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Detecting a Signal... By Measuring the Noise

​The electronic noise of an NMR device is modulated by the nuclear spins of a sample placed inside the detecting coil. Scientists from IRAMIS have succeeded in building an analytical model to interpret this NMR signal of "spin noise". This new approach makes it possible to measure tenuous isotopic effects, and provides NMR with a dramatic gain in "spectroscopic" sensitivity.
Published on 24 January 2017

Although nuclear magnetic resonance (NMR) is a powerful analysis technique, it is not a very sensitive one. It is sometimes difficult to differentiate low-intensity isotopic effects, as they tend to be drowned out by the NMR signal. With this in mind, physicists have proposed a radical new approach: eliminating the nuclear spin excitation radiofrequence pulse and analyzing the electronic noise at the output of the detector! The result is that everything happens as if the "strong" and "weak" signals were separated.

How is that possible? The coil emitting the nuclear spin excitation signal also ensures detection of the spins response (NMR signal). Just like every detector, this coil and its associated preamplifier are affected by an intrinsic electronic noise, even in the absence of any "useful" signal. But this noise signal is sufficient to excite the nuclear spins, the response of which, detected by the coil, amplifies and modulates the fluctuations of the "initial" noise. This non-linear feedback (coupling) between nuclear spins and detecting coil is responsible for NMR signals of spin noise with high resolution, which had been observed before without being correctly interpreted.

A team of scientists has carried out the first analytical modeling of this spectroscopy and has shown that it makes it possible to solve isotopic effects that had never before been observed in NMR. This work paves the way to a completely new NMR spectroscopy.

This study was carried out in collaboration with Johannes Kepler University in Linz, Austria

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