Alain Retournard

Measurement of Hartmann-Hahn cross-polarization dynamics with quenching of proton T1ρ relaxation dependence

A simple modification of the standard cross-polarization method designed for quenching the proton T1 rho dependence when studying polarization transfer is presented. It is demonstrated that by using this simple procedure, new and subtle details of cross-polarization dynamics, previously hidden by the T1 rho(1H) effect, can be observed in dipolar-coupled spin systems.

14N Pulsed nuclear quadrupole resonance. 2. Effect of a single radio-frequency pulse in the general case

A novel theory, based on density operator calculations, is provided for assessing the nuclear quadrupole resonance behaviour of a spin 1 (14N) subjected to a single radio-frequency pulse. It is for a powder sample in zero magnetic field for an electric field gradient tensor without symmetry. A complete set of equations is obtained for the quantities of interest. It is derived from the general Liouville–von Neumann equation and from a proper basis on which the density operator is expanded. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), show that the same nutation curve is expected for the three different transitions which exist when the electric field gradient tensor is without symmetry. This latter nutation curve is, however, different from that which prevails in the case of an axially symmetric tensor, this apparent discrepancy being easily resolved on theoretical grounds. Experimental data (for NaNO2, electric field gradient tensor without symmetry) are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.

On the use of x-ray-excited optical luminescence (XEOL) for the analysis of multisite rare-earth systems

Abstract The realization of a XEOL (X-ray-excited optical luminescence) spectrometer is reported. The aim of the present paper is to show that a comparison between UV-excited emission spectra and XEOL spectra can be helpful in the assignment of some weak transitions for multiple site rare-earth compounds. The case of c-Y 2 O 3 doped with Eu 3+ cations is considered in some detail, as in the XEOL spectrum three lines to be assigned to transitions occurring at the centrosymmetrical Eu 3+ (C 3i ) site are strongly enhanced. Additional site-selective UV-excited emission spectra are also reported supporting the present discussion.

14N pulsed nuclear quadrupole resonance. 1. Nutation experiments in the case of an axially symmetric electric field gradient tensor

An exact theory based on density matrix calculations is provided for assessing the nuclear quadrupole resonance (NQR) behaviour of a spin 1 (14N) subjected to a single radio-frequency pulse. It concerns a powder sample in zero magnetic field with the assumption of an axially symmetric electric field gradient tensor. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.

Structural characterization by X-ray absorption spectroscopy (EXAFS/XANES) of the vanadium chemical environment in Boscan asphaltenes

X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.–visible spectrophotometry was substantially reduced (i.e. <15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.–visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ‘non-porphyrinic vanadium fraction’ from Boscan crudes.

Off-resonance effects and selectivity profiles in pulsed nitrogen-14 nuclear quadrupole resonance

In order to alleviate base-line distortions in nitrogen-14 NQR spectra originating from pulse breakthrough, low power radio-frequency (rf) pulses were applied. It is recalled that the required power is four times lower than that for an equivalent NMR experiment. This is easily explained by the fact that, in NMR, half the amplitude of the rf field is active. Moreover, the selectivity profile (i.e. the peak amplitude as a function of the difference between the carrier frequency and the resonance frequency) exhibits a shape which is, in most cases, more favorable in NQR than in NMR. An appropriate theory has been developed for explaining these experimental observations. It is concluded that low power NQR is perfectly feasible and should even be recommended for most applications, provided that the line-width of the NQR signal is not too large.

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T 1 (T 1: longitudinal relaxation time). The signal growth as a function of T/T 1under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.

NMR data acquisition, data processing, and spectrometer control with an IBM PC compatible

This article describes a system implemented on an IBM compatible personal computer (PC) for acquisition and processing of NMR data and for the spectrometer control. The homemade hardware includes a 12‐bit analog‐digital conversion board and a simplified pulse sequence controller. The software, written in assembly language and in Turbo Pascal, handles these two boards and, thanks to a versatile pulse generator, allows easily to run most of the modern NMR experiments in simple or in quadrature detection with spectral widths up to 25 000 Hz. Owing to its simplicity, its low cost, and its versatility, this system is specially suited for upgrading old NMR spectrometers or can be considered as the basis of a routine spectrometer.

A versatile secondary transmitter unit for nuclear magnetic resonance spectroscopy

A transmitter unit for homo‐ or heteronuclear double‐resonance experiments has been designed so as to be independent of the NMR spectrometer main console. It features capabilities similar to the principal transmitter unit, with respect to rf phases, amplitudes, and pulses. It can be run manually and under computer control and possesses several modulation schemes or sequences specific to liquid‐state or solid‐state NMR spectroscopy. Its general organization as well as particular circuits is described. Its performances are demonstrated by illustrative examples.

Fast bidirectional NMR data transfer between the Bruker aspect 3000 and a personal computer

Abstract A system for transferring data to/from a compatible IBM personal computer (PC) from/to a Bruker Aspect computer is described. The system has the advantage of being relatively inexpensive, simple, reliable, very fast and without limit on the amount of data transferred. The hardware and the software are home-made. The data are transferred via a parallel link. The transfer rate ranges between 22 and 30 kbyte/s, depending on the PC disk access time. The applications of this system are off-line processing of NMR data on any remote computer, archiving and connection to a network.