Indirect detection via the dipolar demagnetizing field

Abstract

A new method for indirectly detecting the magnetization of one spin species via the NMR signal of a second species is described. This method, which does not require that the spins of the two species be chemically coupled or even on the same molecule, relies on the nonlinear effect of the demagnetizing field produced by the nuclei themselves. It involves allowing I spins belonging to one spin species to precess under the influence of the demagnetizing field produced by S spins of a different species. The demagnetizing field generates an echo of the I magnetization. This echo carries information about the state of S magnetization and so offers a method of indirect detection. In certain circumstances this method can yield a signal-to-noise ratio higher than that given by direct detection. To demonstrate the process of echo formation, experiments have been carried out at 11.7 T on an aqueous solution of ¹³C-enriched potassium cyanide. Here the water protons form the I spins, while the ¹³C nuclei in the cyanide group form the S spins. The experimental results are in agreement with the theory outlined in this paper. Experimental methods which allow the application of this form of indirect detection to spectroscopy and imaging are also described.