External high-quality-factor resonator tunes up nuclear magnetic resonance

Abstract

The development of powerful sensors for the detection of weak electromagnetic fields is crucial for many spectroscopic applications, in particular, for nuclear magnetic resonance (NMR). Here we present a comprehensive, new theoretical model for boosting the signalto-noise ratio, validated by liquid-state 1 H, 129 Xe, and 6 Li NMR experiments at low frequencies (20-500 kHz), using an external resonator with a high quality factor combined with a low-quality-factor input coil. In addition to a very high signal-to-noise ratio, this external high quality-factor enhanced NMR exhibits striking features such as a large flexibility with respect to input coil parameters, and a square-root dependence on the sample volume, and signifies an important step towards compact NMR spectroscopy at low frequencies with small and large coils.Nuclear magnetic resonance spectroscopy preferably operates at high magnetic fields (10-24 T) to benefit from high sensitivity and high chemical shift dispersion. The signal is detected by nuclear induction in coils, which are part of a resonant circuit [1][2][3] . NMR operating at high frequencies (~500 MHz) is particularly in demand for microcoil-based spectroscopy with sample volumes smaller than 1 L (ref. 4). Microcoils, with their small inductance (L < 1 nH), cannot readily be tuned to lower resonance frequencies (<1 MHz) owing to the large capacity (C > 1 mF) necessary to fulfil the resonance condition   L C  . Sillerud et al. 5 were the first to address this problem by adding an external inductor to a microcoil for NMR detection at 40 MHz. Coffey et al. 6 reported a very weak frequency dependence of the signal-to-noise ratio (SNR) when comparing hyperpolarized NMR/MRI at 0.047 T and 4.7 T (keeping the polarization constant), with a possibly higher SNR of hyperpolarized low-field NMR/MRI. So far, NMR spectroscopy with microcoil detection and a large SNR have not be realized at very low Larmor frequencies -1000 kHz, where  is the gyromagnetic ratio of the nuclear spin species and B0 is the strength of the static magnetic field. Instead, other unconventional schemes for NMR detection are being explored at low frequencies. Such lowfrequency NMR field sensors are superconducting quantum interference devices (SQUIDs, refs 7,8) and atomic magnetometers 9,10 with sensitivities better than 1 fT/Hz 1/2 . Moreover, a single nitrogen vacancy (NV) centre in diamond can detect 100 out of 10 4 proton spins, which are

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