How Far Can the Sensitivity of NMR Be Increased?

Abstract

Sensitivity enhancement enables the full utilization of NMR for the structural analysis of macromolecular systems. This review covers the methods for the sensitivity enhancement of NMR mainly on biological solid-state systems. Modern solid-state NMR spectroscopy is based on high-sensitivity methods, e.g. high-field superconducting magnets, cross polarization to generate large magnetization of low γ nuclei, proton detection and homo- and heteronuclear decoupling by rf fields and magic-angle spinning. Low temperature reduces the noise in the sample coil and increases the thermal equilibrium polarization. The development of the following methods will further increase the sensitivity, although it may not be straightforward to apply them to high-resolution NMR spectroscopy. Dynamic nuclear polarization, optical pumping, microwave induced optical nuclear polarization, photo-CIDNP, provide highly polarized nuclear magnetization using electron spins. Detection of NMR is another important factor for higher sensitivity. Use of small rf coils improves the sensitivity per sample volume. NMR can be measured by methods other than the generally used Faraday induction. SQUID is employed for NMR at low magnetic fields. NMR of a small number of spins can be detected by optical and electrical methods and also by atomic beams. The high sensitivity of magnetic resonance force detection has been demonstrated by microscopy.