Accurate quantification of aromaticity and nonprotonated aromatic carbon fraction in natural organic matter by 13C solid-state nuclear magnetic resonance

An improved approach for accurately determining the aromatic carbon fraction (f(a)) and nonprotonated aromatic carbon fraction (f(a)N) in natural organic matter by solid-state C-13 NMR is described. Quantitative peak areas are obtained from direct polarization C-13 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 C-13 NMR spectra is solved by a C-13 chemical shift anisotropy (CSA) filter technique. After correction for residual spinning sidebands, an accurate value of the aromaticity f(a) is obtained. To obtain a quantitative faN fraction, dipolar dephasing was adapted for high-speed MAS C-13 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 f(a) = 45 +/- 2% and f(a)N = (0.64 +/- 0.07) x 45%.