Solid-state 13C-NMR dipolar dephasing experiments for quantifying protonated and non-protonated carbon in soil organic matter and model systems

Abstract

Dipolar dephasing was investigated as a means for quantitatively differentiating protonated and non-protonated and molecularly mobile and rigid components in soil organic matter. We experimented on cellulose, hemicelluloses, lignin, a protein, chitin, charcoal, palmitic acid and two waxes which served as model systems for components of soil organic matter. Dipolar dephasing decay curves showed significant deviation from the expected exponential decay (for non-protonated and methyl carbons) and Gaussian decay (for non-methyl protonated carbons), partly due to rotational and dipolar modulation. No signal is observed for most protonated carbon (excluding methyl carbon) for a dipolar dephasing delay of 45 μs, and so we generated three subspectra, representing non-protonated, non-methyl protonated and methyl carbon classes, requiring the acquisition of just the 0- and 45-μs dipolar dephased spectra. This methodology was applied to eight samples of soil organic matter, allowing the determination of relative contributions of overlapping resonances such as C-substituted and H-substituted aromatics (110–145 p.p.m.), carbohydrate anomeric and tannin (90–110 p.p.m.), and amino acid and methoxyl (45–65 p.p.m.). The waxes behaved aberrantly, probably because some of their components are highly mobile. We determined accurately the exponential dipolar dephasing decay rates of non-protonated carbon resonances, free from the interference of rotational and dipolar modulations.