By means of optical pumping with laser light it is possible to enhance the nuclear spin polarization of gaseous xenon by four to five orders of magnitude. The enhanced polarization has allowed advances in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), including polarization transfer to molecules and imaging of lungs and other void spaces. A critical issue for such applications is the delivery of xenon to the sample while maintaining the polarization. Described herein is an efficient method for the introduction of laser-polarized xenon into systems of biological and medical interest for the purpose of obtaining highly enhanced NMR͞MRI signals. Using this method, we have made the first observation of the timeresolved process of xenon penetrating the red blood cells in fresh human blood-the xenon residence time constant in the red blood cells was measured to be 20.4 ؎ 2 ms. The potential of certain biologically compatible solvents for delivery of laser-polarized xenon to tissues for NMR͞MRI is discussed in light of their respective relaxation and partitioning properties.

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...