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Analysis of Humic Acids by Solution and Solid-state Carbon-13 Nuclear Magnetic Resonance¹

ABSTRACT

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) ¹³C nuclear magnetic resonance (NMR). The main objectives of this investigation were: (i) to compare the solution and solid-state ¹³C NMR spectra of the HA's; (ii) to examine effects of different delay times (D₁) on the solution spectra; and (iii) to find reasons for the failure of ¹³C 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 ¹³C NMR at D₁ = 0.74 s were greater than those obtained at D₁ = 1.90 s and by CPMAS ¹³C NMR (where D₁ = relaxation delay). By contrast, proportions of aromatic C and aromaticities were greatest at D₁ = 1.90 s. The highest values for CO₂H groups were obtained by CPMAS ¹³C NMR and approached total acidities; values measured by solution ¹³C NMR at D₁ = 1.90 s were intermediate, while those determined by solution ¹³C NMR with D₁ = 0.74 s were the lowest and closest to CO₂H 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 ¹³C NMR spectra of HA's, signals arising from phenolic C are shifted downfield so that they overlap with resonances of C in CO₂H groups. The net result is that CO₂H groups are overestimated and phenolic OH groups underestimated.

¹ Contribution no. 1557 from the Chemistry and Biology Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario, K1A 0C6, Canada.

² Program Leader, Soil Nitrogen and Organic Matter, and Research Scientist, Nitrogen Fixation, respectively.

Received for publication June 17, 1985.