Ion Mobility Spectrometer (IMS) Sensor Project

PIs: Dr. Molly Gribb, Dr. Sin Ming Loo (BSU); Dr. Herb Hill, (WSU)

Research Staff: Dick Sevier, Kevin Ryan and Dr. Ray Carter, (BSU); Dr. Prabha Dwivedi, (WSU)

Students: Jeremy Aulbach, Brandon Duncan, Kevin Hoffman and Kyle Schwab (BSU)

EPA Awards X-97031101-0 and X-97031102-0

The potential for contamination of groundwater from subsurface chemical sources poses a risk to the nation’s water supply. Monitoring and remediating contaminated sites has proven to be both technically and financially challenging. Because accessing the subsurface is challenging, it is difficult for site managers to proactively develop remediation plans. Inexpensive, minimally invasive real-time instrumentation and sensor systems for detecting and measuring environmental contaminants are needed for characterizing contaminated sites during remediation, and for long-term monitoring at waste sites where remediation is not possible, or economically feasible.

The goal of this project is to develop and fabricate a probe system that utilizes a miniature sensor based on ion mobility spectrometry (IMS) that can yield real-time data about the amount and identity of gaseous volatile organic compounds (VOCs) present in the subsurface. Figure 1 shows a schematic of our IMS system as designed for subsurface use.

Ion mobility spectrometry is used to separate and quantify ions based on the drift time of ions at ambient pressure under the influence of an electric field against a counter-flowing neutral drift gas. Ion mobility spectrometry has been used successfully for the real time detection of volatile chemical warfare agents, explosive vapors, and controlled substances. This research represents a new application of IMS as a method for identifying and measuring VOCs in subsurface soils. Initially, our goal is to measure these compounds in the unsaturated environments. With modification, the probe could also be used to detect contaminants in saturated zones including groundwater.

The compact IMS sensor at the heart of our probe has the resolving capabilities of much larger systems. Our IMS design is based on the use of Macor, a readily available machineable ceramic, to support the needs of our expanding project. Details of the ceramic IMS are shown in Figure 3. The probe internals also include a sampling module to introduce the sample to the IMS, signal amplifier and high voltage systems to support the IMS operation and data collection. A stand-alone uphole system was also developed to provide power to the probe, control the probes operation and to safely deploy the probe below ground.

A major milestone in our project was reached in July 2006 when the probe was deployed about 20 feet below ground surface at a site in southwest Idaho and was successful in detecting contaminants. This event marked the first time an IMS was deployed subsurface. (Fig.4).

Current work on the project is focused on ensuring the probe will work in real world conditions under simulated field conditions. This involves testing in a soil column to investigate the effects of varying environmental and soil conditions on the overall sensor system performance, and to identify experimental parameters that will affect in-situ performance. Results of these tests will allow us to estimate field performance and make any needed final design changes. Deployment of the IMS sensor system will in the field will be continued to correlate field measurements with those made in the lab.