Communication
An alternative tuning approach to enhance NMR signals

https://doi.org/10.1016/j.jmr.2008.04.026Get rights and content

Abstract

By using spin-noise type measurement we show that the resonance frequency of the reception circuit of classical NMR spectrometers does not match the Larmor frequency even if, in emission, the electronic circuit is perfectly tuned at the Larmor frequency and matches the amplifier impedance. We also show that this spin-noise method can be used to ensure a match between the Larmor frequency and the reception circuit resonance frequency. In these conditions, (i) the radiation damping field is in perfect quadrature to the magnetization and (ii) the NMR signal level and potentially the signal-to-noise ratio, are enhanced. This choice induces a change of the probe resonance frequency by several hundreds of kHz for 500 or 700 MHz spectrometer. We show that the resulting mismatch condition for emission can be removed by adding other tuning and matching degrees of freedom located on the excitation line (or by symmetry on the reception line) decoupled to the probe resonance circuit by the crossed diodes.

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:

  • It allows the obtaining of larger signals and potentially larger signal-to-noise ratio than those obtained with the classical tuning procedure;

  • 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).

References (13)

There are more references available in the full text version of this article.

Cited by (0)

View full text