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Description
Introduction
Nuclear spin hyperpolarization (HP) enhances the NMR signal by several orders of magnitude by bringing the spins out of thermal equilibrium, populating one of the spin states in favor of the other. Enhanced spin polarization is especially advantageous at ultra-low magnetic field strengths, where thermal polarization may often result in NMR signal intensities that are indistinguishable from noise.
Among all nuclei, xenon-129 is a remarkable NMR probe as it is inert, soluble in biological tissues, and its hyperpolarization lifetime can last up to a few hours. Xenon-129 hyperpolarization is accomplished via spin-exchange optical pumping (SEOP), whereby spin order of a high-power laser is indirectly transferred to the $^{129}$Xe nucleus. The long-lasting $^{129}$Xe spin order may then be transferred to other fast-relaxing nuclei via the spin polarization-induced nuclear Overhauser effect (SPINOE) [1].
Previous works investigated the transfer of spin order from hyperpolarized $^{129}$Xe to thermally polarized $^{1}$H [2, 3]. Our investigation examines the continuous transfer of spin order from HP $^{129}$Xe to thermally polarized $^{19}$F via SPINOE in the ultra-low field regime. Unlike previous high field investigations, SPINOE enhancements at low field are directly detectable without the need to destroy background thermal polarization. Simultaneous detection of both nuclei enables the determination of cross-relaxation rates and the molecular dynamics of the SPINOE phenomenon.
Methods
All measurements were performed on a lab-built NMR spectrometer operating in the 2 mT regime using a dual-resonance volume coil capable of simultaneously exciting and detecting $^{129}$Xe and $^{19}$F. HP $^{129}$Xe was bubbled into 2 mL hexafluorobenzene (C$_{6}$F$_{6}$) in a 5 mm NMR tube for 6 seconds, prior to acquiring after a delay to allow for spin exchange. The low thermal spin polarization, which at ultra-low field is on the order of 10$^{-9}$, obviates the need for pre-saturation pulses to destroy thermal $^{19}$F signal. An RF train destroyed any remaining $^{129}$Xe and $^{19}$F hyperpolarization prior to subsequent acquisitions.
Results and Discussion
The SPINOE-enhanced $^{19}$F signal was found to persist for over 200 s, in stark contrast to its short T$_{1}$ of 1.1 s at 2 mT. This long-lasting signal enhancement was due to the continuous transfer of polarization from the slowly relaxing $^{129}$Xe spins, the T$_{1}$ of which in hexafluorobenzene is 144 s. The SPINOE-induced NMR signal enhancement was measured to be ε = 178, while the integrated enhancement, achieved across 64 averages, was measured to be ε = 8459. Finally, fitting of the $^{129}$Xe and $^{19}$F signal vs. time curves yielded a σ = $3.40 *10^{-5}$ s$^{-1}$M$^{-1}$ cross-relaxation rate, higher than that previously reported for $^{129}$Xe$-$$^{1}$H spin exchange.
Conclusion
The NMR signal of thermally polarized nuclei is largely undetectable at ultra-low field strengths by standard induction techniques typically used at high field. Here we demonstrate a versatile method to continuously transfer the polarization of $^{129}$Xe, efficiently polarized by SEOP, to other nuclei. This transfer of polarization results in a significantly enhanced NMR signal of the target nucleus and an apparent polarization that lasts beyond the intrinsic T$_{1}$ of the targeted spins.
References
[1] Song, Y.Q. (1999). Spin polarization-induced nuclear Overhauser effect: An application of spin-polarized xenon and helium. Concepts in Magnetic Resonance, 12(1), 6-20.
[2] Appelt, S., Häsing, F. W., Baer-Lang, S., Shah, N. J., & Blümich, B. (n.d.). Proton magnetization enhancement of solvents with hyperpolarized xenon in very low-magnetic fields. Chemical Physics Letters, 348, 263-269.
[3] Song, Y.Q., Goodson, B. M., Taylor, R. E., Laws, D. D., Navon, G., & Pines, A. (1997). Selective Enhancement of NMR Signals for α-Cyclodextrin with Laser-Polarized Xenon. Angewandte Chemie, 36(21), 2368-2370.
