CommunicationAn alternative tuning approach to enhance NMR signals
Introduction
We have recently reported the observation of spontaneous multiple chaotic maser emissions [1] using laser-polarized xenon with magnetization anti-aligned with the static magnetic field. This type of phenomenon corresponds to maximal interaction between the transverse nuclear magnetization and the detection coil through the so-called radiation damping effect. This old-story phenomenon corresponds to a non-linear retroaction on the magnetization: the precessing magnetization creates a current in the coil by induction which, in return, creates an oscillating magnetic field at the exact Larmor frequency that interacts with the nuclear magnetization [2], [3]. The precessing magnetization and the radiation-damping field in the rotating frame are in exact quadrature only when the coil is perfectly tuned at the Larmor frequency [4]. This was the case for the multiple maser experiments but a preliminary key question was how to ensure such a perfect tuning.
In this article, we show that since the excitation and detection pathways are distinct, the parasite capacities and inductances of diodes and wires modify the resonance frequencies of the two circuits. We illustrate this discrepancy by using principles based on spin-noise detection [5], [6], [7], [8], [9], and accordingly an alternative method of frequency tuning, optimized for the reception circuit, is suggested. We also show that when the electronic resonance frequency of the reception circuit matches the Larmor frequency, the NMR signal is improved and the impedance matching of the emission line can however be restored.
Section snippets
Tuning according to the reception channel
Fig. 1 describes a classical NMR probe [10]. The two capacities Ct and Cm ensure that the coil of inductance L and resistance r is tuned at the Larmor frequency of the studied nuclear spins, ω0. On the other hand, two sets of crossed diodes in positions A and B are used to separate the emission and detection circuits. Classical procedures to ensure that the probe is tuned at the Larmor frequency and matched at the amplifier impedance (usually 50 Ω) consist either in using a Balun circuit with a
Discussion
Adjusting the Ct and Cm values of the probe for matching the electronic resonance frequency of the reception circuit to the Larmor frequency exhibits two advantages:
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It allows the obtaining of larger signals and potentially larger signal-to-noise ratio than those obtained with the classical tuning procedure;
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It ensures that radiation-damping field is in quadrature to the transverse magnetization. Hence the resonance line of nuclear magnetization exhibiting radiation damping remains symmetric [4].
Conclusions
We have observed that tuning a probehead according to the emission circuit systematically leads to a mismatch between the reception circuit’s resonance frequency and the Larmor frequency. We have shown that the tuning in reception is possible using a protocol based on the spin-noise measurement method. Using this approach an improvement in signal with respect to the classical approach on the order of 25–30% was achieved, and when the probe is tuned in these conditions, the radiation-damping
Experimental
All experiments reported here were performed at 293 K on a Bruker Avance 700 spectrometer equipped with a TCI cryoprobe. The sample was either the standard 1H signal-to-noise reference sample (ethyl-benzene at 0.1% in deuterated chloroform) from Bruker, a degassed diluted version (0.02% of ethyl-benzene) or a D2O solution containing 0.7 g of saccharose.
The signal-to-noise measurements were performed using the standard protocol (Bruker “sinocal” automatic program). It consisted after signal
Acknowledgment
This research program is supported by ANR (ANR blanche DIPOL).
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