Reduced heterogeneity of a lignite humic acid by preparative HPSEC following interaction with an organic acid. Characterization of size-separates by Pyr-GC-MS and 1H-NMR spectroscopy

Impact of vegetation on sedimentary organic matter composition and polycyclic aromatic hydrocarbon attenuation

Results from natural and engineered phytoremediation systems provide strong evidencethatvegetated soils mitigate polycyclic aromatic hydrocarbon (PAH) contamination. However, the mechanisms by which PAH mitigation occurs and the impact of plant organic matter on PAH attenuation remain unclear. This study assessed the impact of plant organic matter on PAH attenuation in labile and refractory sediments fractions from a petroleum distillate waste pit that has naturally revegetated. Samples were collected in distinct zones of barren and vegetated areas to assess changes to organic matter composition and PAH content as vegetation colonized and became established in the waste pit. Sediments were fractionated into bulk sediment and humin fractions and analyzed for organic matter composition by isotope ratio mass spectrometry (delta (13)C), 13C nuclear magnetic resonance (13C NMR), delta 14C AMS (accelerator mass spectrometry), and percent organic carbon (%TOC). Gas chromatography mass spectrometry (GC/ MS) of lipid extracts of SOM fractions provided data for PAH distribution histograms, compound weathering ratios, and alkylated and nonalkylated PAH concentrations. Inputs of biogenic plant carbon, PAH weathering, and declines in PAH concentrations are most evidentfor vegetated SOM fractions, particularly humin fractions. Sequestered PAH metabolites were also observed in vegetated humin. These results show that plant organic matter does impact PAH attenuation in both labile and refractory fractions of petroleum distillate waste.

Increased Conformational Rigidity of Humic Substances by Oxidative Biomimetic Catalysis

A synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of iron(III) chloride, Fe(TDCPPS)Cl, was employed as a biomimetic catalyst in the oxidative coupling of terrestrial humic materials. High-performance size-exclusion chromatography (HPSEC), solid-state nuclear magnetic resonance (CPMAS-(13)C NMR), electron paramagnetic resonance (EPR), and diffuse reflectance infrared spectroscopy (DRIFT) were used to follow conformational and structural changes brought about in different humic materials by the oxidative coupling. Increase in apparent weight-average molecular weight (Mw(a)) occurred invariably for all humic substances with the oxidative polymerization catalyzed by Fe(TDCPPS)Cl. HPSEC further showed that the polymerization reaction turned the loosely bound humic supramolecular structures into more stable conformations which could no longer be disrupted by the disaggregating effect of acetic acid. DRIFT spectroscopy suggested the formation of new alkyl and aromatic ethers following the oxidative coupling with the biomimetic catalyst. CPMAS-(13)C NMR and EPR spectra suggested a reduced molecular mobility of humic components and enhanced stabilization of free radicals in larger oxidized fragments. All findings concur in indicating that the biomimetic catalysis by Fe(TDCPPS)Cl increased the molecular mass and chemical rigidity of humic materials by formation of intermolecular covalent bonds via a free-radical mechanism. The development of a technology based on oxidative polymerization by biomimetic catalysis may be of importance in controlling the properties and reactivity of humic matter for industrial and environmental applications.

Preliminary study of lake dissolved organic matter in light of nanoscale supramolecular assembly

Large-scale preparative high-performance size-exclusion chromatography (HPSEC) was performed to separate different molecular size fractions in milligram quantities from strongly colored dissolved organic matter (DOM) of a freshwater using a very mild conjugate acid-base pair (10 mM acetic acid-sodium acetate solution at pH 7.0 with an ionic strength of 6 x 10(-5)) as the mobile phase. The homogeneity-uniformity of different molecular size fractions in relation to their combined original mixture was verified by an analytical HPSEC system. In addition to molecular size distribution and basic spectroscopic characteristics, Fourier transform infrared spectroscopy was applied to specify structural features for different size fractions. The results demonstrate clearly that only a very small amount of conjugate organic acid-base pair is required to generate a powerful resolution for a DOM mixture, and very strong treatments with organic acids are not necessarily needed to reach a better SEC resolution. Most essential is the combined outcome of different HPSEC experiments and determined structural functionalities which indicate that almost all original DOM solutes are aggregated mixtures consisting of different associations possessing various molecular size ranges, which can be separated from their integrated whole as nearly homogeneous and uniform species. In summary, the present study strongly speaks for the need to direct the research of natural dissolved and colloidal organic carbon more strongly toward a nanoscale study of supramolecular assemblies. More precise knowledge about the primary, secondary, and tertiary structure of dissolved DOM constituents has its essential function, e.g., for environmental protection and utilization of surface waters.

Comparative study for separation of aquatic humic substances by capillary zone electrophoresis using uncoated, polymer coated and gel-filled capillaries

Several comparative capillary zone electrophoresis (CZE) experiments were carried out by means of uncoated, polyvinyl alcohol (PVA) and polyacrylamide (PAA) coated silica open tubular capillaries and gel-filled capillaries (linear non-cross-linked polyacrylamide, PAGE, by a pre-coated PAA capillary) using different kinds of background electrolytes (BGEs) and organic modifiers for characterization of aquatic dissolved humic matter (DHM). Organic compounds, such as acetic acid, acetate buffer, methanol, ethylene glycol, acetonitrile, dimethylsulphoxide, 5 M urea and sodium dodecyl sulphate (SDS) were tested as sample modifiers to improve the separative power. The fractionation mode by a PVA coated open tubular capillary using 40 mM phosphate buffer at pH 6.8 and 5 M urea-water as the sample modifier turned out to be fairly practical as well as its PAA homologue. Linear non-cross-linked PAGE with 10% gel concentration and 5 M urea-water as the sample modifier using 40 mM phosphate buffer at pH 6.8 produced the most reliable results as to the adaptation of physical gels, especially if the interactions of humic solutes with the gel matrix are not critical. The addition of SDS in the linear PAGE gel increased the interaction of humic solutes with the gel matrix but also improved the separative power and strengthened the chaotropic effect of the urea modifier.

