Evaluating the Effect of Fluorescence on the Raman Spectroscopic Analysis of Chlorinated Solvents

“Monitored natural attenuation” of chlorinated solvents is presently seen as one of the most technically feasible and economic means to manage remediation of sites contaminated with these compounds. Optical spectroscopic analysis methods offer the fieldability and analytical potential to enable cost-effective long-term monitoring of the attenuation process. However, their value in these settings is often compromised because aqueous media in the hydrogeological environment tend to contain naturally occurring non target fluorophores, which adversely affect the quality of in situ optical spectroscopic measurements. The purpose of this research was to characterize the influence of three different fluorophore emissions on time resolved Raman spectroscopic (TRRS) analysis of aqueous trichloroethylene (TCE) samples. The investigated fluorophores included naturally occurring humic acid (C₉H₉NO₆), and the anthropogenic compounds, fluorescein (C₂₀H₁₂O₈) and rhodamine B (C₂₈H₃₁CIN₂O₃). Key characteristics investigated included the form of the fluorescence spectra, their mean decay times, and the TRRS signal-to-noise ratios (SNR) at varying counting-times achievable in the presence of each fluorophore for both direct monitoring of TCE at the 381 cm^-1 (δ skeletal) Raman line and indirect TCE monitoring via observation of the water (OH stretching) Raman line at 3393 cm^-1. Results indicate that optimal SNR for TRRS measurements of fluorophore doped TCE samples is likely obtained with counting times less than ½ of the fluorescence lifetime of the involved fluorophore.