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Advanced Solid-State NMR Characterization of Marine Dissolved Organic Matter Isolated Using the Coupled Reverse Osmosis/Electrodialysis Method
journal contribution
posted on 2012-06-05, 00:00 authored by Jingdong Mao, Xueqian Kong, Klaus Schmidt-Rohr, Joseph J. Pignatello, E. Michael PerdueAdvanced 13C solid-state techniques were employed
to
investigate the major structural characteristics of two surface-seawater
dissolved organic matter (DOM) samples isolated using the novel coupled
reverse osmosis/electrodialysis method. The NMR techniques included
quantitative 13C direct polarization/magic angle spinning
(DP/MAS) and DP/MAS with recoupled dipolar dephasing, 13C cross-polarization/total sideband suppression (CP/TOSS), 13C chemical shift anisotropy filter, CH, CH2, and CHn selection, two-dimensional 1H–13C heteronuclear correlation NMR (2D HETCOR), 2D HETCOR combined
with dipolar dephasing, and 15N cross-polarization/magic
angle spinning (CP/MAS). The two samples (Coastal and Marine DOM)
were collected at the mouth of the Ogeechee River and in the Gulf
Stream, respectively. The NMR results indicated that they were structurally
distinct. Coastal DOM contained significantly more aromatic and carbonyl
carbons whereas Marine DOM was markedly enriched in alkoxy carbon
(e.g., carbohydrate-like moieties). Both samples contained significant
amide N, but Coastal DOM had nitrogen bonded to aromatic carbons.
Our dipolar-dephased spectra indicated that a large fraction of alkoxy
carbons were not protonated. For Coastal DOM, our NMR results were
consistent with the presence of the major structural units of (1)
carbohydrate-like moieties, (2) lignin residues, (3) peptides or amino
sugars, and (4) COO-bonded alkyls. For Marine DOM, they were (1) carbohydrate-like
moieties, (2) peptides or amino sugars, and (3) COO-bonded alkyls.
In addition, both samples contained significant amounts of nonpolar
alkyl groups. The potential sources of the major structural units
of DOM were discussed in detail. Nonprotonated O-alkyl carbon content
was proposed as a possible index of humification.