Configuration and Performance of a Mobile 129Xe Polarizer

A stand-alone, self-contained and transportable system for the polarization of ¹²⁹Xe 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 ¹²⁹Xe 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 ¹²⁹Xe 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 ¹²⁹Xe 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 ¹²⁹Xe more widespread.     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.     

Time resolved spectroscopic NMR imaging using hyperpolarized Xe

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.     

Noninvasive Measurements of Gas Exchange in a Three-Dimensional Fluidized Bed by Hyperpolarized 129Xe NMR

We present a novel nuclear magnetic resonance (NMR) technique that provides a noninvasive, direct measurement of gas exchange in a three-dimensional gas-fluidized bed of solid particles. The NMR spectrum of hyperpolarized ¹²⁹Xe gas in an Al₂O₃ particle bed displays three resolved peaks corresponding to xenon in bubbles, the interstitial spaces (emulsion), and adsorbed on particles. Modified NMR exchange and saturation recovery sequences, together with data analysis based on an exchange-coupled set of Bloch equations, yield gas exchange rate constants between the emulsion and adsorbed phases, and between the bubble and emulsion phases. The results are in approximate agreement with previously unverified predictions from well-known models of fluidized bed behavior. Incorporation of NMR imaging methodologies would straightforwardly allow similar measurements on a spatially resolved basis. Authors' address: Ross W. Mair, Harvard Smithsonian Center for Astrophysics, MS 59, 60 Garden Street, Cambridge, MA 02138, USA     

129Xe NMR – highly polarized thin films

We studied the macroscopic effects of nuclear magnetization. Highly polarized xenon is often used to increase the sensitivity in NMR investigations of porous media, diluted liquids or for imaging in the gas phase. In the condensed phase, however, highly nuclear spin polarized xenon also possesses a sizable magnetization due to the nuclear spin density. This results in an additional magnetic field, that is used to measure the polarization of the sample, when only the particle density is known. Here we find P_z≈0.8 corresponding to a spin temperature of 0.5 mK. We use isotopically enriched xenon with a ¹²⁹Xe abundance of 0.71. At high abundance of ¹²⁹Xe and high nuclear polarization the dipolar linewidth is considerably reduced. We find for small angle excitation a reduction from 650 Hz to 400 Hz. We investigate this using a thin film geometry. The susceptibility effects of the substrate and the Xe film are treated. The macroscopic angle between the normal of the film and the external field strongly changes the polarization induced line shift and line width. The first follows an expected cos²θ dependence with an understood amplitude the latter however is not understood up to now. Relaxation of ¹²⁹Xe in the condensed film is observed to be T₁=15±1.8 min, much faster than expected. To cite this article: P. Gerhard et al., C. R. Physique 5 (2004).     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

Optical pumping system design for large production of hyperpolarized

We present a design for a spin-exchange optical pumping system to produce large quantities of highly polarized 129Xe. Low xenon concentrations in the flowing gas mixture allow the laser to maintain high Rb polarization. The large spin-exchange rate between Rb and 129Xe through the long-lived van der Waals molecules at low pressure, combined with a high flow rate, results in large production rates of hyperpolarized xenon. We report a maximum polarization of 64% achieved for a 0.3 l/h Xe flow rate, and maximum magnetization output of 6 l/h at 22% polarization. Our findings regarding the polarization dependence on temperature, nitrogen partial pressure, and gas mixture flow velocity are also reported.     

T1-relaxation of 129Xe on metal single crystal surfaces-multilayer experiments on iridium and monolayer considerations

The surface of a typical laboratory single crystal has about 10(15) surface atoms or adsorption sites, respectively, and is thus far out of reach for conventional NMR experiments using thermal polarization. It should however be in reach for NMR of adsorbed laser polarized (hyperpolarized) 129Xe, which is produced by spin transfer from optically pumped rubidium. With multilayer experiments of xenon adsorbed on an iridium surface we do not only demonstrate that monolayer sensitivity has been obtained, we also show that such surface experiments can be performed under ultra high vacuum conditions with the crystal being mounted in a typical surface analysis chamber on a manipulator with far-reaching sample heating and cooling abilities. With only four spectra summed up we present an NMR signal from at most 4x10(14) atoms of 129Xe, four layers of naturally abundant xenon, respectively. The fact that no monolayer signal has been measured so far is explained by a fast Korringa relaxation due to the Fermi contact interaction of the 129Xe nuclei with the electrons of the metal substrate. T(1)-relaxation times in the order of several ms have been calculated using all electron density functional theory for several metal substrates.     

