Humic substances molecular weight distribution

Characterization of Samples. Fifty-five samples of dissolved humic substances were isolated and characterized during the first five sampling cruises. Characterizations of samples obtained during the first two sampling cruises included elemental analyses, molecular-weight distributions, acid-base titrimetry, solution-density determinations, stable-carbon-isotope determinations, and determinations of 13C NMR and lH NMR spectra. The... 

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). 

Shaw, P. J., R. I. Jones, and H. De Haan. 2000. The influence of humic substances on the molecular weight distributions of phosphate and iron in epilimnetic lake waters. Freshwater Biology 45 383-393. 

See, J. H., and Bronk, D. A. (2005). Changes in molecular weight distributions, C N ratios, and chemical strucmres of estuarine humic substances with respect to season and age. Mar. Chem. 97, 334-346. 

Collins M. R., Amy G. L., and Steelink C. (1986) Molecular weight distribution, carboxylic acidity, and humic substances... 

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. 

Adsorption of aquatic humic substances onto surfaces changes their molecular weight distribution pattern. However, there is apparently little knowledge of how the chemical composition and the microbial availability is altered by this process under environmental conditions. Photolysis by UV radiation also results in changes of the molecular weight distribution, but, there is some contradictory evidence concerning which molecular weight fractions are affected most. 

Since its introduction some years ago gel permeation chromatography has become a powerful tool in the study of naturally occurring polymers. While primarily devised and used for studying proteins, the technique has been applied to a wide variety of materials and has been used in the study of humic substances since the early 1960s. Gel permeation chromatography is a rapid, cheap, and very versatile technique. It can be used as a method for separation, purification, and fractionation as well as for determinations of molecular weights and molecular weight distributions of polymer systems. A review of the principles and applications of the technique is provided by Fisher (1969). 

Kemp, A. L. W. and Wong, H. K. T. (1974). Molecular-weight distribution of humic substances from Lakes Ontario and Erie sediments. Chem. Geol. 14, 15-22. 

Wrobel, K., Sadi, B. B. M., Wrobel, K., Castillo, J. R., and Caruso, J. A., Effect of metal ions on the molecular weight distribution of humic substances derived from municipal compost ultrafiltration and size exclusion chromatography with spectrophotometric and inductively coupled plasma-MS detection. Anal. Chem., 75, 761-767, 2003. 

Beckett R., Hart B.T. (1988), Molecular weight distributions of humic substances using flow field-flow fractionation, Proc. Science, Technology and Utflisation of Hunuc Acids, CSIRO North Ryde, Nov 88, 42-52. 

Rice J.A., Weil D.A. (1994), Absolute molecular-weight distribution of fuhdc acid. Humic Substances in the Global Environment and Implications on Human Health, (Ed. N. Senesi, T.M. Miano), Elsevier Science, 355-359. 

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. 

Comparison of humic substances from different lakes indicates a high variation in concentration, composition, and molecular weight. The extent to which differences in methodology contribute to this variation has not been evaluated. Temporal and spatial distributions of dissolved humic substances and humic-associated organic substances are presented for five representative lakes. General parameters (UV absorbance, DOC measurements with or without fractionation on the basis of molecular size) do not adequately reflect the dynamic nature of various humic substances in lake ecosystems. 

The task, however, of determining molecular weights for humic substances has not been simple. Humic substances comprise one of the most widely distributed classes of natural products on Earth. Defining the chemistry and understanding the nature of humic substances have long been hampered because most humic substances are not discrete chemical entities, but are a complex mixture of organic substances (Kononova, 1966) with a wide range of molecular sizes (polydisperse). 

The extent of competition may also be a function of the adsorbate molecular size, correlated with the activated carbon pore size distribution. Activated carbon fibers tliat are exclusively microporous (more than 96 % of micropore volume) present a selectivity property for pesticides or phenol in the presence of higher molecidar weight compounds like humic substances, due to the direct connection of micropores to their external surface [41]. Using granular activated carbon, which does not have this molKUilar sieve property, a 20 to 70 % reduction in adsorption is obtained for atrazine in raw water compared with equilibria in distilled water [42]. 

Pore size distribution Pore diameters tontrol which sizes of molecules are accessible to them. - micropores (c 2 nm) for microorganics, pesticides mesopores (2-50 nm) for higher molecular weight compounds like dyes, humic substances... 

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