Acquisition and Interpretation of Liquid-state 1H NMR Spectra of Humic and Fulvic Acids

Fourier Transform nuclear magnetic resonance (NMR) spectrometers have become available to many researchers studying humic substances over the last decade. As a result, liquid-state proton (1H) NMR spectrometry has been commonly used to determine the nonexchangeable proton distribution in humic and fulvic acids. The high sensitivity of the 1H nucleus to NMR spectrometry allows spectra to be obtained on a relatively small quantity of sample (10-100 mg) in a short time (10-30 min). 1H NMR spectrometric profiles of humic substances are useful to environmental scientists in determining the source, properties, and degree of transformation (humification) of organic matter that is operationally classified as humic substances. These 1H NMR spectrometric profiles, which provide information about hydrogen distributions in humic substances, are more useful for structural and biogeochemical studies when combined with 13C NMR spectra, which provide information on carbon distributions, and infrared spectra, which provide information on oxygen distributions. These three spectra, in conjunction with elemental composition, molecular weight, and titrimetric data, can then be synthesized to provide average structural characteristics that can be related to source, properties, and degree of humification of the organic material being studied. Special challenges, that are not met when obtaining the spectra of pure compounds, are encountered in obtaining 'H NMR spectra of natural humic substances. These challenges include (1) lack of complete dissolution of macromolecular humic substances at the high concentrations required for NMR studies; (2) significant concentrations of exchangeable protons giving broad peaks that obscure portions of the spectrum; (3) broad peaks of non-exchangeable protons over the entire spectrum that cause difficulties in correct phasing; (4) unstable structures that oxidize, hydrolyze, and structurally rearrange at the high pH conditions under which humic substances are the most soluble; and (5) the presence of unusual structures that complicate straightforward assignment of structure from handbook data. The purposes of this chapter are to describe methods of sample preparation and to provide generally applicable structural assignments whereby 1H NMR spectra suitable for quantitative studies of humic substance structure may be obtained and interpreted.