Optimization of Raman Spectroscopy for Speciation of Organics in Water

Abstract

We describe herein a method for determining constants for simultaneously occurring, site-specific “microequilibria” (as with tautomers) for organics in 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 charge-coupled device (CCD)-based, dispersive Raman spectrometer. The success of the method depends on accurate quantification of small spectral changes that are monotonic with temperature due to changes in relative concentration of equilibrium components. The method assumes that we can neglect intensity 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 assumes that we can neglect the temperature dependence of the Raman spectrum of an individual conformer. We have investigated these assumptions and found that we can typically reduce frequency and intensity fluctuations to tolerable levels by normalizing all spectra on the basis of the atmospheric N₂ stretching band at 2331 cm⁻¹, which is observed in all of our spectra. Further, we have found that we can typically neglect the temperature dependence of Raman spectra if areas of depolarized bands are used in the modeling.