Spin prepolarization with a compact superconducting magnet

Abstract. Compact benchtop NMR systems provide excellent and affordable access to good-quality NMR spectroscopy. Nevertheless, such systems are limited by low polarization levels, resulting in low signal-to-noise ratios compared to those of high-field systems. We show here that polarization levels can be significantly improved by using a medium-homogeneity high-field magnet as a spin prepolarizer. For this type of brute-force hyperpolarization we employ a cryogen-free 5 T superconducting magnet. Because such systems typically lack shielding and thus have noticeable stray fields, samples can be transferred adiabatically from the prepolarizer to the bore of a commercial benchtop NMR system. By adjusting the physical separation between the two magnets, and hence ensuring a sufficiently strong stray field during sample transfer, we report a ¹H polarization enhancement of up to a factor of 2.62 as a first demonstration of the utility. By employing 2G-HTS magnets, higher magnetic fields would become possible while minimizing the size and stray field of the magnet, so that the polarization levels can be further increased in a foreseeable future with moderate effort. In a follow-up paper, we aim to explore some of the advantages of the prepolarization approach.