ABSTRACT
The application of solid state cross polarisation (CP) nuclear magnetic resonance (NMR) techniques to the study of humic substances has brought about one of the great advances in the elucidation of their complex structures. The two most significant causes of loss of carbon signal in NMR spectra of humic substances are the presence of paramagnetic species and unfavourable cross polarisation time constants. In addition to inorganic paramagnetic species, the possibility of stable organic radicals affecting the CP process should not be overlooked. It has been shown that stable organic radicals can be present in humic acid and there is a wealth of literature on the presence of stable radicals in coal. Perdue has published a method of estimating the constraints on the possible range of structural groups in humic substances, based on the elemental analysis and other properties of the substance.
