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

Abstract

The compositions of humic acids (HAs) from various Histosols in North America and Europe, of similarly treated plant‐extracted materials (PEMs), of coal‐extracted humic acids, and of International Humic Substances Society (IHSS) Florida peat were quantified by solid‐state 13C nuclear magnetic resonance (NMR). In order to obtain quantitative intensities, the peak areas in direct‐polarization 13‐kHz magic‐angle spinning (DPMAS) 13C NMR spectra were corrected for incomplete relaxation by factors measured in cross‐polarization spin‐lattice relaxation time (CP/T1) experiments with total sideband suppression (TOSS). The elemental compositions (%C, %H, %O + N) of a peat sample, 8 HAs and 2 PEMs were estimated from the NMR results and compared with chemical analyses, as well as solution NMR for two of the HAs. The results are in good agreement, which shows that DPMAS corrected by CP/T1–TOSS permits quantitative characterization of HAs and PEMs. The compositions of the PEMs deviate significantly from those of the Histosol HAs. The compositions in terms of nine types of chemical groups were computed. The investigated HAs consist of more than 60% of aromatic and CO carbons (including both carbonyl and carboxyl groups). Previous cross‐polarization magic‐angle spinning (CPMAS) NMR experiments have significantly underestimated the ratio of sp2– to sp3–carbons; in particular, the true COO carbon fraction is a factor of two larger than estimated by CPMAS NMR. In spite of their wide range of geographical origins, the compositions of the Histosol HAs appear to be relatively uniform, suggesting that the search for a general model of Histosol HA structure is worthwhile. Eight models proposed in the literature do not reproduce the experimentally determined compositions, but a few models show promising partial agreement.

Abundant and Stable Char Residues in Soils: Implications for Soil Fertility and Carbon Sequestration

“…The 90°pulse length was 4 μs. The recycle delay was 40 s for Hatahara and 26 s for Lago Grande, which is >4 times the measured spin−lattice relaxation time 23,24 (see also Figure S2 in the Supporting Information). A spectrum with 100-s recycle delay experiments confirmed complete relaxation, as did >95% observability in spin counting analysis.…”

Effects of humic acid and lipid on the sorption of phenanthrene on char

“…Mao et al (2002) reported that the sp 2 carbons (110-220 ppm) or -COOH tend to be underrepresented by CP MAS 13 C NMR due to their longer relaxation times (T 1 , spin-lattice relaxation time). According to this observation, -COOH of the lipid may be in rigid domains, corresponding to longer T 1 Mao et al, 2000). When lipid was coated on char, a small peak at 167 ppm (-COOH) appeared and even more intensive with increasing coated lipid, indicating that some rigid domains changed to more mobile domains, corresponding to the reduced T 1 of 13 COOH.…”

Carbon-13 solid-state NMR of soil organic matter - using the technique effectively

“…The purpose is not to repeat what is available elsewhere, such as reviews of applications, noted previously, or basic theory and factors affecting quantitative reliability of NMR spectra (Preston et al 1984;Axelson 1985;Wilson 1987Wilson , 1991Fründ et al 1994a, b;Kinchesh et al 1995;Conte et al 1997a, b;Veeman 1997;Oades 1999, 2000a, b;Hu et al 2000;Mao et al 2000). The second purpose is to raise awareness of the need to develop NMR infrastructure and expertise in SOM studies in a variety of contexts.…”