Optimization of continuous flow pump cells used for the production of hyperpolarized <Superscript>129</Superscript>Xe: A theoretical study

Laser-polarized xenon can be used to increase the sensitivity of NMR experiments by 4–5 orders of magnitude, enabling numerous applications in physics, chemistry, biology, and medicine. For the production of hyperpolarized (hp) ¹²⁹Xe, continuous flow pump cells are commonly used. Based on a 3D numerical model consisting of eight partial differential equations [1], we describe the optimization of continuous flow cells with respect to several experimental parameters in this contribution. We also compare our simulations with data from literature. PACS 32.80.Bx     

A rapid and precise probe for measurement of liquid xenon polarization

The relaxation time of liquid (129)Xe is very long (>15 min) and the signal at thermal equilibrium is weak. Therefore, determination of the absolute polarization enhancement of hyperpolarized (129)Xe by direct measurement is tedious. We demonstrate a fast and precise alternative, based on the dipolar field created by liquid hyperpolarized (129)Xe contained in a cylindrical sample tube. The dipolar field is homogeneous in the bulk of the tube and adds to the external field, causing a shift in the Larmor frequencies of all nuclear spins. We show that the frequency shift of the proton in CHCl(3) (chloroform), which dissolves homogeneously in xenon over a fairly broad temperature range, is an excellent probe for (129)Xe polarization. Frequency measurements are precise and the experiment is much faster than by direct measurement. Furthermore the (129)Xe polarization is minimally disturbed since no rf pulses are applied directly to (129)Xe and since chloroform is a fairly weak source of (129)Xe relaxation. The experiments are reproducible and require only standard NMR instrumentation.     

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.     

Mobile high resolution xenon nuclear magnetic resonance spectroscopy in the earth's magnetic field

Conventional high resolution nuclear magnetic resonance (NMR) spectra are usually measured in homogeneous, high magnetic fields (>1 T), which are produced by expensive and immobile superconducting magnets. We show that chemically resolved xenon (Xe) NMR spectroscopy of liquid samples can be measured in the Earth's magnetic field (5 x 10(-5) T) with a continuous flow of hyperpolarized Xe gas. It was found that the measured normalized Xe frequency shifts are significantly modified by the Xe polarization density, which causes different dipolar magnetic fields in the liquid and in the gas phases.     

Investigating hyperpolarized Xe and CPMAS for spin polarization transfer to surface nuclei: a model study

Several experimental techniques have been developed to utilize spin-polarized xenon gas for sensitivity and selectivity enhancement in surface studies using solid-state NMR. Although previously reported as a viable spin polarization transfer mechanism, the details of high-field cross-polarization (CP) have not been thoroughly investigated. We recently reported observations of CP from an adsorbed layer of hyperpolarized xenon (HP Xe) to a variety of surface nuclei at temperatures as high as 323 K [J. Am. Chem. Soc. 105 (2001) 1412]. In this paper, we investigate many of the issues associated with HP Xe surface CP studies, including polarization transfer kinetics and the effects of temperature on the dynamics. Protonated and methylated silica samples are used as model systems for comparison. A comparison of the rate analysis data from CP and SPINOE (Spin Polarization-Induced Nuclear Overhauser Effect) experiments provides information on the origin of the difference in polarization transfer efficiencies between the two techniques. Lineshape analysis of ¹H spectra for CP and SPINOE experiments demonstrates the difference in selectivity of methods due to longer SPINOE evolution times that lead to greater spin diffusion. The results of this work help to assess the viability of HP Xe CP as a surface analysis technique.     

