Optimization of Raman spectroscopy for speciation of organics in water

We describe herein a method for determining constants for simultaneously oc curring, site-specific "microequilibria" (as with tautomers) for organics i n water. The method is based in part on modeling temperature-variant Raman spectra according to the van't Hoff equation. Spectra are measured with a c harge-coupled device (CCD)-based, dispersive Raman spectrometer. The succes s of the method depends on accurate quantification of small spectral change s that are monotonic with temperature due to changes in relative concentrat ion of equilibrium components. The method assumes that we can neglect inten sity and frequency fluctuations in spectra collected over a period of days that are not due to chemical compositional changes (e.g., frequency shifts due to ambient instrument temperature fluctuations). Also, the method assum es that we can neglect the temperature dependence of the Raman spectrum of an individual conformer. We have investigated these assumptions and found t hat we can typically reduce frequency and intensity fluctuations to tolerab le levels by normalizing all spectra on the basis of the atmospheric N-2 st retching band at 2331 cm(-1), which is observed in all of our spectra. Furt her, we have found that we can typically neglect the temperature dependence of Raman spectra if areas of depolarized bands are used in the modeling.