Alternative probe tuning for signal enhancing and spin-noise studies
Initial remarks: The classical procedure to tune a probe consists in minimizing the power reflected to the amplifier and corresponds to a tuning according to the emission circuit. The reception circuit being different, when the signal of the magnetization is acquired, the circuit is no longer well tuned.
Aim: Obtaining a probe perfectly tuned at the Larmor frequency according to the reception circuit.
Principle: By monitoring the probe noise level at the output of the probe through the preamplifier, it becomes possible to tune it at the Larmor frequency according to the reception circuit.
Principle of an NMR probe. Classical tuning: according to the emission through A. Alternative tuning: according to the reception through B.
Advantages:
- Since the observed signal is dependent on the quality of the probe during the acquisition time, larger signals are observed and potentially larger signal-to-noise ratio.
- If the probe is perfectly tuned in reception, the radiation damping field is effectively perpendicular to the transverse magnetization. Optimizing solvent suppression is thus easier.
- For spin-noise measurements pure Lorentzian shapes are obtained.
Improvements allowed by tuning according to the reception:Left Improvement in signal amplitude allowed by the tuning according to the reception (B) relative to the classical procedure (A). Right Spin-noise power spectra of a concentrated saccharose sample at 700 MHz. A Tuning according to the emission. B Tuning according to the reception: Pure absorption Lorentzian signals and average noise level higher, i.e. perfect tuning
Optimal conditions: Obviously, when your probe is tuned according to the reception circuit it is no longer tuned and matched according to the emission circuit and thus amplifier mismatch can be expected on special experiments. It is nevertheless possible to restore this tuning and matching condition by inserting before the cross-diodes a tuning circuit.
To go further:
- Reference: D. J.Y. Marion and H. Desvaux, Journal of magnetic Resonance, 193, (2008) 153-157.
- Automatic procedure to perform this noise tuning: Pulse program, Scilab procedure to treat the data, help file
Beside the detection of laser-polarized xenon signal by nuclear spin-noise, we have an on-going collaboration with the group of Pr. N. Müller at University J. Kepler (Linz) for optimizing the spin-noise detection and looking to the key properties of this phenomenon which appears to be strongly correlated to radiation damping.