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SPECIATION OF ORGANICS IN WATER WITH RAMAN SPECTROSCOPY: UTILITY OF IONIC STRENGTH VARIATION
Citation:
Collette, T W. AND T. L. Williams. SPECIATION OF ORGANICS IN WATER WITH RAMAN SPECTROSCOPY: UTILITY OF IONIC STRENGTH VARIATION. Presented at Federation of Analytical Chemistry and Spectroscopy Societies Conference, Providence, RI, October 13-17, 2002.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
We have developed and are applying an experimental and mathematical method for describing the micro-speciation of complex organic contaminants in aqueous media. For our case, micro-speciation can be defined as qualitative and quantitative identification of all discrete forms of a chemical that co-exist as partners in coupled equilibria. We believe that this information is required to provide an accurate description of the fate and effects of many organic contaminants, both in natural ecosystems and in living organisms. An important example of this phenomenon is the co-existence of a zwitterion form and a true neutral form for molecules such as amino acids, hydroxypyridines, and atrazines.
Our experimental method has thus far been based on collecting Raman spectra as a function of temperature. The zwitterion/neutral concentration ratio changes as a function of temperature according to the van't Hoff equation and can be fully described, in the simplest cases, using nonlinear modeling of the experimental data. However, the approach is not without limitations and constraining assumptions. For example, the temperature range is limited to the liquid state of water. The change in concentration ratio over this temperature range may not be dramatic enough to model with certainty in some cases. Also, the results may be confounded if multiple conformers of a given ionization form are present. Furthermore, the convenience, speed, accuracy, and precision of temperature control are limited.
We have explored several ways to improve this Raman-based method. For example, we have used a high pressure sample chamber to increase the upper temperature limit. Also, we have investigated other domains in which the zwitterion/neutral concentration ratio varies; perhaps the most promising is ionic strength. While the zwitterion/neutral concentration ratio does not change as a function of pH, it does vary in a definable way as a function of ionic strength. In this talk we will describe our efforts to improve speciation measurements, focusing on preliminary results with ionic strength variation. In addition to alleviating some of the problems with the temperature-variant approach, the ionic strength-variant approach may also yield some new information that is of considerable thermodynamic importance.