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Laser Technique Reveals Metal Contamination in Soil

Photonics Spectra
May 1997
Kate Leggett

Commercial ships require routine repainting, and when the old paint is sandblasted from the anchors and hulls, lead and other metals contaminate the soil. Environmental assessment of these sites typically involves collecting soil samples at various depths, followed by chemical analysis at a lab.
Dr. Stephen Lieberman's group at the Naval Command, Control and Ocean Surveillance Center in San Diego proposed a faster, less costly detection method using laser-induced breakdown spectroscopy (LIBS). In LIBS measurement, the high-power infrared emission from a pulsed Nd:YAG laser is focused to a small spot and generates powers in excess of several gigawatts per square centimeter. It rapidly heats, vaporizes and ionizes a small amount of the sample, making possible spectroscopic analysis of the subsequent laser-induced plasma emission.
The time-resolved emission of this plasma is initially characterized by a broadband continuum. As the plasma cools, this broadband background decays, leaving only longer-lived, spontaneous emission lines at discrete wavelengths that characterize the elemental species present within the plasma. Time-gated detection techniques allow isolation of the atomic emissions from the broadband background.
With LIBS, background interference is minimal, and because all species in the excitation volume enter the plasma together, simultaneous multicomponent analysis is possible. Spectral results are immediately available, so analysis of the results and site delineation decisions can be carried out in real time. In addition, heavy metals such as lead and chromium can be detected at sub-parts-per-million levels, making it suitable for an in situ probe.
Because of the high power densities required to initiate the spark, this technique usually involves the delivery of the light through air. For this application, the group leader proposed a single fused-silica fiber to deliver light to the soil samples and to route the emitted light from the plasma back to an above-ground detector. The group would route the fiber through a hole in a cone penetrometer -- a probe that is hydraulically pressed into the ground more than 100 ft -- to deliver the light directly to the site in question.
To test the theory, the team deployed the prototype fiber optic LIBS (FOLIBS) to measure the spark initiation threshold needed for different soils, including standard sea sand and soils from locales in California and Arizona. The group observed that the threshold was 0.5 GW/cm2 regardless of soil type.
The researchers calibrated the FOLIBS instrumentation using chromium- and lead-contaminated sand to determine the minimum metal concentration that could be detected, which fell in the 0.5-ppm range -- below the Environmental Protection Agency's site screening level. The instrument was then used on site at the 32nd Street naval shipyard in San Diego.
With initial results more promising than anticipated, efforts are in place to validate the technology not only in California, but throughout the US. The group would like to see FOLIBS put to use detecting concentrations of heavy metals in the soil of firing ranges, battery dumps, industrial waste treatment plants and old mine sites.

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