Broadband excitation and detection of cross-relaxation NMR spectra

Abstract

Nuclear magnetic resonance cross-relaxation spectroscopy maps the proton solidstate NMR spectrum onto the proton water NMR signal of aqueous heterogeneous materials ( Z-3). Application of a relatively long, low-intensity RF pulse, off resonance from the water Larmor frequency but within the linewidth of the solid, partially saturates the nuclear magnetism of the solid protons. Magnetic cross relaxation between the immobile and mobile protons transfers the saturation of the solid protons to the water protons, decreasing the magnitude of the water NMR signal. A cross-relaxation spectrum is obtained by saturating the immobile protons at many discrete frequencies and plotting the ratio of the water signal with and without RF saturation as a function of the saturation frequency. The intensity and the width ofthe cross-relaxation spectrum are a function of the longitudinal, transverse, and cross-relaxation rates of the liquid and solid components, the mole fraction of liquid to solid protons, and the magnitude and duration of the saturating RF pulse. This Communication describes a method for obtaining the entire cross-relaxation spectrum of a spatially homogeneous material with two acquisitions: RF saturation on and RF saturation off. The pulse sequence is demomtrated schematically in Fig. I. A magnetic field gradient and RF pulse are applied to the sample concurrently. The gradient creates a continuum of frequencies across the !;ample and establishes at once all conditions of off-resonance saturation for the RF pulse. The decrease in water magnetization by cross relaxation as a function of off-resonance frequency is stored in the sample as a function of the position in the sample and detected by recording a onedimensional image with the readout gradient applied along the same axis as the frequency-dispersal gradient. The cross-relaxation spectrum is created by dividing the image obtained with RF saturation turned on by one obtained with RF saturation turned off. The pulse sequence shown in Fig. 1 represents the simplest implementation of broadband cross-relaxation spectroscopy. The sequence may be modified to include slice selection, spin-echo detection, and imaging by phase encoding in either or both of the two spatial dimensions which are not needed to record the cross-relaxation spectrum. In addition the magnitude of the readout gradient may be decreased to lower the bandwidths of the receiver and increase the signal-to-noise of the crossrelaxation spectrum.