Abstract

Time-integrated laser-induced fluorescence (LIF) spectroscopy and fiber optics provide a powerful tool for the diagnostics of various environmental pollutants in different media [1]. The method can easily be adapted for practical applications, e.g. in the trace analysis of oil contaminations in the soil or in the water. Since petroleum products are complicated mixtures of poly- and monocyclic aromatic hydrocarbons their time-resolved fluorescence spectra are described by a sum of up to three exponentials with characteristic intensities a_i and decay times τ_i. For excitation in the uv (337 nm) and observation of the fluorescence at 400 nm decay times τ_p ≥ 50 ns are obtained for pure oil samples. However, for the same conditions most water and soil samples show very short fluorescence decay times with typical τ_m≤10 ns [2]. Therefore, time-integrated detection of "early" and "late" fluorescence with respect to the laser pulse can be used very efficient for qualitative and quantitative diagnostics. On the basis of experimental results a computer simulation is developed to show the dependence of the time evolution of the LIF-signals when the concentration of contaminations with long decay times (e.g. polycyclic aromatic hydrocarbons (PAH)) is changed in a polluted water sample [3]. In fig. 1a results of this computer simulation are shown. The left curve corresponds to the fluorescence from dissolved organic matter (DOM) which is always present in natural water. For times t > 60 ns after the laser pulse no fluorescence is obtained. However, when the number density of PAH in the water sample is increased a significant fluorescence signal can be measured for times t>60 ns. Then, time- integrated detection of fluorescence in the time gates 0 to 100 ns (I₁) and 100 to 200 ns (I₂) with respect to the laser pulse is very efficient method to measure even very low concentrations of PAH in the oil water mixtures. The ratio I₂/I₁ can be used for qualitative and quantitative diagnostics (refer to fig. 1b).

© 1994 Optical Society of America