Effects of variations in contact times and copper contents in a 13C CPMAS NMR study of samples of four organic soils

Abstract

Contact time, T c, was varied from 0.5 to 4 ms in a 13C CPMAS NMR study of samples of four organic soils differing only in copper content and degree of decomposition. Initial polarization was rapid, with maximum intensity reached at or before 0.5 ms contact time and a large decrease in intensity by 4 ms contact time. The distribution of intensity was determined by dividing the spectra into four regions, corresponding approximately to aliphatic, carbohydrate, aromatic and carboxyl carbons. Three samples, with up to 1150 p.p.m. Cu, showed small increases in aromaticity with increasing T c, with the greatest effect between 0.5 and 1 ms, and fairly constant intensity distributions between 1 and 4 ms. The sample with 2920 p.p.m. Cu showed much greater increase in aromaticity with increasing T c and a decrease in the 50–110 p.p.m. region, which is dominated by carbohydrate signals. T 1 ϱ H was negatively correlated with increasing Cu content for all four regions of the spectra, but the effect was much larger and the correlation coefficient much higher for the carbohydrate region. A T c of 1 ms appears suitable for organic soil samples; however, caution must be exercised in drawing conclusions from intensity data, especially where samples contain high concentrations of metals. These data also indicate a possible preferential localization of Cu in the carbohydrate or hydrophilic portions of the organic matter; 13C CPMAS NMR may provide a novel, non-perturbing method to study metal binding in organic soils.