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Combined Spectroscopies Simplify Oil Detection

Photonics Spectra
Sep 1999
Jörg Schwartz

Mineral oils and fuels keep the world going, but they may also severely contaminate the ground and water. Laser-induced fluorescence spectroscopy has become a reliable tool for detecting such contamination, particularly for in situ soil screening. Now researchers at the Universität Erlangen-Nürnberg have improved this process with the introduction of a relatively simple but effective calibration method.

Backscattered light from light sand is much more intense than that from dark soil. This has been a considerable problem for in situ screening using laser-induced fluorescence because it produces a significantly variable fluorescence signal, noted Matthias Lemke and Lars Schober, scientists involved in the study. But by incorporating diffuse-reflectance spectroscopy into the technique, the researchers have been able to compensate for variances in soil types and to simplify calibration.

Laser-induced fluorescence uses ultraviolet emission (typically a frequency-quadrupled Nd:YAG) to excite polycyclic aromatic components in various soil samples. Diesel fuel -- a typical contaminant -- produces a broad spectrum with a peak at 360 nm. The intensity of this fluorescence is proportional to the degree of contamination, with a correction factor depending on the type of ground material.

Straightforward calibration

The researchers applied diffuse-reflectance spectroscopy and found a linear relationship between the soil reflectance at 360 nm and the corresponding laser-induced fluorescence correction factor. This enables researchers to measure the absolute concentrations of contaminants from laser-induced fluorescence measurements with simultaneous reflection analysis. "Due to this correlation, we were able to find a very straightforward calibration method," Lemke said.

The US Environmental Protection Agency has qualified the use of the method for qualitative and semi-quantitative measurements for pedospheric applications, according to Lemke. The detection limit is typically below 200 ppm, depending on the soil. Regulation of ground contamination starts at values of 1000 ppm (at which point closer investigation is recommended), and remediation is recommended for levels above 5000 ppm. A mobile measurement system using the new method is being developed by Optimare GmbH, a manufacturer in Wilhelmshaven.

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