A comprehensive liquid-state heteronuclear and multidimensional NMR study of Laurentian fulvic acid

The Boreal forest fulvic acid known as Laurentian fulvic acid (LFA) has been interrogated by state of the art heteronuclear and 2D high resolution NMR techniques. It is shown that one can obtain very highly resolved and informative spectra of a traditionally fractionated material. It was possible to observe a proton coupled system of up to seven bonds in the TOCSY spectrum and 329 peaks in the 1H,13C-HSQC spectrum. It is found that the majority of the nitrogen in this sample is in the form of ammonium cations. From the combination of inverse-gated decoupling, APT, and INEPT 13C spectra of LFA it can be concluded that while the aromatic moieties of LFA are highly unfunctionalized, the carbohydrate moieties are highly functionalized. Proton coupled networks are observed in the TOCSY spectrum between and within the aliphatic, functionalized aliphatic, and heteroatom substituted regions and, to a lesser extent, also between the amine/aromatic and heteroatom substituted regions. The HMBC spectrum confirms that both the aliphatic and heteroatom moieties are highly functionalized with carboxylic and alcoholic functional groups, while the aromatic moieties are very sparsely functionalized with phenolic and carboxylic functionalities. Additionally, specific model molecular structures have been identified which are consistent with experimental evidence and are in full agreement with our previously derived meso-model based on solid-state 13C NMR data. Finally, some of the shortcomings of 2D liquid-state NMR for the characterization of humic materials are addressed.

Capillary zone electrophoresis of natural organic matter

This paper presents an in-depth look at the use of capillary electrophoretic (CE) techniques for the fingerprinting and characterization of humic substances and natural organic matter. These materials are highly heterogeneous in structure and show all characteristics of mixtures unliked in analytical chemistry. The electrophoretic approach, however, allows the determination of mobility distributions in different solution conditions, representative of the effective charge and size distribution status of the components present. A tabulated review covers over 50 references on the subject and highlights the possibilities and problems encountered in the analysis of such polydisperse materials with CE methods. In a second part of the article the consequences of experimental and buffer parameters on the behavior of humic materials in CE are presented.

On-line coupling of size exclusion chromatography with electrospray ionization-tandem mass spectrometry for the analysis of aquatic fulvic and humic acids

A method was developed for the analysis of humic and fulvic acids by size-exclusion chromatography-electrospray ionization-tandem mass spectrometry using a completely volatile eluent. Humic and fulvic acids were separated into three peaks. These fractions occupied different mass ranges and showed differences in the fine structure of their mass spectra. The low-molecular-weight (LMW) fraction of fulvic acids is most sensitively determined by ESI-MS, and it appears that previous results obtained by infusion-ESI-MS were primarily determined by this fulvic acid fraction. The average molecular weight of this fractions turned out to be lower than that reported from infusion-ESI-MS measurements. Its scan spectra and the product ion spectra of some of its molecular anions perfectly match those previously obtained from whole fulvic acid mixtures. Obviously, a class of well-defined polycarboxylated molecules exist that occurs in all fulvic acid fractions thus far investigated. With decreasing elution time and increasing molecular weight, detection by ESI-MS loses sensitivity as compared to the parallel UV recording, and the fine structure of the scan spectra becomes increasingly uniform for both fulvic and humic acids. The average molecular weight of the HMW fraction exceeds those values calculated from infusion experiments. Scan spectra and product ion spectra of the high-molecular-weight (HMW) fraction of both the humic and the fulvic acids suggest that the HMW fraction consists of several subunits that originate from the LMW fraction.

Separation methods in the chemistry of humic substances

Separation methods are widely used to isolate humic substances (HSs), to fractionate them before further investigation, and to obtain information about their structure and properties. Among the chromatographic methods, techniques based on a size-exclusion effect appear to be most useful, as they allow us to relate elution data to the molecular mass distribution of HSs. The limitations of this approach are discussed in this review. Gas chromatography with mass spectrometric detection is typically used to identify the products of pyrolysis or thermochemolysis of HSs; this technique is considered most important in the structural investigation of HSs. Electrophoretic methods (especially capillary zone electrophoresis) provide detailed characterization of HSs, but it is very difficult to relate the electrophoretic data to any specific subfraction, structure or properties of HSs. The electrophoretic patterns are often called "fingerprints" and can potentially be used for the identification and classification of HSs. This is limited, however, by the great diversity of the procedures employed and by the low degree of harmonization--no data on reproducibility and between-laboratory comparability are available. The same holds true, to a certain degree, for most methods utilized for the characterization of HSs. Separation methods play an important role in the examination of the interactions of HSs with heavy metals and other chemical pollutants. They allow us to determine binding constants and other data necessary to predict the mobility of chemical pollutants in the environment.