Zero- to Ultralow-field Nuclear Magnetic Resonance

TL;DR

Zero- to ultralow-field NMR enables chemical analysis and new experiments by operating at microtesla fields, offering advantages like portability, low cost, and unique capabilities due to the absence of strong magnetic fields.

Contribution

This paper reviews recent advances in ZULF NMR, highlighting its unique features, practical benefits, and potential future research directions.

Findings

ZULF NMR operates at microtesla fields, revealing spin interactions suppressed in high-field NMR.

It allows high-resolution spectroscopy in conductive and heterogeneous samples.

Recent improvements in magnetometers and hyperpolarization have enhanced ZULF NMR's accessibility.

Abstract

Zero and ultralow-field nuclear magnetic resonance (ZULF NMR) is an NMR modality where experiments are performed in fields at which spin-spin interactions within molecules and materials are stronger than Zeeman interactions. This typically occurs at external fields of microtesla strength or below, considerably smaller than Earth's field. In ZULF NMR, the measurement of spin-spin couplings and spin relaxation rates provides a nondestructive means for identifying chemicals and chemical fragments, and for conducting sample or process analyses. The absence of the symmetry imposed by a strong external magnetic field enables experiments that exploit terms in the nuclear spin Hamiltonian that are suppressed in high-field NMR, which in turn opens up new capabilities in a broad range of fields, from the search for dark matter to the preparation of hyperpolarized contrast agents for clinical imaging. Furthermore, as in ZULF NMR the Larmor frequencies are typically in the audio band, the nuclear spins can be manipulated with d.c. magnetic field pulses, and highly sensitive magnetometers are used for detection. In contrast to high-field NMR, the low-frequency signals readily pass through conductive materials such as metals, and heterogeneous samples do not lead to resonance line broadening, meaning that high-resolution spectroscopy is possible. Notable practical advantages of ZULF NMR spectroscopy are the low cost and relative simplicity and portability of the spectrometer system. In recent years ZULF NMR has become more accessible, thanks to improvements in magnetometer sensitivity and their commercial availability, and the development of hyperpolarization methods that provide a simple means to boost signal strengths by several orders of magnitude. These topics are reviewed and a perspective on potential future avenues of ZULF-NMR research is presented.