Surface-Enhanced NMR Using Continuous-Flow Laser-Polarized Xenon

Abstract

Sensitivity and selectivity pose constant challenges in NMR studies of surfaces. 1 The number of nuclei residing on the surface of a material may be too small for NMR observation, and potentially detectable signals may not be discernible from the spectrum of the bulk material. Some progress has been achieved in recent years by the use of laser-polarized xenon, either by freezing the gas onto the solid surface followed by high-field cross-polarization (CP) 2 or by nuclear Overhauser cross-relaxation from adsorbed polarized xenon, 3 a process dubbed SPINOE 4 (spin polarization induced nuclear Overhauser effect). For both approaches, experiments are typically performed with one batch of isotopically enriched laser-polarized Xe, with subsequent polarization decay either by relaxation in the adsorbed states or by radio frequency driven transfer of the spin polarization. In either case, loss of polarization prevents signal accumulation and the multiple pulse manipulation of spins necessary for spectral resolution. A method for rapid production and continuous delivery of polarized xenon gas to surfaces would clearly be advantageous.In this communication, we report the use of a fast optical pumping process, combined with a closed gas-circulation NMR probehead designed to deliver a continuous flow of laserpolarized Xe to an Aerosil surface. The technology for the production of 129 Xe with nuclear spin polarization several orders of magnitude higher than thermal Boltzmann levels is now well established. 5 In the present study, we exploited the pressure broadening of the rubidium D1 transitions, and recently available high-power diode laser arrays, to pump much denser Rb vapors ([Rb] ) 4 × 10 14 cm -3 ), thereby enabling spin-exchange time constants on the order of tens of seconds. 6 The high-pressure optical pumping cell was connected to a gas recirculation system capable of delivering a continuous stream of the polarized xenon gas to the NMR sample, as shown schematically in Figure 1. This new methodology allowed the observation of difference SPINOE's that selectively highlight surface spins, thereby enhancing both sensitivity and selectivity for surface-resolved NMR spectroscopy.Aerosil300 was obtained from Degussa Inc. and is reported to have a surface area of 300 m 2 /g with about 2.5 hydroxyl protons/nm 2 . Sample (150 mg) was packed into a 10 mm U-shaped sample tube placed in a double-tuned probe ( 1 H: 178.02 MHz; 129 Xe: 49.24 MHz) built on the basic principle of lumped circuit elements published previously. 7 The sample tube and the optical pumping cell were constructed of glass and subsequently glued to 1 / 8 in. copper tubing. The optical pumping cell was placed in the 150 G fringe field of a 4.2 T superconducting magnet; this assembly was then attached to the recirculation and vacuum pumps, as well as the sample probe. This entire assembly was evacuated to below 10 -5 Torr for several hours to remove physisorbed water and oxygen; during evacuation, the sample was kept at 50°C. The evacuated system was then fi...

Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging

“…Hyperpolarized (hp) 3 He and hp 129 Xe (both I ϭ 1͞2) have both been used in a wide range of NMR and magnetic resonance imaging (MRI) experiments that are otherwise impossible (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). In particular hp 3 He has been applied for medical MRI diagnosis of pulmonary diseases (16)(17)(18)(19).…”

“…However, 129 Xe adds a further parameter not available by 3 He, namely, the chemical shift that allows for insights into the local environment of the xenon atoms (22)(23)(24)(25)(26)(27). The 129 Xe chemical shift has been used extensively for research in materials science, engineering, and xenon dissolved in liquids including human blood (3,5,6,8,9,12,28). The chemical shift obtained from hp 129 Xe can be used to generate an in vivo MRI contrast that is a probe for gas perfusion in lungs (i.e., exchange of the gaseous xenon with the lung parenchyma) (29).…”

Amplification of xenon NMR and MRI by remote detection

“…with a total pressure of 7 atm was used. Polarizer, encoding, and detection sites were connected in a closed loop, maintaining a constant pressure of 7 atm (27). This setup was used only for the remote imaging experiment.…”

129Xenon NMR: Review of recent insights into porous materials

“…Prompt implementation of continuous flow systems greatly improved the convenience and productivity of HP experiments regardless the fact that polarization levels in batch systems were much higher than those obtained under flow conditions. The first paper reporting continuousflow SEOP in aerosil surface was published by Haake et al [67] In such systems a XeHe mixture is passed first through the pressurized polarizing cell containing Rb vapor and then through the sample. The mixture containing ca.…”