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

The effect of cation (Zn2+, Cu2+, Pr3+) amendment on the solid state C-13 n uclear magnetic resonance (NMR) spectral properties of organic materials wa s investigated. The organic materials were chosen to represent structures f ound in natural organic matter (NOM) from soils, waters, sediments, sewage sludges and plant residues, and included cellulose, pectin, chitin, collage n, a commercial humic acid, and charcoal. Cation amendment was shown to hav e little effect on the observability of C-13 NMR signal, except for the par amagnetic amended pectin samples, for which observability was decreased fro m near 100% in the unamended sample to 19% for the Cu2+ amended sample and 71% for the Pr3+ amended sample. NMR relaxation parameters (T1 rhoH, T1H) w ere more sensitive to cation amendment. For a number of the samples, a decr ease in relaxation rate (increase in T1 rhoH and T1H) was observed on amend ment 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 T1 rhoH and T1H) was generally observed on amendment with Cu2+. The effects amendment with Pr3+ varied. T1H was more sensitive to the presence of para magnetic species than was T1 rhoH. These results suggest that bound paramag netic cations will only decrease the observability of C-13 NMR signal in NO M samples (or domains within NOM samples) at high paramagnetic cation conce ntrations (>3%). There is great potential for the use of paramagnetic catio n amendment to differentiate relaxation rates of domains within NOM samples , subspectra for which can then be generated using the proton spin relaxati on editing (PSRE) technique.