Measurement of Trace Environmental Contaminants Using Cavity Ringdown Spectroscopy

Environmental contamination has become a significant threat to the health and well-being of mankind as well as to the environment, prompting the establishment and implementation of stringent environmental regulations. The ability to accurately detect and quantify contaminants, such as mercury (Hg), uranium (U), and volatile organic compounds (VOCs), in real-time, in situ is of significant importance to monitoring and remediation efforts. In an effort to develop a real-time, fast-response detector that is portable, highly sensitive, and cost efficient, this research explored the feasibility of utilizing cavity ringdown spectroscopy (CRDS) in conjunction with various plasma sources and vacuum cavities to accurately detect trace quantities of contaminants. The feasibility of detecting Hg with a low power, low temperature candle-shaped microwave-induced plasma (MIP) and a copper surfatron microwave cavity with various plasma discharge tube configurations in conjunction with cavity ringdown spectroscopy (MIP-CRDS) is discussed. Detection limits were on the order of 221 ppt Hg in the vapor phase for the candle-shaped MIP and improved by a factor of 10 with the tube-shaped plasma. The ability to detect elemental Hg naturally-evaporating from contaminated soils and solutions was evaluated, and 10’s of ppt were consistently obtained. Additionally, the fine structure of the Hg 253.65 nm transition was observed with each iteration of this approach. The potential of effectively generating uranium atoms and ions with a low-power, lowlow rate microwave-induced plasma was evaluated. Uranium emission spectra covering 320 – 430 nm were obtained, labeled, and compared to the available literature values. Calibration curves were generated, and the detection limits were determined to be ~0.4 ppm. The feasibility of measuring U incorporating diode laser-plasma-CRDS was explored. The preliminary studies clearly show the ability to detect U vapor with this technique and sub-ppm detection limits were obtained. A continuous wave cavity ringdown spectroscopy system (CW-CRDS) incorporating commercially available telecommunications diode lasers was constructed, and the overall sensitivity of this system was evaluated by utilizing the absorption of the asymmetric C-H stretch overtones of several VOCs, including benzene, chlorobenzene, 1,2-dichlorobenzene, toluene, and acetone. Detection limits are determined to be in the ppb’s for each of the organics examined