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OPTIMIZATION OF MODERN DISPERSIVE RAMAN SPECTROMETERS FOR MOLECULAR SPECIATION OF ORGANICS IN WATER
Citation:
Collette, T W. AND T. L. Williams. OPTIMIZATION OF MODERN DISPERSIVE RAMAN SPECTROMETERS FOR MOLECULAR SPECIATION OF ORGANICS IN WATER. Presented at Federation of Analytical Chemistry and Spectroscopy Societies Conference, Vancouver, Canada, October 23-29, 1999.
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:
Pesticides and industrial chemicals are typically complex organic molecules with multiple heteroatoms that can ionize in water. However, models for understanding the behavior of these chemicals in the environment typically assume that they exist exclusively as neutral species -- primarily due to a lack of experimental data on speciation of organics in water. It is clear that this erroneous assumption contributes large uncertainty in chemical exposure assessments when one considers the degree to which chemical behavior (e.g., sorption to soil) differs for different ionization species of the same chemical (e.g., a cation and a neutral). Unfortunately, it has heretofore been prohibitively difficult to study organic chemical speciation in water -- particularly when multiple species exist with the same net ionization charge (such as with zwitterions and tautomers).
We have recently developed a method, based on Raman spectroscopy, by which simultaneously occurring site-specific "micro-equilibrium" constants (such as with zwitterions and tautomers) can be determined with confidence. The method involves nonlinear regression modeling of temperature-variant spectral data according to the Gibbs-Helmholtz equation. Success of the method depends on capturing small changes in the observed series of spectra that are due to a systematic change (as a function of temperature) in relative concentration of the species that compose the equilibrium "mixture". In this talk, we will briefly describe the theoretical basis for this method, detail the arduous steps required to obtain an acceptable set of spectra, explore the limitations of assumptions inherent to the method, and make the case that Raman spectroscopy is the ideal tool for this type of environmental investigation.