Summary Laser‐optical measurements and fibre optics are potentially attractive tools for applications in soil science because of their great sensitivity and selectivity and their capabilities for on‐line and in situ analysis. We have investigated laser‐induced breakdown spectroscopy (LIBS) for the quantitative detection of metal ions on the surface of natural soil samples from two sites (Hohenschulen and Oderbruch, Germany). The LIBS technique allows the spatially resolved investigation of adsorption and desorption effects of ions in soil. A frequency doubled (532 nm) and Q‐switched Nd:YAG laser with a pulse duration of 8 ns is focused on the soil surface and induces a plasma. Typical power densities are 150 mJ mm−2. The plasma emission is recorded in time and spectrally resolved by a gateable optical multichannel analyser (OMA). A delay time of about 500 ns between laser pulse and OMA gate was used to resolve single atomic and ionic spectral lines from the intense and spectrally broad light that is emitted by the plasma itself. The dependency of the LIBS signal of a single spectral line on the amount of water in the sample is investigated in detail. The results indicate that quenching of water in the plasma plume reduces the line intensities, while the interaction with aquatic colloids increases the intensity. The two processes compete with each other, and a non‐linear correlation between measured line intensities and the amount of water in the sample is obtained. This is verified by a simple computer simulation and has to be taken into account for the quantitative interpretation of LIBS signals, e.g. when absolute concentrations are estimated. In the present investigation natural calcium concentrations < 2 μg kg−1 were measured with the LIBS technique in the samples for the two test sites. In addition, measurements were made with dry and water‐saturated BaCl2 mixed soil samples, and no significant difference in the detection limit for barium was obtained.
Laser-induced fluorescence (LIF) spectroscopy in combination with fiber optics is shown to be a powerful tool for qualitative and quantitative diagnostics of environmental pollutants in water and soil. Timeintegrated data accumulation of the LIF signals in early and late time windows with respect to the excitation pulse simplifies the method so that it becomes attractive for practical applications. Results from field measurements are reported, as oil contaminations under a gas station and in an industrial sewer system are investigated. A KrF-excimer laser and a hydrogen Raman shifter can be applied for multiwavelength excitation. This allows a discrimination between benzene, toluene, xylene, and ethylbenzene aromatics and polycyclic aromatic hydrocarbon molecules in the samples under investigation. For a rough theoretical approach, a computer simulation is developed to describe the experimental results.
Time-resolved laser-induced fluorescence spectroscopy and fiber optics are shown to be promising tools for the detection of environmental pollutants in water and soil. Time-integrated data accumulation of fluorescence intensities in an "early" and in a "late" time window with respect to the excitation pulse simplifies the method in such a way that it becomes very attractive for practical applications. Results from field measurements are reported while on-line oil concentrations in an industrial oil separator are monitored for process control. For UV laser excitation at 337 nm and recording LIF signals at 400 nm, typical detection limits of the present setup are 0.5 mg of engine oil/L in water and 5 mg of engine oil/kg in soil. The selectivity of the method can be improved significantly when a multiwavelength laser excitation in the ultraviolet spectral range between 240 and 360 nm is applied. Thus, a separation between different classes of aromatic components in petroleum products is possible.
Time-resolved laser-induced fluorescence spectroscopy and fiber optics are applied for the detection of aromatic hydrocarbons in oil contaminated water and soil samples. Time-integrated data accumulation of fluorescence intensities in an "early" and in a "late" time window with respect to the exciting laser pulse simplifies the method in such a way that it becomes very attractive for practical applications. For ultraviolet laser excitation at 337 nm and recording fluorescence signals at 400 nm, typical detection limits of the present set-up are 0.5 mg engine oil/L in water and 5 mg engine oil/kg in soil. A discrimination between BTXE-aromatics and PAHs in oil polluted soil or water samples is possible, when more than one laser wavelength in the ultraviolet is used for the excitation. The possibilities for a thermal discrimination between different aromatics are also discussed.
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