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Quantitative Characterization of Humic Substances by Solid-State Carbon-13 Nuclear Magnetic Resonance

^a Dep. of Polymer Sci. and Eng., Univ. of Massachusetts, Amherst, MA 01003 USA
^b Chemistry Dep. and the Barnett Institute, Northeastern Univ., Boston, MA 02115 USA

View larger version (15K): [in a new window]   Fig. 1 Nuclear magnetic resonance (NMR) pulse sequences used in this work. (a): direct-polarization magic-angle spinning (DPMAS) (b) cross-polarization spin-lattice relaxation time (CP/T1)–total sideband suppression (TOSS)

View larger version (24K): [in a new window]   Fig. 2 Sketch of the related changes of signal intensity with time for (a) cross-polarization spin-lattice relaxation time-total sideband suppression (CP/T1– TOSS) with T1–filter time t±z; (b) direct-polarization magic-angle spinning (DPMAS) with recycle delay trecy

View larger version (29K): [in a new window]   Fig. 8 Series of cross-polarization-total sideband suppression (CP–TOSS) spectra of humic acids (HAs) of different origins, International Humic Substances Society (IHSS) Florida peat and plant-extracted materials (PEMs), as indicated

View larger version (32K): [in a new window]   Fig. 3 Fully and almost fully relaxed direct-polarization magic-angle spinning (DPMAS) spectra of five humic acids (HAs) and International Humic Substances Society (IHSS) Florida peat as indicated on the right. The recycle delays are given on the left. Note the large intensity of the sp2 carbons (which still tend to be underrepresented due to their longer relaxation times)

View larger version (27K): [in a new window]   Fig. 4 Two cross-polarization spin-lattice relaxation time (CP/T1)–total sideband suppression (TOSS) spectra of humic acids (HAs) with different relaxation rates: A = IHSS-LEON, fully relaxed at filter time t±z = 5 s; B = Florida HA, nearly fully relaxed at filter time t±z = 5 s; C = Amherst HA, relaxed nearly half at filter time t±z = 5 s; D = German HA, relaxed more than half at filter time t±z = 25 s

View larger version (31K): [in a new window]   Fig. 5 Deconvolution of the incompletely relaxed direct-polarization magic-angle spinning (DPMAS) spectrum of Amherst humic acid (HA). The numbers above the spectrum refer to the bands corresponding to the ranges listed in Table 2

View larger version (28K): [in a new window]   Fig. 6 The correlation curves for %C, %H, and %(O + N) between chemical analyses and nuclear magnetic resonance (NMR) estimation. Diamond: International Humic Substances Society (IHSS) Florida peat; Circles: soil humic acids (HAs); triangles: coal HAs; squares: plant-extracted materials (PEMs). The solid lines represent the relationship y = x

View larger version (22K): [in a new window]   Fig. 7 Spectra of low-ash and high-ash Florida humic acid (HA). (a) Absolute-intensity comparison. The reduced intensity of the high-ash sample represents carbon loss (invisible carbon) due to paramagnetic species (see text). (b) Comparison of lineshapes after matching the intensities of the highest peaks

View larger version (36K): [in a new window]   Fig. 9 Bar graph plot of the composition of five peat humic acids (HAs), as obtained from direct-polarization magic-angle spinning (DPMAS) corrected by cross-polarization spin- lattice relaxation time-total sideband suppression (CP/T1– TOSS). Bar graphs of the compositions calculated from six models of HA structures are also shown. The bars (from left to right) represent nine chemical-shift ranges (identified by numbers as defined in Table 2): 1 (ketone–quinone), 2 (quinone–carboxyl), 3 (phenol), 4–5 (aromatic), 6 (complex aromatic–anomeric), 7 (carbohydrate), 8 (ether–methyne), 9 (complex aliphatic), 10–11 (simple aliphatic)

View larger version (28K): [in a new window]   Fig. 10 Bar-graph plot of the composition of two plant-extracted materials (PEMs), International Humic Substances Society (IHSS) Florida peat, and three commercial humic acids (HAs), as obtained from direct-polarization magic-angle spinning (DPMAS) corrected by cross-polarization spin- lattice relaxation time-total sideband suppression (CP/T1–TOSS). On the right, bar graphs of compositions calculated from two models of humic acid (HA) structures are also shown. The chemical-shift ranges are the same as in Fig. 9

View larger version (36K): [in a new window]   Fig. 11 The sp2 /sp3 carbon ratio in International Humic Substances Soceity (IHSS) Florida peat, plant-extracted materials (PEMs), humic acids (HAs) and models of HA structure. The uncertain assignment of the 96–108 ppm region of several models and the variability in Schulten and Schnitzer's model are taken into account by error bars. In Stevenson's model, the amino acid sidechains (labeled as -R in his model) were not counted due to their vague definition