Spin-Diffusion Limitations for High-Sensitivity NMR Techniques

Abstract

The experimental sensitivity attainable in employing such high-sensitivity NMR techniques as double nuclear resonance and rotating-frame relaxation can be limited by spin-diffusion rates within the bulk detected spin system. We develop a diffusion kernel solution to the driven spin-diffusion equation and calculate the detected-system magnetization decay rate induced by a dilute concentration of pumping centers. Our results show that spin-diffusion suppression of the induced decay rate depends only upon local parameters, is independent of concentration of pumping centers, and, in all systems whose experimental study has thus far been reported, would produce at most a 10% effect. These particular conclusions apply only to the case of completely dispersed pumping centers, but the general approach can be extended to treat aggregated systems. Our description of the spin-diffusion behavior yields results in agreement with existing experimental data, but in disagreement with previous theoretical treatments.