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.

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Carbon nanotubes provided by different manufacturers and synthesized by a variety of methods were subjected to the same oxidative purification procedure. Electron spin resonance ͑ESR͒ was used to investigate changes in the electronic structure before and after purification and exposure to hydrogen gas at a pressure of 136 kPa. The ESR signal in single-wall carbon nanotubes was due to paramagnetic impurities and diminished in intensity upon hydrogen adsorption. The conduction electrons of multiwall carbon nanotubes gave rise to signals with Dysonian line shapes. Here, the signal intensity increased upon hydrogen adsorption and the asymmetry parameter as well as the g factor were affected, suggesting a decrease in band gap. In samples with large metal content a ferromagnetic resonance was observable which disappeared upon purification. Some samples yielded no observable ESR signal due to an increased relaxation time of the electrons upon interaction with residual metal catalyst particles and possibly a large proportion of semiconducting nanotubes.