Suitability of Different 13C Solid-state NMR Techniques in the Characterization of Humic Acids |
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| Authors: | Jingdong Mao Weiguo Hu Guangwei Ding Klaus Schmidt-Rohr Geoffrey Davies Elham Ghabbour |
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| Institution: | 1. Department of Chemistry , Iowa State University , Ames , IA , 50011 , USA;2. Department of Polymer Science and Engineering , University of Massachusetts , Amherst , MA , 01003 , USA;3. Department of Plant and Soil Sciences , University of Massachusetts , Amherst , MA , 01003 , USA;4. Chemistry Department and the Barnett Institute , Northeastern University , Boston , MA , 02115 , USA |
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| Abstract: | Qualitative and quantitative analyses of humic acids (HAs) with five different 13C solid-state NMR techniques were assessed using HAs of various origins and locations. The NMR techniques compared are: (1) direct polarization/magic angle spinning (DP/MAS) at 13 kHz, (2) conventional cross polarization (CP)/MAS at 5 kHz, (3) ramp-CP/MAS at 8 kHz, (4) CP/total sideband suppression (TOSS) at 4.5 kHz, and (5) DP/MAS corrected by CP/spin-lattice relaxation with TOSS. The spectra from the five techniques were first compared qualitatively. Then, each spectrum was divided into eight regions for quantitative evaluation. DP/MAS spectra were used as quantitative references. Ramp-CP/MAS and CP/TOSS spectra gave consistently better results than those of the conventional CP/MAS spectra at a 13C frequency of 75 MHz, which were incorrect due to spinning sidebands. CP/MAS at low magnetic fields (22.6 and 50.6 MHz 13C frequency) indicated improved integration results but lower resolution. Correction factors calculated by comparison with DP/MAS will be useful to convert the non-quantitative peak areas in the CP/TOSS and ramp-CP/MAS spectra into more quantitative results. |
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| Keywords: | Solid-state Nmr Quantitative |
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