Molecular weight humic substance analysis

A problem for both humic- and yellow substances is that for these groups of experimentally defined components of different sources, each analysis will be ambiguous in terms of relative composition and molecular weight distribution. Additionally it appears that almost every scientist working in this field has developed his own extraction procedure (Weber and Wilson, 1975 Mantoura and Riley, 1975 a Schnitzer, 1976 Stuermer and Harvey, 1977). Different extraction times and -procedures result in different compositions of the organic constituents (Laane and Kramer, 1984). Soil humic-and fulvic acids, often used for studies on the interaction with trace elements, and those derived from water have certainly not the same composition and contain not the same distribution of functional groups. Therefore, results should be compared with care (Buffle, 1980 Buffle et al., 1984). 

Note Humic substances (HS) are operationally defined as DOC that is retained on XAD-2 or XAD-8 resins. UDOM is operationally defined as DOC that is retained by a membrane with a 1 nm pore size and 1000 Da molecular weight cutoff. 13C-NMR spectroscopy was used for analysis of carbon functional groups, which are presented as a percentage of the total organic carbon in the sample. THNS, total hydrolyzable neutral sugars THAA, total hydrolyzable amino acids. 

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. 

Size exclusion chromatography (SEC) has been used to measure molecular weight (MW) distribution of humic substances (3, 6-9). Coupled with detection methods such as molecular fluorescence spectroscopy and dissolved organic carbon analysis (7), electrochemical detection (9), and atomic emission spectroscopy (5), SEC has been used extensively to study humic-metal complexes. A major disadvantage of SEC is that it does not provide adequate resolution for separating humic materials as they do not appear to be made up of distinct fractions with large differences in MW. 

The fact that a humic substance is not a pure compound, but is a heterogenous mixture of many compounds with generally similar chemical properties, places an important constraint on all these characterization methods. Examples of the multicomponent mixture problem are presented in the chapters discussing interpretation of elemental analysis, determination of molecular weight, analysis of potentiometric data, and interpretation of infrared and other spectroscopic data. 

Humic substances are from terrestrial sources in overlying soils. Humic substances in the Biscayne aquifer are quite similar to those in surface water in elemental analysis, carbohydrate content, color, molecular weight, C/... 

The determination of molecular sizes and weights for humic substances is a task complicated by the nature of these materials. A brief overview of the methods most commonly used to determine molecular sizes and weights of humic substances has been presented in this chapter. Many of the methods discussed are powerful techniques that can yield information about the molecular size, shape, and weight of humic substances. No single method alone, however, is sufficient to provide a complete understanding of these molecular characteristics. Meaningful and accurate conclusions can only be made by using the data provided by different methods of analysis. 

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