Hyperpolarized ¹³¹Xe NMR spectroscopy

Hyperpolarized (hp) ¹³¹Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4 T) was obtained after separation from the rubidium vapor of the spin-exchange optical pumping (SEOP) process. The SEOP was applied for several minutes in a stopped-flow mode, and the fast, quadrupolar-driven T₁ relaxation of this spin I = 3/2 noble gas isotope required a rapid subsequent rubidium removal and swift transfer into the high magnetic field region for NMR detection. Because of the xenon density dependent ¹³¹Xe quadrupolar relaxation in the gas phase, the SEOP polarization build-up exhibits an even more pronounced dependence on xenon partial pressure than that observed in ¹²⁹Xe SEOP. ¹³¹Xe is the only stable noble gas isotope with a positive gyromagnetic ratio and shows therefore a different relative phase between hp signal and thermal signal compared to all other noble gases. The gas phase ¹³¹Xe NMR spectrum displays a surface and magnetic field dependent quadrupolar splitting that was found to have additional gas pressure and gas composition dependence. The splitting was reduced by the presence of water vapor that presumably influences xenon-surface interactions. The hp ¹³¹Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp ¹³¹Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I≥1/2 nuclei is presented.