capacities may be useful for measuring changes in gas exchange associated with inflammation and other lung diseases.

NMR velocity mapping of gas flow around solid objects

We present experimental visualizations of gas flow around solid blunt bodies by NMR imaging. NMR velocimetry is a model-free and tracer-free experimental means for quantitative and multi-dimensional flow visualization. Hyperpolarization of (129)Xe provided sufficient NMR signal to overcome the low density of the dilute gas phase, and its long coherence time allows for true velocity vector mapping. In this study, the diverging gas flow around and wake patterns immediately behind a sphere could be vectorally visualized and quantified. In a similar experiment, the flow over an aerodynamic model airplane body revealed a less disrupted flow pattern.

Two-compartment radial diffusive exchange analysis of the NMR lineshape of Xe129 dissolved in a perfluorooctyl bromide emulsion

Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s.

Micropores in Crystalline Dipeptides as Seen from the Crystal Structure, He Pycnometry, and 129Xe NMR Spectroscopy

Eight crystalline dipeptides were studied: AV (Ala-Val), VA (Val-Ala), AI (Ala-Ile), VV (Val-Val), IA (Ile-Ala), IV (Ile-Val), VI (Val-Ile), and LS (Leu-Ser) (all LL isomers). The first seven form an isostructural series (space group P6(1)), whereas LS has a different structure (P6(5)). All structures display H-bonded tubular assemblies of the dipeptide molecules resulting in open ultramicropores in the form of isolated one-dimensional (1D) channels. The total porosity of the materials ranges from 4 to 12% (micropore volume from 0.04 to 0.12 cm(3)/g). Calculations based on the crystal structures, He pycnometry, and solid-state (129)Xe NMR methods were used to obtain a comprehensive description of the geometry and properties of the micropores. The following order was established for the channel diameter: AV > VA > AI > VV > IA > IV > VI, with >5 A for AV and <4 A for VI; LS is close to AI. The observed sorption behavior cannot be described adequately based on the crystal structure and can only be understood if one takes into account the dynamics of the host matrix. The pores are chiral, with the center of the channel describing a right-handed helix (left-handed for LS). The following order was established for the channel helicity: VA > IA > IV > AV approximately AI approximately VV > VI > LS, with a helix diameter of approximately 2 A for VA, IA, and IV and approximately 1 A or less for the remaining dipeptides. A comparison of the dipeptides studied with other supramolecular materials is given and the potential for applications is discussed.

Analysis of porosity in porous silicon using hyperpolarized 129Xe two-dimensional exchange experiments

The porosity in porous silicon was characterized using hyperpolarized (HP) xenon as a probe. HP xenon under conditions of continuous flow allows for the rapid acquisition of xenon NMR spectra that can be used to characterize a variety of materials. Two-dimensional exchange spectroscopy (EXSY) (129)Xe NMR experiments using HP xenon were performed to obtain exchange pathways and rates of xenon mobility between pores of different dimensions within the structure of porous silicon and to the gas phase above the sample. Pore sizes are estimated from chemical shift information and a model for pore geometry is presented.

Nuclear Magnetic Resonance Studies of Resorcinol−Formaldehyde Aerogels

In this article, we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, [resorcinol]/[catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized (129)Xe NMR in combination with solid-state (13)C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both coadsorption of chloroform and 2D EXSY (129)Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY (129)Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micropores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (V(g)/S) ratios for aerogels are measured for the first time without the use of geometric models. The V(g)/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in V(g)/S.

