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

The quantitative determination of chlorinated hydrocarbons and aromati c substances in water by a fiber-optic near-infrared (NIR) evanescent wave absorbance sensor, whose sensing element consists of a silicone-c lad quartz glass fiber coiled on a stainless steel/Teflon(R) support, is described. The silicone cladding acts as a hydrophobic membrane, th at enriches non-polar organic species, which absorb energy from the ev anescent wave penetrating into the cladding. NIR evanescent wave spect ra of aqueous solutions of dichloromethane, trichloroethene and chloro benzene are compared with conventional transmission spectra of the pur e compounds. Calibration functions for different aromatic substances a re presented. The limits of detection, which are in the low ppm or eve n sub-ppm range are given for relevant organic species. The sensitivit y for a compound depends mainly on its distribution coefficient in the water/silicone system. The uptake of the compounds in the sensor clad ding is completely reversible with tau90 values ranging from 0.5 to 71 min depending on the molecular interaction of the substance and the s ilicone matrix. Coupled to a tungsten halogen lamp and an NIR spectrop hotometer by all-silica optical fibers the sensor can be used for remo te 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.