Elemental analyses humic substances

Before discussing the analysis of any of the other elements it is necessary to discuss the critical importance of the determination of water in these samples. The water determination is crucial to the correct determination of carbon, hydrogen, and oxygen, the most abundant elements in humic substances, and there are different approaches to the determination of water from which to choose. 

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

The concentration of humic acid was determined by TOC analysis. TOCs for the humic stock solutions were normally 2.3 0.1 g/L. Elemental analysis showed that the powdered humic substance was 52 carbon weight. This result indicates that the theoretical maximum of TOC for an 800-mL solution prepared from 4.0 g of humic substances is 2.6 g/L. 

Elemental Analysis. The elemental analyses are presented in Table IV. The atomic ratios H/C for all drinking water samples (nos. 1-10) were between 1.28 and 1.39. These values were comparable to humic acid derived from lake sediments. However, H/C ratios were much lower when compared to the chlorinated model humic substances (e.g., 1.04-1.08 for CFH-1 and CFH-2). Bromine was present in almost negligible quantities, whereas Cl varied between 0.3 and 2.4, and S varied between 0.9 and 2.7 in the drinking water organic matter. All fractions from drinking water showed similar elemental composition. However, they differed from the elemental composition of the CFH samples in all respects, especially in chlorine content. 

Huffman, E. W. D., and Stuber, H. A. (1985). Analytical methodology for elemental analysis of humic substances. In Humic Substances in Soil, Sediment, and Water. Geochemistry, Isolation, and Characterization, Aiken, G. R., McKnight, D. M., Wershaw, R. L., and Mac-Carthy, P., eds., John Wiley Sons, New York, pp. 433 455. 

Elemental analysis and the detailed characterization require the extraction and purification of soil organic matter. International Humic Substances Society (2008)... 

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/... 

Insofar as humin is an insoluble macromolecular residue, it has mostly been examined by techniques amenable to solid materials (i.e., elemental analysis, infrared, solid-state NMR, and ESR spectroscopy). Degradative techniques such as oxidation, reduction, and pyrolysis have also been employed. All these methods have been used for the study of humic substances and excellent reviews of the various methods are provided by Schnitzer and Khan (1972, 1978) as well as by Stevenson (1982). 

Use of aqueous solutions of sodium hydroxide for the extraction of humic substances was first described by Achard (1786). Such solutions have been the solvents of choice by most workers since that time. Comparison of elemental analysis data (Table 5) for the 2.5M EDA- and for sodium hydroxide-extracted humic substances show that the carbon and nitrogen contents of the EDA-soluble substances were significantly higher than for those extracted with sodium hydroxide. The reverse was true for the oxygen contents of these substances. 

Analytical Methodology for Elemental Analysis of Humic Substances... 

Given the importance of elemental data in interpretive studies, it is necessary to ask questions about the accuracy of the reported results. Very often, elemental data are not obtained by the investigator studying the sample but, rather, by a separate in-house or eommercial laboratory. It is imperative that the investigators know the limitations of the results they interpret and how to obtain the best possible results from their laboratories. The purpose of this study was to evaluate the elemental analysis of humic substances as accomplished by the currently common methods. We intended to learn if there are any special problems in attaining accurate data for humic substances, to establish some of the precision limits being attained by laboratories, and, finally, to make recommendations about optimum, or perhaps unacceptable, approaches for elemental analysis of these substances. We know of no published evaluation of the elemental analysis of humic substances. 

The sources and handling of samples chosen for this study were known to be carefully and accurately documented. They include one aquatic fulvic acid with a low ash content, one high ash aquatic humic acid, and one low ash soil humic acid. The laboratories chosen include three widely known commercial laboratories in the United States which specialize in organic elemental analysis and one government laboratory with extensive experience in elemental analysis of humic substances. 

The purpose of this section is to provide a brief review of the methods and techniques commonly used in the elemental analysis of humic substances with special emphasis placed on areas that may cause difficulties in their analysis. There are few specific references to methods of elemental analysis of humic substances. A computerized search of Chemical Abstracts since 1966 revealed no references to techniques of elemental analysis when elemental analysis was cross-referenced with humic or fulvic acid materials. In general, the methods of analysis have been developed to be applicable to a wide range of organic materials. However, it should be pointed out that most methods have been validated on the basis of the analysis of stable, nonhy-groscopic, nonvolatile, pure compounds and not heterogeneous mixtures. 

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