The Effect of Turbidity on Raman Spectroscopic Analysis of Aqueous Chlorinated Samples

Abstract

As attention has shifted to the use of monitored natural attenuation to remediate sites contaminated with chlorinated solvents, optical spectroscopic methods have gained attention as a potential means to enable long-term monitoring. However, previous field research has shown that a number of geoenvironmental factors adversely influence the quality of in-situ optical spectroscopic measurements, including the presence of (non-target) fluorophores, presence of multiple chemical compounds, and the hydrogeological environment of the site. The purpose of this study was to evaluate and characterize the influence of turbidity on the capability of a Time Resolved Raman Spectroscopy (TRRS) system to observe chlorinated solvents in aqueous samples via laboratory tests conducted on liquid standards of trichloroethylene. Key issues investigated included turbid constituent particle size, mineralogy, and turbidity level. Samples were created by doping aqueous chlorinated solvent samples with fine silica flour and Namontmorillonite ranging from 0.02-2μm, with the ultimate aim of understanding how variations in turbidity influence TRRS measurement. Preliminary data indicate that turbidity and particle size consistently influence optical spectroscopic measurements of the TCE contaminant in aqueous solution, with a general trend of exponential decreases in SNR with linear increases in turbidity. Overall, a loss in SNR of ~50% was observed at a turbidity value of ~40 NTU. This observation indicates that absolute Raman intensity in turbid media likely needs to be corrected for the influence of suspended particulate materials in solutions.