FIBEROPTIC EVANESCENT-WAVE SENSOR FOR IN-SITU DETERMINATION OF NONPOLAR ORGANIC-COMPOUNDS IN WATER

The quantitative determination of chlorinated hydrocarbons and aromatic substances in water by a fiber-optic near-infrared (NIR) evanescent wave absorbance sensor, whose sensing element consists of a silicone-clad quartz glass fiber coiled on a stainless steel/Teflon(R) support, is described. The silicone cladding acts as a hydrophobic membrane, that enriches non-polar organic species, which absorb energy from the evanescent wave penetrating into the cladding. NIR evanescent wave spectra of aqueous solutions of dichloromethane, trichloroethene and chlorobenzene are compared with conventional transmission spectra of the pure compounds. Calibration functions for different aromatic substances are presented. The limits of detection, which are in the low ppm or even sub-ppm range are given for relevant organic species. The sensitivity for a compound depends mainly on its distribution coefficient in the water/silicone system. The uptake of the compounds in the sensor cladding is completely reversible with tau90 values ranging from 0.5 to 71 min depending on the molecular interaction of the substance and the silicone matrix. Coupled to a tungsten halogen lamp and an NIR spectrophotometer by all-silica optical fibers the sensor can be used for remote and continuous in situ monitoring of non-polar organic contaminants in drainage or industrial waste waters, with distances of up to a few hundred meters between instrument and probe.