Alain Louis-Joseph

Neutralization of radiation damping by selective feedback on a 400 MHz NMR spectrometer

SummaryRadiation damping is a phenomenon well known among NMR spectroscopists of proteins as a source of undesirable features, especially in high-field and high-Q probe NMR. In this paper, we present an electronic neutralization network which dramatically reduces radiation damping. It detects the radiation field profile and feeds back into the probe an rf field with identical amplitude and opposite phase. Experimental results of a practical implementation carried out on a 400 MHz Bruker spectrometer are shown.

Probing Structure and Dynamics of Bulk and Confined Crude Oils by Multiscale NMR Spectroscopy, Diffusometry, and Relaxometry

We propose using a set of noninvasive multiscale NMR techniques for probing the structure and dynamics of bulk and confined crude oils with and without asphaltene. High-field 1D (1)H and (13)C NMR spectroscopies evidence the proton species and the amount of asphaltene and give an average chain length for the hydrocarbon aliphatic chains. Two-dimensional (1)H diffusion-ordered NMR spectroscopy (DOSY) spectra allow us to identify two populations of hydrocarbons characterized by two distributions of translational diffusion coefficients in the presence of asphaltene and a single one without asphaltene. A detailed analysis of the distributions of longitudinal, T1, relaxation times measured at different magnetic fields is proposed in terms of highly skewed bimodal (or monomodal) log-normal distributions, confirming the two environments in the presence of asphaltene and a single one without asphaltene. We show that these distributions are similar to the gas and gel permeation chromatography distributions, thus showing a connection of the hydrocarbon dynamics with their chain lengths. The remarkable observed features of the nuclear magnetic relaxation dispersion (NMRD) profiles of <1/T1> for bulk and confined crude oils with and without asphaltene are interpreted with an original relaxation model of intermittent surface dynamics of proton species at the proximity of asphaltene nanoaggregates and bulk dynamics in between clusters of these nanoaggregates. This allows us to probe the 2D translational diffusion correlation time and the time of residence of hydrocarbons in the proximity of the asphaltene nanoaggregates. Provided that the diffusion of the hydrocarbons close to the asphaltene nanoaggregates is three times smaller than the bulk diffusion, as the DOSY experiments show, this time of residence gives an average radius of exploration for the 2D hydrocarbon diffusion, r2D ≈ 3.9 nm, of the same order of magnitude as the aggregate sizes found by J. Eyssautier with SAXS and SANS in asphaltene solutions and by O. C. Mullins with the observation of gravitational gradients of asphaltenes in oilfield reservoirs.

Bayesian signal extraction from noisy FT NMR spectra

SummaryThe statistical interpretation of the histogram representation of NMR spectra is described, leading to an estimation of the probability density function of the noise. The white-noise and Gaussian hypotheses are discussed, and a new estimator of the noise standard deviation is derived from the histogram strategy. The Bayesian approach to NMR signal detection is presented. This approach homogeneously combines prior knowledge, obtained from the histogram strategy, together with the posterior information resulting from the test of presence of a set of reference shapes in the neighbourhood of each data point. This scheme leads to a new strategy in the local detection of NMR signals in 2D and 3D spectra, which is illustrated by a complete peak-picking algorithm.

Amplification of radiation damping in a 600-MHz NMR spectrometer: Application to the study of water-protein interactions.

SummaryA new application of a recently developed electronic radiation-damping (RD) control system is presented. It is possible to amplify radiation damping so as to make the water magnetization return back to its equilibrium direction in a time shorter than the characteristic RD time. Certain types of experiments involving radiation damping as a selective inversion pulse can be significantly improved by this new method. Moreover, amplification of RD is shown to improve water suppression and consequently the dynamics of 2D NOESY experiments on proteins.

Self-sustained Maser oscillations of a large magnetization driven by a radiation damping-based electronic feedback

In this paper, the dynamics of a magnetization undergoing a radiation-damping based feedback radio-frequency field is investigated both theoretically and experimentally. It is shown that due to the presence of T1 relaxation the evolution equations predict the existence of self-sustained maser pulses. This phenomenon is a consequence of the competition between two different processes, namely, T1 relaxation and a precession about a magnetization-dependent radio-frequency field. Experiments show the existence of periodic revivals of the free induction decay over unusually long periods of time, on the order of tens of seconds.

A NEW CONCEPT FOR SELECTIVE EXCITATION IN NMR

Abstract The new NMR concept of selective excitation generated by pulses whose envelope is slaved to that of a signal originating from the sample is described. It is shown that these kinds of pulses exhibit interesting features such as a sort of auto-calibration of the pulse length and phase. These particular features are discussed and experimental evidence is given. Illustration of the use of slaved pulses is demonstrated in the case of 1D transient NOE by selective inversion.

Generating spin turbulence through nonlinear excitation in liquid-state NMR

Chaotic dynamics of a water magnetization in a 600 MHz NMR spectrometer was generated by a radiation damping-based electronic feedback. Erratic induction signal was observed for several tens of seconds. The analysis of the data shows that this chaotic behaviour can be ascribed to spin turbulence in the sample and that a simpler model based on the three-dimensional Bloch equations modified to include a feedback field may not account for the experimental data.

Enhancement of water suppression by radiation damping-based manipulation of residual water in Jump and Return NMR experiments

Abstract It is shown in this Letter that amplification of radiation damping obtained by electronic feedback between consecutive transients of a Jump–Return (JR) experiment, residual in-plane water magnetization is suppressed, leading to substantial enhancement of water suppression. Application of this approach to RNA spectroscopy, where JR is particularly useful, is shown.

On the possibility of performing self-calibrated selective π/2 pulses in nuclear-magnetic resonance

In this paper, a generalization of the concept of selective “slaved” pulses in NMR spectroscopy introduced recently by the authors is presented. The shape of these pulses is slaved to that of a signal produced by the sample by the action of an electronic feedback loop. The theoretical analysis is based on results of the theory of nonlinear dynamical systems, which predicts the possibility of performing 90° self-calibrated radiofrequency pulses, through the simultaneous action of a constant radio-frequency field and of a magnetization dependent field of the kind above. Experimental demonstrations show that rotation of the magnetization onto the xy-plane is achieved, regardless of its initial position. Moreover, it is shown to remain stable for more than 50 ms, without significant loss of intensity.