Nonlinear detection of secondary isotopic chemical shifts in NMR through spin noise

Abstract

The detection of minor species in the presence of large amounts of similar main components remains a key challenge in analytical chemistry, for instance, to obtain isotopic fingerprints. As an alternative to the classical NMR scheme based on coherent excitation and detection, here we introduce an approach based on spin-noise detection. Chemical shifts and transverse relaxation rates are determined using only the detection circuit. Thanks to a nonlinear effect in mixtures with small chemical shift dispersion, small signals on top of a larger one can be observed with increased sensitivity as bumps on a dip; the latter being the signature of the main magnetization. Experimental observations are underpinned by an analytical theory: the coupling between the magnetization and the coil provides an amplified detection capability of both small static magnetic field inhomogeneities and small NMR signals. This is illustrated by two-bond 12C/13C isotopic measurements.

Spin‐Noise‐Detected Two‐Dimensional Nuclear Magnetic Resonance at Triple Sensitivity

“…The concentration was 0.65 mol L À1 for the d-glucose (as in the original experiment [1] )a nd 3.20 mol L À1 for d-fructose. We note that spin-noise-detected experiments involving coherence transfer are best conducted in the regime of positive spin noise signal [8,26] to mitigate quenching by radiation damping. The validity of this condition was tested before the 2D experiments by recording one dimensional spin noise spectra.…”

“…During evolution the amplitudes or phases of the coherences are modulated with a characteristic frequency, which may be a chemical shift, coupling constant or a linear combination of these . In spin‐noise‐detected experiments, as schematically drawn in Figure b, no RF irradiation is applied to the detected spins ( 1 H in most cases), as both excitation and detection rely exclusively on intrinsic random spin fluctuations, which are usually called spin noise and are intrinsic to the tightly coupled system consisting of the nuclear spins and the resonance RF circuit . For each repetition of the 2 D pulse sequence, two input and output noise blocks are acquired and cross‐correlated as demanded by the nature of the stochastic excitation process.…”

“…Recent advances in the field, especially the wide availability of cryogenically cooled probes, [5] allowed the measurement of spin noise on many spectrometers andr enewedt he interesti nt his phenomenon. [6][7][8] Even if practical applicationsi ns pectroscopy are still very limited, the properties of spin noise seem ap romising approach for addressing certain types of problems.E xtensive researchi no ne-dimensional spin noise spectroscopy gave rise to variousn ovel application in the fieldso fs pectroscopy and imaging. [7] Spin noise is also relevant for interference-free investigationso fs pin systems.…”

“…[3,10] Our focus is set on spectroscopic applications for liquid samples. [1,[7][8][9][11][12][13] To date, spin-noise-detected NMR spectroscopy is still in an early stage of development and will require additional hardware advances for implementation at nano-scale. However, in magnetic resonance force microscopy (MRFM) spin noise detectionh as become the state-of-the art.…”

“…[15] In spinnoise-detected experiments,a ss chematically drawn in Figure 1b,n oRF irradiationi sa pplied to the detected spins ( 1 Hi n most cases), as both excitation and detection rely exclusively on intrinsic random spin fluctuations, which are usually called spin noise and are intrinsic to the tightly coupled system consisting of the nuclear spins and the resonance RF circuit. [8] For each repetition of the 2D pulse sequence, two input and output noise blocks are acquired and cross-correlated as de-mandedb yt he nature of the stochastic excitation process. Cross-correlation is achieveda sd etailed in the paper by Chandra et al [1] and illustrated in the table of contents graphics.…”