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.     

Surface SelectiveH/Si CP NMR by NOE Enhancement from Laser Polarized Xenon

The surface proton spin polarization created by the spin-polarization-induced nuclear Overhauser effect from optically polarized xenon can be transferred in a subsequent step by solid-state cross polarization to another nuclear spin species such as²⁹Si. The technique exploits the dipolar interactions of xenon nuclear spins with high γ nuclei such as¹H, and is experimentally simpler than direct polarization transfer from¹²⁹Xe to heteronuclei such as¹³C and²⁹Si.     

Measurement of diffusion coefficients in polystyrene using xenon-129 NMR

A sample of polystyrene beads, 18 μm in diameter, has been sealed in an NMR tube under 10 atm of xenon gas. Two dimensional,¹²⁹Xe NMR spectra show cross peaks between the resonances corresponding to xenon in the free gas and the sorbed state, indicating that appreciable exchange occurs during the mixing time of the NMR experiment. Selective saturation of the free gas resonance attenuates the integrated intensity of the sorbed xenon resonance as a function of saturation time, thus allowing the accurate measurement of the exchange rates between the gas and the sorbed states. A model has been developed using a slightly modified form of Crank’s treatment of diffusion in a sphere which allows for the accurate determination of the diffusion coefficient for xenon in the sorbed state. The diffusion coefficient for xenon in polystyrene at 25°C is determined to be 2.9·10^?9 cm²/s.     

Intravascular delivery of hyperpolarized129Xenon forin vivo MRI

The nuclear polarization of¹²⁹Xe and³He can be enhanced by several orders of magnitude by using optical pumping techniques, thus allowing NMR detection of xenon and helium in very low concentrations. The benefits of optically enhanced magnetic resonance (MR) are already exploited in MR imaging of the lungs using optically polarized³He. The high solubility of xenon in blood and lipids suggests a variety ofin vivo MR applications, for instance perfusion measurements or functional MR studies. This article reviews some current work directed towards delivery of optically polarized xenon forin vivo MR applications.     

129Xe NMR in study of tissues and plants

The high sensitivity of the¹²⁹Xe nuclear magnetic resonance (NMR) chemical shift to the environment was used for characterization of biological tissues and plants. The xenon gas was dissolved under moderate pressure by means of a special device in small pieces of human and animal tissues (heart, muscle, lung, kidney, liver, spleen, brain, sinew, cartilage and hypodermic fat) or plants (leaves, stems, grains, fruits) and the NMR parameters were measured in vitro. The observed line with the chemical shift ～ 180 ppm was attributed to the xenon located in various cellular structures such as lipid shells, intracellular formations. A xenon spectrum in the lungs obtained in vitro coincides with that in the lungs of a mouse measured in vivo by other investigators. The NMR parameters were found to reveal noticeable distinctions between normal and tumour-affected tissues. The analysis of the¹²⁹Xe NMR spectra of the sinew and the cartilage revealed the dependence of the magnetic parameters on the age of the substance. This fact could be accounted for by the changes of the absorption ability of a biological system due to age transformations. The results obtained in comparison with biochemical data reveal the promissory outlook of¹²⁹Xe NMR for the investigation of the state of biological tissues and for medical diagnostics.     

Frequency-selective laser optical pumping and spin exchange of cesium with129Xe and131Xe in a high magnetic field

We report the experimental results of frequency-selective laser optical pumping and spin exchange of Cs with¹²⁹Xe and¹³¹Xe in a high magnetic field of 11.74 T. Our results show that hyperpolarized¹²⁹Xe and¹³¹Xe nuclear magnetic resonance (NMR) signals exhibit alternating phases when the laser frequency for pumping the cesium atoms is changed, which is explained on the basis of the high-field optical pumping of Cs. We obtain about 3% polarization of the¹²⁹Xe. The electron-spin polarization of the Cs atoms has been measured to be about 22% with a simple NMR method.