Monitoring Made Simple

Groundwater in many U.S. historical legacy sites is contaminated with dense non-aqueous phase liquids, most of which are chlorinated hydrocarbons such as trichloroethylene (TCE). Due to migration in the subsurface over time, it is practically impossible to completely remove these contaminants. In order to assess the status of the pollutants and to address public concern, technologies for long-term monitoring of groundwater contaminants are needed.

Current monitoring methods usually involve complex procedures such as installation of groundwater wells, sampling and shipping of the groundwater with efforts to protect sample integrity and laboratory analysis using gas chromatography-mass spectrometry (EPA Method 8260). These methods are labor intensive, expensive and slow. Because these procedures are delayed and performed at different locations, contaminants can escape, giving misleading site characterization. Therefore, it is desirable to have technologies that combine sampling and detection of contaminants with high reliability and sensitivity.

With support of the Strategic Environmental Research and Development Programs, a research team led by Oak Ridge National Laboratory (ORNL), Oak Ridge, Tenn., delivered a field-deplorable technology called membrane-extraction ion mobility sensing. The proprietary sensor is a single-step compact device capable of detecting most aqueous chlorinated hydrocarbons, TCE, tetrachloroethene (PCE) and 1, 2-dichloroethene (DCE) with high reliability and high sensitivity.

How it Works
A membrane extraction assembly is combined with a miniature ion mobility spectrometry (IMS) analyzer. Membrane tubes are made of funct-ionalized PDMS and are used for converting chlorinated solvents from water to vapor. Dry air with optimized flow carries the vapor to the IMS analyzer specialized for accepting the contaminants. The IMS analyzer instantly gives a qualitative and quantitative response to the contaminants that originated from the water.

Building on ORNL’s recent system reported in Analytical Chemistry (Analytical Chemistry, 2010, 82, 4089–4096), the identification power was further increased by incorporating Sionex’s chromatographic differential mobility spectrometry (DMS) and increased sensitivity with the use of nanoporous materials as a preconcentrator. The new detection limits for TCE and PCE are about 0.5 parts per billion in volume (ppbv), namely, 0.8 g/L. The detection dynamic range is 0.5 ppbv to 2 parts per million in volume. The system has survived the presence of co-contaminants and foreign contaminants with reproducibility for six months.

The sensor has been tested for contaminants in groundwater at the Y-12 National Security Complex sites. In its GW225 well, TCE, carbon tetrachloride (CCl4) and ethanol were identified as major contaminants and PCE and chloroform (CHCl3) as minors. The test result is better than the result obtained with EPA 8260 GC-MS method because: sample contaminants (TCE) were not lost through diffusion during sampling, containing and shipping; and the high sensitivity due to use of the preconcentrator.

Multiple Applications
The sensor can also be configured to monitor well, tap or river water or other water suspected of having an undesirable or possibly illegal level of contamination. The sensor was demonstrated for testing water from the water fountain in the ORNL laboratory. CHCl3, bromodichloromethane (CHBrCl2) and dibromochloromethane (CHBr2Cl) were seen in the tested water, while no such chemicals were found in Crystal Spring bottled water.

The immediate benefit of this new technology would be reducing the cost of long-term monitoring of contaminants in groundwater. It was estimated that the cost can be reduced by up to 80% while the identification and sensitivity remain high because the ORNL sensor performs both sampling and detection in one step, eliminating shipping steps and saving money. Another benefit is fast detection. The test duty cycle is about three minutes, not monthly.

Author’s note: Contributors to the study are Jun Xu, Yongzhai Du, William Whitten, David Watson and Wei Zhang of ORNL; Erkinjun Nazarov, formerly of Sionex Corp.; and Haiyang Li of the Dalian Institute of Chemical Physics, Chinese Academy of Science. UT-Battelle manages ORNL for the Department of Energy’s Office of Science.