title: Accurate quantification of aromaticity and nonprotonated aromatic carbon fraction in natural organic matter by 13C solid-state nuclear magnetic resonance.
abstract: 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) x 45%.
authors.affiliation: Department of Chemistry, Gilman Hall, Iowa State University, Iowa 50011, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Mao
authors.forename: K
authors.initials: K
authors.lastname: Schmidt-Rohr
mesh_terms: D002244:Carbon / Q000032:analysis*; D002247:Carbon Isotopes / Q000032:analysis; D004785:Environmental Pollutants / Q000032:analysis*; D006841:Hydrocarbons, Aromatic / Q000032:analysis*; D009682:Magnetic Resonance Spectroscopy; D009930:Organic Chemicals; D015203:Reproducibility of Results
chemical_list: D002247:Carbon Isotopes; D004785:Environmental Pollutants; D006841:Hydrocarbons, Aromatic; D009930:Organic Chemicals; D002244:Carbon