Effect of paramagnetic cations on solid state ¹³C nuclear magnetic resonance spectra of natural organic materials

Abstract

The effect of cation (Zn²⁺, Cu²⁺, Pr³⁺) amendment on the solid state ¹³C nuclear magnetic resonance (NMR) spectral properties of organic materials was investigated. The organic materials were chosen to represent structures found in natural organic matter (NOM) from soils, waters, sediments, sewage sludges and plant residues, and included cellulose, pectin, chitin, collagen, a commercial humic acid, and charcoal. Cation amendment was shown to have little effect on the observability of ¹³C NMR signal, except for the paramagnetic amended pectin samples, for which observability was decreased from near 100% in the unamended sample to 19% for the Cu²⁺ amended sample and 71% for the Pr³⁺ amended sample. NMR relaxation parameters (T_1p H, T₁H) were more sensitive to cation amendment. For a number of the samples, a decrease in relaxation rate (increase in T_lpH and T₁H) was observed on amendment with Zn²⁺. This was ascribed to a decrease in molecular motion due to the chelating effects of Zn²⁺. An increase in relaxation rate (decrease in T_1pH and T₁H) was generally observed on amendment with Cu²⁺. The effects of amendment with Pr³⁺ varied. T₁H was more sensitive to the presence of paramagnetic species than was T_1pH. These results suggest that bound paramagnetic cations will only decrease the observability of ¹³C NMR signal in NOM samples (or domains within NOM samples) at high paramagnetic cation concentrations (>3%). There is great potential for the use of paramagnetic cation amendment to differentiate relaxation rates of domains within NOM samples, subspectra for which can then be generated using the proton spin relaxation editing (PSRE) technique.

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