Aquatic samples, humic compound

At present, soil derived humic matter and fulvic acids extracted from freshwater are available commercially and are commonly used to test techniques for DOM detection and also used as model compounds for trace metal chelation studies. The results obtained using these model compounds are frequently extrapolated to the natural environment and measurements on "real" samples provide evidence that this DOM is a good model compound. In the past, some investigators also made available organic matter isolated from marine environments using C18 resins. While these compounds come from aquatic sources, this isolation technique is chemically selective and isolates only a small percentage of oceanic DOM. Reference materials are not currently available for these compounds, which inhibits study of the role they play in a variety of oceanographic processes. 

As an approach to investigating the complex chemistry of natural foams, humic substances (compounds sufficiently nonpolar at pH 2.0 to be isolated by reverse phase on XAD-8 and recovered in 0.1 N sodium hydroxide) were isolated from aquatic foam and associated stream water for chemical characterization and investigations into surfactant behavior. Humic substances were chosen because they represent natural organic compounds present in natural waters that are sufficiently nonpolar at pH 2.0 to be isolated by XAD-8 adsorption. As surfactants also possess moderately nonpolar characteristics it follows that humic substances may contain a significant surfactant component. We hypothesized that foam would be enriched in humic substances compared to stream samples and would show increased hydrophobicity, aliphaticity, and decreased carboxylation in order to sustain surface-active behavior. 

Silver-membrane and glass-fiber filters are recommended for studying particulate organic carbon, because organic contamination of the sample is avoided (Cranston and Buckley, 1972). These filters are also recommended for the isolation of aquatic humic substances, because interactions with the filter by dissolved organic compounds are minimized, and the potential for organic contamination of the sample is slight. 

Fractionation techniques for aquatic humic substances have not been developed to the same extent as concentration and isolation techniques. Many organic fractionation techniques presuppose a concentrated sample, but aquatic humic substances exist naturally at dilute concentrations in the presence of greater suspended sediment and inorganic solute concentrations. Now that efficient preparative concentration and isolation techniques for aquatic humic substances have been developed, as reported by Aiken in Chapter 14 of this book, renewed emphasis can be given to group fractionations with the ultimate hope that the chromatographic separation of aquatic humic substances into individual compounds can be achieved. 

The preparative fractionation scheme will apply the chromatography developed for the analytical scheme to larger samples. From 10 to 20 mg of the pure compound isolated from aquatic humic substances needs to be the objective of the preparative fractionation. The preparative fractionation might have to begin with as much as a kilogram of initial material. After structural determination of a number of aquatic humic substance components, the analytical fractionation, combined with spectral determinations. 

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