In-situ fluorescence measurements of aromatic organic ground water contaminants do not always agree with gas chromatographic methods. Dissolved oxygen quenching of fluorescence may be an interferant in field measurements. Two standard fluorescent aromatics, quinine sulfate and naphthalene, were evaluated in this study. Over the range of dissolved oxygen concentrations expected to be encountered in the field, no effects of oxygen quenching on fluorescence of these compounds was observed. Quenching of quinine sulfate fluorescence by sodium chloride was observed using this system. Sodium chloride quenching was shown to follow the Stern-Volmer relation.
Prior work has reported on the usefulness of fiber optic sensors in the detection of dissolved aromatic organic ground water contaminants such as the benzene, toluene, ethylbenzene, and xylenes (BTEX) fraction of petroleum fuels. Our device is a laser fluorimeter using fiber optic sensors for in situ measurements. Fluorescence intensity and lifetime can be measured at any wavelength in order to compare concentrations and discriminate certain compounds. Our instrument configuration uses a pulsed Nd:YAG laser twice frequency doubled to provide 266 nm excitation light. Excitation light, and subsequent induced fluorescence, are carried to the location of interest by a pair of 600 micron core fused silica optical fibers. Fluorescence is measured using a photomultiplier (PMT). PMT output goes either into a high-speed oscilloscope for fluorescence lifetime measurements or into a gated integrator for fluorescence intensity measurements. The same system can be used to detect aromatic contaminants in the vapor phase. Phenol, toluene, and xylene have been tested in the vapor phase. Concentrations below 10 micrograms phenol per liter air are detectable. In the vapor phase, the water Raman line seen around 295 nm in aqueous solution is insignificant, allowing a greater wavelength range to be scanned by the detector. Fluorescence spectra, fluorescence lifetimes, and fluorescence versus concentration information are presented. Applications of this approach to vadose zone ground water monitoring are discussed.
Prior work has reported on the usefulness of fiber optic sensors in detection of aromatic organic ground water contaminants such as the benzene toluene ethylbenzene and xylenes (BTEX) fraction of petroleum fuels. Our device is essentially a laser fluorimeter using fiber optic sensors for in-situ measurements. Both field and laboratory work have exhibited limits in the dynamic response range of fluorescence signal versus concentration when excitation occurs in the ultraviolet (266 nm). Potential causes of the observed shallow fluorescence versus concentration response include self-absorption at high concentration and stray light or electronic noise at low concentrations. The observed wide dynamic range resulting from visible excitation (532 nm) is used as a basis for comparison with UV performance. Selfabsorption phenomena are evident from the data at high contaminant concentration but practical applications more often are concerned with low concentrations. Lower limits of detection observed in UV excitation experiments are evaluated as to the possible sources of stray light including fiber luminescence coupling between the excitation and collection fibers in the sensor elastic scattering and reflection signals in the excitation module and spectral impurities in the laser excitation light from the laser flashlamp. Coupling and fiber luminescence in the sensor have been evaluated and resolved the excitation module has been redesigned to reduce potential scattering and the laser source continues to be investigated. 1 . LASER-INDUCED FLUORESCENCE TECHNIQUE Our research group has been
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