Passive shimming of the fringe field of a superconducting magnet for ultra-low field hyperpolarized noble gas MRI

Hyperpolarized noble gases (HNGs) provide exciting possibilities for MR imaging at ultra-low magnetic field strengths (<0.15 T) due to the extremely high polarizations available from optical pumping. The fringe field of many superconductive magnets used in clinical MR imaging can provide a stable magnetic field for this purpose. In addition to offering the benefit of HNG MR imaging alongside conventional high field proton MRI, this approach offers the other useful advantage of providing different field strengths at different distances from the magnet. However, the extremely strong field gradients associated with the fringe field present a major challenge for imaging since impractically high active shim currents would be required to achieve the necessary homogeneity. In this work, a simple passive shimming method based on the placement of a small number of ferromagnetic pieces is proposed to reduce the fringe field inhomogeneities to a level that can be corrected using standard active shims. The method explicitly takes into account the strong variations of the field over the volume of the ferromagnetic pieces used to shim. The method is used to obtain spectra in the fringe field of a high-field (1.89 T) superconducting magnet from hyperpolarized 129Xe gas samples at two different ultra-low field strengths (8.5 and 17 mT). The linewidths of spectra measured from imaging phantoms (30 Hz) indicate a homogeneity sufficient for MRI of the rat lung.

Xe NMR lineshapes in channels of peptide molecular crystals

To further an understanding of the nature of information available from Xe chemical shifts in cavities in biological systems, it would be advantageous to start with Xe in regular nanochannels that have well known ordered structures built from amino acid units. In this paper, we report the experimental observation of Xe NMR lineshapes in peptide channels, specifically the self-assembled nanochannels of the dipeptide L-Val-L-Ala and its retroanalog L-Ala-L-Val in the crystalline state. We carry out grand canonical Monte Carlo simulations of Xe in these channels to provide a physical understanding of the observed Xe lineshapes in these two systems.

Hyperpolarized xenon in NMR and MRI

Hyperpolarized gases have found a steadily increasing range of applications in nuclear magnetic resonance (NMR) and NMR imaging (MRI). They can be regarded as a new class of MR contrast agent or as a way of greatly enhancing the temporal resolution of the measurement of processes relevant to areas as diverse as materials science and biomedicine. We concentrate on the properties and applications of hyperpolarized xenon. This review discusses the physics of producing hyperpolarization, the NMR-relevant properties of 129Xe, specific MRI methods for hyperpolarized gases, applications of xenon to biology and medicine, polarization transfer to other nuclear species and low-field imaging.

Electric field effects on the shielding constants of noble gases: A four-component relativistic Hartree-Fock study

Second derivatives of nuclear shielding constants with respect to an electric field, i.e., shielding polarizabilities, have been calculated for the noble gas atoms from helium to xenon. The calculations have been carried out using the four-component relativistic Hartree-Fock method. In order to assess the importance of the individual relativistic corrections, the shielding polarizabilities have also been calculated at the nonrelativistic Hartree-Fock level, with spin-orbit and scalar (Darwin and mass-velocity) effects having been established by perturbative methods. Electron correlation effects have been estimated using the second-order polarization propagator approach. The relativistic effects on the tensor components of the shielding polarizabilities are found to be larger and changing less regularly with the atomic number than for the shielding constant itself. However, there is a partial cancellation of the contributions to the parallel and perpendicular components of the shielding polarizability and as a consequence the mean shielding polarizability is far less affected than the individual components.

Time resolved spectroscopic NMR imaging using hyperpolarized 129Xe

We have visualized the melting and dissolution processes of xenon (Xe) ice into different solvents using the methods of nuclear magnetic resonance (NMR) spectroscopy, imaging, and time resolved spectroscopic imaging by means of hyperpolarized 129Xe. Starting from the initial condition of a hyperpolarized solid Xe layer frozen on top of an ethanol (ethanol/water) ice block we measured the Xe phase transitions as a function of time and temperature. In the pure ethanol sample, pieces of Xe ice first fall through the viscous ethanol to the bottom of the sample tube and then form a thin layer of liquid Xe/ethanol. The xenon atoms are trapped in this liquid layer up to room temperature and keep their magnetization over a time period of 11 min. In the ethanol/water mixture (80 vol%/20%), most of the polarized Xe liquid first stays on top of the ethanol/water ice block and then starts to penetrate into the pores and cracks of the ethanol/water ice block. In the final stage, nearly all the Xe polarization is in the gas phase above the liquid and trapped inside the pores. NMR spectra of homogeneous samples of pure ethanol containing thermally polarized Xe and the spectroscopic images of the melting process show that very high concentrations of hyperpolarized Xe (about half of the density of liquid Xe) can be stored or delivered in pure ethanol.

