Effect of residual ashes on CPMAS-C-13 NMR spectra of humic substances from volcanic soils

Potentially paramagnetic elements such as Fe, Cu, and Mn, did not affect NM R sensitivity and T-1 rho(H) values of humic, materials from volcanic soils , despite a large ash content in the extracts. T-1 rho(H) values increased with increasing C/H ratios, thereby confirming a relationship between the n umber of protons bound to carbons and process of relaxation in solid humic materials. Differences in T-1 rho(H) values were explained by molecular com position of the samples, Few carbon-bound protons and high molecular rigid ity in highly aromatic humic matter reduced the efficiency of the relaxatio n. Humic samples richer in polar carbons showed variable T-1 rho(H) values due to different molecular rigidity conferred by intermolecular hydrogen bo nds. Not only the number, but also the distribution of protons in humic mol ecules appeared to affect T-1 rho,(H) values.