State of the art of CPMAS 13C-NMR spectroscopy applied to natural organic matter

A number of different techniques are presently available for characterizing humified natural organic matter (NOM). Carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR) in the solid state using cross-polarization (CP) and magic angle spinning (MAS) represents the most powerful experimental approach used to collect direct information on the structural and conformational characteristics of humic carbon backbones. Despite the problems due to the presence of paramagnetic impurities and the generally low organic matter concentration, cross-polarization magic angle spinning 13C-NMR spectroscopy (CPMAS 13C-NMR) is applied in soil chemistry and geochemistry mainly because of its relevance in studying bulk samples such as soils and sediments. For example, CPMAS 13CNMR has been used to study the role of aluminosilicates in humic substances formation], and that of inorganic terrestrial constituents in the stabilization of NOM. Moreover, CPMAS 13C-NMR spectroscopy was also employed to understand structural differences between coals, tar, and char, to monitor structural changes of different coals during thermal degradation, to determine the extent of condensation in various lignins, and to describe structures of lignins in soft- and hardwoods. Cross-polarized 13C-NMR spectroscopy in the solid state was also successfully applied to describe the composition of purified humic substances and to better understand their genesis, transformation and decomposition, and their interactions with organic pollutants. While the main aim of CPMAS 13C-NMR spectroscopy is to provide fast quantitative information on humic matter composition, a debate on the real quantitativeness of cross-polarized NMR spectroscopy is arising based on the assumption that low protonated or quaternary carbons may not be completely detected. Direct excitation of carbons is suggested as the only valid alternative to CPMAS 13C-NMR for quantitative purposes, although this requires longer machine time. Since very general information on basic NMR theory can be found in many textbooks and reviews, the present paper intends to address only the current issues of CPMAS 13C-NMR spectroscopy applied to NOM, to suggest future possible developments of such techniques related to NOM, and to briefly summarize the most common experimental procedure used to obtain reliable solid state CPMAS 13C-NMR spectra of NOM extracted from terrestrial samples.