Molecular Weight Characteristics of Humic Substances from Different Environments As Determined by Size Exclusion Chromatography and Their Statistical Evaluation

Abstract

Recorded molecular weights (MWs) for humic substances (HS) range from a few hundred to millions of daltons. For purposes of defining HS as a specific class of chemical compounds, it is of particular importance to ascertain if this broad range of MWs can be attributed to actual variability in molecular properties or is simply an artifact of the analytical techniques used to characterize HS. The main objectives of this study were (1)to establish if a preferential range of MWs exists for HS and (2) to determine any consistent MW properties of HS. To reach the goal, we have undertaken an approach to measure under standardized conditions the MW characteristics of a large set of HS from different natural environments. Seventy-seven humic materials were isolated from freshwater, soil, peat, and coal, such that each possessed a different fractional composition: humic acid (HA), fulvic acid (FA), and a nonfractionated mixture of HA and FA (HF). Size exclusion chromatography (SEC) was used as the analytical technique to determine molecular weight characteristics. The MW distributions were characterized by number (Mn) and weight (Mw) average MW, and by polydispersity. The complete range of Mw values varied within 4.7-30.4 kDa. The maximum Mw values were observed for peat HF and soil HA, whereas the smallest weights were measured for river water HF. Maximum values of polydispersity (3.5-4.4) were seen for peat HF and soil HA, while much lower values (1.6-3.1) were found for all preparations isolated with XAD-resins. Statistical evaluation showed consistent Mw and Mn variations with the HS source, while polydispersity was mostly a function of the isolation procedure used. A conclusion was made that HS have a preferential range of MW values that could characterize them as a specific class of chemical compounds.

Atmospheric HULIS: How humic-like are they? A comprehensive and critical review

“…SEC characterization of complex multicomponent mixtures of polyfunctional polyelectrolytes such as humic substances is well-known to be difficult due to the preponderance of separation mechanisms other than size exclusion (e.g., electrostatic and molecular interactions with the stationary phase). At this time, after years of application of SEC to humic materials, optimal elution and detection systems, as well as molecular size interpretations of chromatograms, are still quite controversial (Chin et al, 1994;Peuravuori and Pihlaja, 1997;Perminova et al, 1998;DeNobili and Chen 1999;Perminova 1999;Piccolo et al 1999;Kudryavtsev et al 2000;Perminova et al, 2003;Peuravuori and Pihlaja, 2004) etc. A recent study by Samburova et al (2005a) of WSOC extracted from urban aerosol from Zurich exemplifies this difficulty, displaying differences in estimated molecular size by nearly an order of magnitude as a function of the polymer standard applied.…”

Characterization of high molecular weight compounds in urban atmospheric particles

“…Several authors have analyzed this class of compounds, mainly using size exclusion chromatography methods (SEC) (Fuzzi and Zappoli, 1996;Zappoli et al, 1999;Krivácsy et al, 2000) or spectroscopic methods such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), UV or fluorescence spectroscopy (Havers et al, 1998;Zappoli et al, 1999;Kiss et al, 2003). SEC is also a widely used and well established technique in the field of humic and fulvic acid analysis (Rausa et al, 1991;Piccolo et al, 2001;Perminova et al, 2003;Janos, 2003).…”

Peat as a feed supplement for animals: a literature review

“…Chromatographic analyses showed that the molecular weight of humic substances is primarily affected by their source (freshwater, soil, peat and coal). The maximum molecular weight and polydispersity values were observed for peat humic and fulvic acid, and soil humic acid (Perminova et al, 2003).…”

“…During humification stable humic substances are formed by microbial transformation of non-humic substances (Table 1) such as hemicellulose, cellulose, lignin, pectins, bitumens, waxes, resins, nitrogenous materials, lipids, amino acids, non-saturated and saturated fatty acids, organic sulphur, various types of carbohydrates, starch compounds, ethereal oils, balsam, bioterin and tannic acid (Andriesse, 1988;Hruska, 1988;Riede et al, 1992;Banaszkiewicz and Drobnik, 1994;Anonym, 2002). Humic substances (Table 2) such as humic, fulvic, ulmic acids, and humins are the major part of humified peat (Hruska, 1988;Hruska, 1999;Anonym, 2002;Kocabagli et al, 2002;Janos, 2003;Perminova et al, 2003). They constitute a dark brown non-soluble fraction of peat with an extremely high molecular weight, responsible for the capability to retain water, friability and electrostatic conductivity.…”