A new approach to the use of high-field instruments for
cross-polarization (CP) magic angle spinning (MAS)
13C
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 13C NMR approach
to
coals.
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