CHARACTERIZATION OF A FIBEROPTIC EVANESCENT-WAVE ABSORBENCY SENSOR FOR NONPOLAR ORGANIC-COMPOUNDS

A fiber-optic evanescent field absorbance sensor (EFAS) is described, for which the sensing element consists of a commercially available sil icone-clad quartz glass fiber, coiled on a Teflon(R) support. The poly dimethylsiloxane cladding fulfills various functions. It protects the brittle fiber core against fracture induced by mechanical stress. More over, as a lower-refractive-index medium, it causes total reflection i n the fiber and acts as a hydrophobic membrane that enriches nonpolar organic compounds, whereas polar species like water cannot penetrate. Coupled to an NIR spectrometer, the sensor has a potential for remote in situ measurements of organic pollutants in drainage waters originat ing from contaminated areas. In this study aqueous solutions of typica l drainage-water contaminants like dichloromethane, chloroform, and tr ichloroethylene were measured in the 900-2100 nm spectral range. The i nfluence of refractive index, fiber length and diameter, bend radius, polysiloxane swelling, and ambient temperature on the sensor signal is described and qualitatively compared with theoretical predictions. Ki netics measurements are presented, which allow explanation of the diff usion mechanism of CHCl3 enrichment in the polysiloxane cladding. The data show that the rate-determining step for penetration of this subst ance into the sensor polymer layer can be described mainly by film dif fusion through the aqueous boundary layer. In most cases no remarkable influence of gel diffusion in the polysiloxane membrane was observed.