Mobile High Resolution Xenon Nuclear Magnetic Resonance Spectroscopy in the Earth’s Magnetic Field

Conventional high resolution nuclear magnetic resonance (NMR) spectra are usually measured in homogeneous, high magnetic fields ($>1\mathrm{T}$), which are produced by expensive and immobile superconducting magnets. We show that chemically resolved xenon (Xe) NMR spectroscopy of liquid samples can be measured in the Earth’s magnetic field ($\sim 5\times {10}^{-5}\mathrm{T}$) with a continuous flow of hyperpolarized Xe gas. It was found that the measured normalized Xe frequency shifts are significantly modified by the Xe polarization density, which causes different dipolar magnetic fields in the liquid and in the gas phases.

Figure 1

Experimental setup for the Earth’s field NMR.

Figure 2

Xe NMR spectroscopy in the Earth’s field. (a) ${}^{129}\mathrm{Xe}$ FID (left) and corresponding spectrum (right) measured with Xe gas flowing above $2{\mathrm{cm}}^3$ of liquid toluene. (b) ${}^{129}\mathrm{Xe}$ FID as in (a) but without gas flow. Compared to (a) the transverse relaxation time of the Xe gas ($T_2\sim 80\mathrm{s}$) is about 6 times longer. (c) The Fourier transform of (b) (solid line) shows two separated lines ($\sim 8\times {10}^{-3}\mathrm{Hz}$ FWHM) corresponding to Xe gas (left) and Xe in toluene (right). The dotted line displays the spectrum of Xe in ethanol. For all experiments the flip angle of the dc excitation pulse was 90°.

Figure 3

Accuracy of measured Xe chemical shifts. (a) Xe spectra in ethanol measured over a time period of 30 min with 90° pulses. The density of Xe gas (Xe in ethanol) is 0.07 (0.18) amagat. (b) Xe chemical shift of toluene (circles) and of ethanol (triangles). The solid lines indicate the chemical shift measured at 7 T.

Figure 4

Xe NMR spectroscopy at high Xe polarization densities. (a) Enhanced view of the liquid toluene sample in the Earth’s magnetic field and sketch of the Xe dipolar field antiparallel and parallel to the Earth’s field vector. (b) Dotted line: Reference ${}^{129}\mathrm{Xe}$ spectrum after 90° pulse with low ${}^{129}\mathrm{Xe}$ polarization density ($0.1\mathrm{\text{amagat}}\times P_{\mathrm{Xe}}$ in the liquid). Dashed (solid) line: ${}^{129}\mathrm{Xe}$ spectrum in the presence of high ${}^{129}\mathrm{Xe}$ polarization density ($\sim 1\mathrm{\text{amagat}}\times P_{\mathrm{Xe}}$ in the liquid) after a 30° pulse for antiparallel (parallel) orientation of the Xe polarization vector. A positive and a negative shift as well as a broadening of the Xe peaks in toluene are observed. (c) Time dependence of the observed normalized Xe frequency shift which converges to the chemical shift of toluene (192 ppm) for both orientations of the Xe polarization. Solid line: Exponential fit. (d) Exponential fit (solid line) of the measured linewidth of Xe gas (triangles) and Xe in toluene (circles).