JINGDONG MAO, WEIGUO HU, GUANGWEI DING, KLAUSS CHMIDT-ROHR, GEOFFREY DAVIES, ELHAM A. GHABBOUR and BAOSHAN XING( 2002). Suitability of different 13c solid-state Nmr techniques in the characterization Of humic acids.ISSN 0306-7319 print: ISSN 1029-0397
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/MASand CP/TOSS spectra gave consistently better results than those of the conventional CP/MASspectra at a 13C frequency of 75 MHz, which were incorrect due to spinning sidebands. CP/MASat low magnetic fields (22.6 and 50.6MHz 13C frequency) indicated improved integration results but lower resolution. Correction factors calculated by comparison with DP/MASwill be useful to convert the non-quantitative peak areas in the CP/TOSSand ramp-CP/MAS spectra into more quantitative results.
Keywords: (Solid-state NMR; Quantitative 13C NMR; Nuclear magnetic resonance; Cross-polarization (CP) magic-angle spinning (MAS) NMR; Structure of humic acids.)
