Fiber Optic Probe Identifies Hazardous Contaminants

Barbara Grant

WASHINGTON -- Hazardous waste site assessment and remediation are receiving increased attention these days, and a new development from the US Naval Research Laboratory (NRL) should enhance these efforts.
A fiber optic infrared reflectance probe that can detect trace amounts of toxic substances in subsurface samples has been developed by a team of NRL researchers with support from the US Army Environmental Center in Aberdeen, Md. The probe, which can detect hydrocarbon concentrations as low as 100 parts per million, will undergo testing later this year.
According to Frank Bucholtz, a member of the NRL team, this device is a significant improvement over conventional methods of determining concentrations of chlorinated solvents such as trichloroethylene (TCE). "Current techniques use laser-induced fluorescence to identify contaminants, but these do not work well on nonaromatic hydrocarbons such as TCE, whose fluorescence is very weak," he said.
The fiber optic reflectance probe is placed inside a cone penetrometer for subsurface sampling. The penetrometer, a hollow 23/4-in.-diameter tube with a point at one end, is driven into the ground and measurements are made at various depths. The probe works by directing blackbody radiation from a nichrome wire source through a window in the penetrometer onto the sample. The instrument collects, collimates and then focuses the sample's diffuse reflectance onto a 12-m IR-transmitting chalcogenide fiber that telemeters the signal to the surface. A conventional Fourier transform infrared spectrometer (FTIR) analyzes the signal, and a computer produces a three-dimensional map of the contaminant area and depth in real-time.
The spectral region between 2.5 and 4.0 µm has been of greatest interest for contaminant assessment, said Bucholtz. In this portion of the spectrum, many organic contaminants have strong absorption features, and FTIR techniques can identify multiple compounds. The probe can also determine soil type and water content, he added.