Characterization of humic substances by advanced solid state NMR spectroscopy: Demonstration of a systematic approach |
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Authors: | Jingdong Mao Na ChenXiaoyan Cao |
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Affiliation: | Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Blvd, Norfolk, VA 23529, USA |
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Abstract: | Characterization of humic substances is challenging due to their structural complexity and heterogeneity. Solid state nuclear magnetic resonance (NMR) is regarded as one of the best tools for elucidating structures of humic substances. The primary solid state NMR technique that has been used so far is the routine 13C cross polarization-magic angle spinning (CP-MAS) technique. Although this technique has markedly advanced our understanding of humic substances, the full potential of NMR for characterizing humic substances has yet to be realized. Recent technical developments and applications of advanced solid state NMR have revealed the promise to provide deeper insights into structures of humic substances. In this paper, we summarized and demonstrated the systematic solid state NMR protocol for characterization of humic substances using a humic acid as an example. This protocol included (1) identification of specific functional groups using spectral editing techniques, occasionally assisted by 1H13C two-dimensional heteronuclear correlation (2D HETCOR) NMR, (2) quantification of specific functional groups based on direct polarization-magic angle spinning (DP-MAS) and DP-MAS with recoupled dipolar dephasing, combined with spectral editing techniques, (3) determination of connectivities and proximities of specific functional groups by 1H13C 2D HETCOR or 2D HETCOR combined with spectral editing techniques, and (4) examination of domains and heterogeneities by 1H13C 2D HETCOR with 1H spin diffusion. We used a soil humic acid as an example to demonstrate how this protocol was applied to the characterization of humic substances step by step. Afterwards, based on typical 13C NMR spectra of humic substances we described how we could combine different NMR techniques to identify specific functional groups band by band from downfield to upfield. Finally, we briefly mentioned the potential new NMR techniques that could be developed to enrich the current systematic protocol. This systematic protocol is not only applicable to humic substances but also to other natural organic matter samples. |
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Keywords: | NMR, nuclear magnetic resonance 2D HETCOR, two dimensional 1H 0" alt=" single bond" src=" http://cdn.els-cdn.com/sd/entities/sbnd" class=" glyphImg" >13C heteronuclear correlation DP-MAS, direct polarization/magic angle spinning DP, direct polarization DD, dipolar dephasing FT-IR, Fourier transform infrared NOM, natural organic matter HA, humic acid CP-MAS, cross polarization-magic angle spinning CP, cross polarization TOSS, total suppression of sidebands CP-T1-TOSS, cross polarization spin lattice relaxation time-total sideband suppression CP-TOSS, cross polarization-total sideband suppression CSA, chemical shift anisotropy SPIDER, saturation-pulse induced dipolar exchange with recoupling FSLG, frequency switched Lee-Goldburg REDOR, rotational echo double resonance TPPM, two-pulse phase-modulated LGCP, Lee-Goldburg cross polarization DEPT, distortionless enhancement by polarization transfer HSQC, heteronuclear single quantum coherence HMQC, heteronuclear multiple quantum coherence HMBC, heteronuclear multiple bond correlation SUPER, separation of undistorted powder patterns by effortless recoupling FSLG, frequency switched Lee-Goldburg |
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