In Situ NMR Spectroscopy of Combustion

The first successful in situ studies of free combustion processes by one- and two-dimensional NMR spectroscopy are reported, and the feasibility of this concept is demonstrated. In this proof-of-principle work, methane combustion over a nanoporous material is investigated using hyperpolarized (hp)-xenon-129 NMR spectroscopy. Different inhomogeneous regions within the combustion cell are identified by the xenon chemical shift, and the gas exchange between these regions during combustion is revealed by two-dimensional exchange spectra (EXSY). The development of NMR spectroscopy as an analytical tool for combustion processes is of potential importance for catalyzed reactions within opaque media that are difficult to investigate by other techniques.

High capacity production of >65% spin polarized xenon-129 for NMR spectroscopy and imaging

A rubidium spin exchange optical pumping system for high capacity production of >65% spin polarized 129Xe gas is described. This system is based on a fiber coupled multiple laser diode array capable of producing an unprecedented 210 W of circularly polarized light at the pumping cell with a laser line width of 1.6 nm. The 129Xe nuclear spin polarization is measured as a function of flow rate, pumping cell pressure, and laser power for varying pumping gas compositions. A maximum 129Xe nuclear polarization of 67% was achieved using a 0.6% Xe mixture at a Xe flow rate of 2.45 sccm. The ability to generate 12% polarized 129Xe at rates in excess of 1L-atm/h is also demonstrated. To achieve production of 129Xe gas at even higher polarization will rely on further optimization of the pumping cell and laser beam geometries in order to mitigate problems associated with temperature gradients that are encountered at high laser power and Rb density.

Barley viability during storage: use of magnetic resonance as a potential tool to study viability loss

Malting-quality barley samples of the varieties Harrington, Manley, and TR118, each from two locations in Saskatchewan, were collected directly from the producers and sent to China for storage. At regular intervals samples were shipped back to Canada for analysis consisting of germination studies, alpha-amylase tests, magnetic resonance imaging (MRI), and magnetic relaxation (NMR) studies. Samples showing a decrease in germinative energy and elevated levels of alpha-amylase also showed a rapid uptake of water in the area between the embryo and the endosperm as observed by MRI. Using NMR relaxation experiments, viable and nonviable barley samples could be distinguished after 2 h of imbibition.

In situ switching of sorbent functionality as monitored with hyperpolarized (129)Xe NMR spectroscopy

In this contribution, we demonstrate that a material (organic zeolite mimetic coordination polymer [CuL(2)], where L = L(-) = CF(3)COCHCOC(OCH(3))(CH(3))(2)) can be endowed with its functionality in situ under molecular-level control. This process involves the isomerization of the ligands followed by phase interconversion from a dense to an open, porous form. The porous (beta) form of the complex reveals zeolite-like behavior but, unlike zeolites and many other hard porous frameworks, porosity may be created or destroyed at will by the application of suitable external stimuli. Contact with methylene chloride vapor was used to switch on the sorbent functionality, whereas switching off was accomplished with a temperature pulse. The transformations between functionally inactive alpha and active beta forms, as well as the amount of vacant pore space, were monitored in situ by observing the NMR spectrum of hyperpolarized (HP) Xe atom probes. For methylene chloride, the chemical shift of the coabsorbed HP Xe correlated directly with the amount of adsorbate in the pore system of the open framework, illustrating the use of HP Xe for following sorption kinetics. The adsorption of propane, as an inert adsorbate, was also monitored directly with (1)H NMR, with HP Xe and by BET measurements, revealing more complex behavior.