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Accurate Quantification of Aromaticity and Nonprotonated Aromatic Carbon Fraction in Natural Organic Matter by 13 C Solid-State Nuclear Magnetic Resonance

2004; American Chemical Society; Volume: 38; Issue: 9 Linguagem: Inglês

10.1021/es034770x

1520-5851

Jingdong Mao, Klaus Schmidt‐Rohr,

Solid-state spectroscopy and crystallography

An improved approach for accurately determining the aromatic carbon fraction (fa) and nonprotonated aromatic carbon fraction (faN) in natural organic matter by solid-state 13C NMR is described. Quantitative peak areas are obtained from direct polarization 13C nuclear magnetic resonance (NMR) under high-speed magic angle spinning (MAS). The problem of overlap between aromatic and alkyl carbon resonances around 90−120 ppm in 13C NMR spectra is solved by a 13C chemical shift anisotropy (CSA) filter technique. After correction for residual spinning sidebands, an accurate value of the aromaticity fa is obtained. To obtain a quantitative faN fraction, dipolar dephasing was adapted for high-speed MAS 13C NMR; the separation of the signals of nonprotonated alkyl and aromatic carbons was achieved by CSA filtering plus dipolar dephasing. The method is demonstrated on a peat humic acid, yielding fa = 45 ± 2% and faN =(0.64 ± 0.07) × 45%.