A modified cross-polarization magic angle spinning C-13 NMR procedure for the study of humic materials

A new approach to the use of high-field instruments for cross-polarization (CP) magic angle spinning (MAS) C-13 NMR for analysis of humic materials is described. This technique consists of using a high sample spinning rate and a ramp CP pulse sequence, which, among other advantages, addresses chemical shift anisotropy effects. The theoretical aspects of high spinning rates on line broadening for nonrigid solids are reviewed. Also, the ramp CP pulse sequence and its implications for humic materials are discussed. It is shown that the highest resolution and most informative spectra can be obtained via a high-field instrument if the sample is spun at a rate higher than 0.25 of the anisotropy of the chemical shift of the aromatic moieties and a ramp CP pulse sequence is used. This study raises concerns that previous methods may have underestimated the amount of aromatic carbons in humic materials, Because of the similarities between humic materials and coals, this work may also have implications in the CP-MAS C-13 NMR approach to coals.