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Paramagnetic Effects on Solid State Carbon-13 Nuclear Magnetic Resonance Spectra of Soil Organic Matter

Department of Soil and Water, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond SA 5064, Australia

* Corresponding author (ronald.smernik{at}adelaide.edu.au)

Received for publication June 2, 2000. The effects of paramagnetic species on solid state ¹³C nuclear magnetic resonance (NMR) spectra were quantified in a series of doping experiments. The degree of signal loss caused by paramagnetic metals was shown to depend not only on the quantity, but also on the nature of the paramagnetic species, as well as the intimacy of contact with the organic substrate and the type of NMR experiment. Two mechanisms of signal loss were distinguished—signal loss via loss of magnetic field homogeneity, which affects all ¹³C nuclei in a sample, and signal loss via interaction between electronic and nuclear spins, the effects of which were localized to the close environment of the paramagnetic species. Loss of field homogeneity is important for manganese species, but not for copper species, and is equally important for both cross polarization and Bloch decay experiments. The interaction between electronic and nuclear spins is highly dependent on the spin-lattice relaxation rate constant of the free electron (T₁e), as cations with very short T₁e values (e.g., Pr³⁺) cause less signal loss than cations with longer T₁e values (e.g., Cu²⁺, Mn²⁺). Cross polarization spectra are shown to be more susceptible than Bloch decay spectra to this mechanism of signal loss. Signal loss and increased relaxation rates brought about by paramagnetic species can be used to provide information on soil organic matter (SOM) heterogeneity in the submicron range. This is demonstrated for SOM doped with paramagnetic cations where selective signal loss and increased relaxation rates are used to determine the nature of cation exchange sites.

Abbreviations: CP, cross polarization • CPMAS, cross polarization with magic angle spinning • NMR, nuclear magnetic resonance • PSRE, proton spin relaxation editing • SOM, soil organic matter

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