Relaxation effects in a system of a spin-1/2 nucleus coupled to a quadrupolar spin subjected to RF irradiation: evaluation of broadband decoupling schemes

Resolution enhancement using a new multiple-pulse decoupling sequence for quadrupolar nuclei

A new decoupling composite pulse sequence is proposed to remove the broadening on spin S=1/2 magic-angle spinning (MAS) spectra arising from the scalar coupling with a quadrupolar nucleus I. It is illustrated on the (31)P spectrum of an aluminophosphate, AlPO(4)-14, which is broadened by the presence of (27)Al/(31)P scalar couplings. The multiple-pulse (MP) sequence has the advantage over the continuous wave (CW) irradiation to efficiently annul the scalar dephasing without reintroducing the dipolar interaction. The MP decoupling sequence is first described in a rotor-synchronised version (RS-MP) where one parameter only needs to be adjusted. It clearly avoids the dipolar recoupling in order to achieve a better resolution than using the CW sequence. In a second improved version, the MP sequence is experimentally studied in the vicinity of the perfect rotor-synchronised conditions. The linewidth at half maximum (FWHM) of 65 Hz using (27)Al CW decoupling decreases to 48 Hz with RS-MP decoupling and to 30 Hz with rotor-asynchronised MP (RA-MP) decoupling. The main phenomena are explained using both experimental results and numerical simulations.

GAVA: spectral simulation for in vivo MRS applications

An application that provides a flexible and easy to use interface to the GAMMA spectral simulation package is described that is targeted at investigations using in vivo MR spectroscopic methods. The program makes available a number of widely used spatially localized MRS pulse sequences and NMR parameters for commonly observed tissue metabolites, enabling spectra to be simulated for any pulse sequence parameter and viewed in an integrated display. The application is interfaced with a database for storage of all simulation parameters and results of the simulations. This application provides a convenient method for generating a priori spectral information used in parametric spectral analyses and for visual examination of the effects of difference pulse sequences and parameter settings.

Line narrowing of I = 12 spins coupled to quadrupolar nuclei in liquids: effects of weak decoupling fields

In this paper an exact description of the observed transverse magnetization of spin 12 nuclei, coupled to quadrupolar spins which are subjected to RF irradiation, is presented. It is shown that for on-resonance CW decoupling at weak to intermediate irradiation levels, the transverse decay of the spin 12 magnetization is modulated with a period of 1/nu(2), where nu(2) is the amplitude of the decoupling irradiation. When the spin 12 signal is created as a spin echo, and the quadrupolar resonance continuously irradiated during the echo evolution, the echo amplitudes experience much stronger modulation with period 2/nu(2). In previous treatments of such spin systems, the regime of weak decoupling power was usually neglected, and approximate analytical expressions seeking to define the "adequate" or "minimal" decoupling power, necessary to achieve the collapse of the spin 12 multiplet into a single narrow line, were derived. It is demonstrated here, both by experiments and by simulations using a full Redfield formalism, that simple analytical predictions for the T(2) decay of the spin 12 magnetization are still possible, even when the scalar relaxation is not in the "fast exchange" limit and the transverse decay is considerably modulated due to insufficient decoupling power. In this case, the expected single exponential decay rate is obtained for the nonmodulated component of the signal. The theoretical solution for spin I = 12 coupled to S = 3 is derived, and results for the proton decay in (10)B-enriched sodium borocaptate in aqueous solution are presented. The effects of irradiation by several composite pulse decoupling sequences are also considered. Copyright 1999 Academic Press.