Structural analysis of humic substances

A New Approach to the Structural Analysis of Humic Substances in Water and SoUs... 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

Hertkorn, N. (2006). Molecular level structural analysis of natural organic matter and of humic substances by NMR spectroscopy. Habilitation thesis, Technical University of Munich, Munich, Germany, 285 pp. 

In order to investigate the properties of individual fractions of humic substances, various modes of high performance liquid chromatography (HPLC) have been employed. Hydrophobic interaction chromatography (5) has proved to be an effective separation technique, resulting in five distinct humic fractions from one sample. Structural analysis of these fractions was subsequently performed by infrared and nuclear magnetic resonance spectroscopy, and molecular weight distribution was also measured. 

Analysis of functional groups such as carboxyl, phenol, carbonyl, or meth-oxyl (Table 2) increases our understanding of the chemical structure of humic substances and can be used to explain the behavior of humic substances in various humification processes (Gjessing, 1976). Carboxyl and phenolic hydroxyl groups clearly predominate, although in some cases methoxyl groups are quantitatively important as well (Muenster, 1982). 

The most serious problem in the study of humic substances is the lack of reproducibility of analytical results. One would expect soil humates to vary with soil type, aquatic humates to vary with water sources, and coal humates to vary with coal rank. But even within one well-defined source, the elemental composition will vary between samples, depending on extraction and fractionation procedures. There are cases in which the same authors have used the same source and the same extraction procedure and have obtained significantly different elemental analyses. Before any meaningful structural conclusions can be deduced from elemental analysis, a rational definition of humic substances will have to be established (MacCarthy, 1976 Malcolm and MacCarthy, 1979). 

This chapter examines the fundamental structural properties of organic molecules that directly or indirectly affect pKa values of organic acids, making it possible to estimate theoretical upper limits for concentrations of common acidic functional groups, to assess methods of quantitative analysis of acidic functional groups in humic substances, and to examine several models that have been proposed for the description of proton binding by humic substances. Some of the concepts that appear obvious in the context of this chapter have been included for the simple reason that the literature on acidity of humic substances reveals that these points have frequently been either misunderstood or neglected. 

The development of methods of analysis of triazines and their hydroxy metabolites in humic soil samples with combined chromatographic and ms techniques has been described (78). A two-way approach was used for separating interfering humic substances and for performing structural elucidation of the herbicide traces. Humic samples were extracted by supercritical fluid extraction and analyzed by both hplc/particle beam ms and a new ms/ms method. The new ms/ms unit was of the tandem sector field-time-of-flight/ms type. 

Wilson, M. A., GiUiam, A. H., and Collin, P.J. (1983). Analysis of the structure of dissolved marine humic substances and their phytoplanktonic precursors using and C NMR. Chem. Geol. 40, 187—201. 

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