Inferential monitoring of chlorinated solvents through Raman spectroscopic observation of the vibrational modes of water

Abstract

Recent improvements in diode laser, fiber optic, and data acquisition technology have rejuvenated interest in field applications of Raman spectroscopy in a wide range of settings. One such application involves the observation of chlorinated solvents to facilitate the practice of "monitored natural attenuation." In this context, this manuscript focuses on means to improve the sensitivity of in-situ Raman analysis of chlorinated solvents. In particular, the work explores the performance limits of a Time-Resolved Raman Spectroscopy (TRRS) system employed to observe chlorinated solvents in aqueous samples via laboratory tests conducted on both liquid standards of trichloroethylene (TCE) and simulated biodegraded field samples. Quantitative assessment of TCE in solution is carried out through both direct observation of TCE Raman functional groups (381 cm(-1) (δ skeletal), 840 cm(-1) (νCCl) and 1242 cm(-1) (δCH)) and indirect observation of the broad OH stretching (2700-3800 cm(-1)) Raman modes of water. Results from tests on simple solutions show that the TRRS system can detect TCE at aqueous concentrations as low as 70 ppm by directly monitoring the 381 cm(-1) TCE line, whereas observation of the OH stretching line of water (3393 cm(-1)) provides an indirect indication of TCE presence with nearly a 9× improvement in detection level. This unique and counterintuitive mechanism to detect the presence of chlorinated compounds in solution takes advantage of the influence of chlorine on the vibrational modes of water. This influence, which is believed to be attributed to the formation of hydrogen bonds and their resultant interactions with the solvation shell, may serve as a more sensitive and robust indication of the presence of aggregate chlorinated solvent contamination in aqueous systems. Tests performed on simulated biodegraded field samples demonstrate that the indirect detection mechanism is apparent even in complex samples representative of typical field conditions.

Assessment and Correction of Turbidity Effects on Raman Observations of Chemicals in Aqueous Solutions

“…Although at any given TCE concentration turbidity level can be expected to influence the Raman return of the water line in the same manner as the TCE Raman line, the water Raman line cannot be used as an intrinsic calibration in this case due to the interactions that are known to occur between the hydrogen bonds in water and the chlorine comprising the chlorinated solvent. 46 Thus, an alternative mechanism to determine in situ turbidity would be required to identify the appropriate correction factor from Fig. 9 that could be employed to adjust field measurements.…”

“…TCE, a chlorinated hydrocarbon and member of a group of compounds known as dense nonaqueous phase liquids, was employed in a narrower set of tests as an analyte known to influence the Raman vibrational modes of water. 45,46 A spectrometric grade (purity > 99.5%) of the compound obtained from Alfa Aesar was used as received for this work.…”