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Analysis of Humic Acids by Solution and Solid‐state Carbon‐13 Nuclear Magnetic Resonance
Author(s) -
Schnitzer M.,
Preston C. M.
Publication year - 1986
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1986.03615995005000020015x
Subject(s) - chemistry , magic angle spinning , nmr spectra database , chemical shift , spectral line , solid state nuclear magnetic resonance , analytical chemistry (journal) , magic angle , carbon 13 nmr , nuclear magnetic resonance spectroscopy , resonance (particle physics) , carbon 13 , nuclear magnetic resonance , solid state , crystallography , stereochemistry , organic chemistry , physics , particle physics , astronomy , quantum mechanics
Eighteen humic acids (HA's) were characterized by chemical analyses, infrared (IR) and electron spin resonance spectra, and by solution and solid‐state cross polarization magic‐angle spinning (CPMAS) 13 C nuclear magnetic resonance (NMR). The main objectives of this investigation were: (i) to compare the solution and solid‐state 13 C NMR spectra of the HA's; (ii) to examine effects of different delay times (D 1 ) on the solution spectra; and (iii) to find reasons for the failure of 13 C NMR to detect the presence of significant concentrations of phenolic C in unmethylated HA's. Compared to solution spectra, CPMAS spectra consisted of relatively few, broad bands, indicative of considerable overlapping of individual signals, especially in the aliphatic regions. Proportions of aliphatic C determined by solution 13 C NMR at D 1 = 0.74 s were greater than those obtained at D 1 = 1.90 s and by CPMAS 13 C NMR (where D 1 = relaxation delay). By contrast, proportions of aromatic C and aromaticities were greatest at D 1 = 1.90 s. The highest values for CO 2 H groups were obtained by CPMAS 13 C NMR and approached total acidities; values measured by solution 13 C NMR at D 1 = 1.90 s were intermediate, while those determined by solution 13 C NMR with D 1 = 0.74 s were the lowest and closest to CO 2 H concentrations determined chemically. Because the presence in HA's of significant concentrations of phenolic OH groups has been confirmed by several independent methods, it is possible that in 13 C NMR spectra of HA's, signals arising from phenolic C are shifted downfield so that they overlap with resonances of C in CO 2 H groups. The net result is that CO 2 H groups are overestimated and phenolic OH groups underestimated.