Characterization of a Fiber-Optic Evanescent Wave Absorbance Sensor for Nonpolar Organic Compounds

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Abstract
A fiber-optic evanescent field absorbance sensor (EFAS) is described, for which the sensing element consists of a commercially available silicone-clad quartz glass fiber, coiled on a Teflon® support. The polydimethylsiloxane cladding fulfills various functions. It protects the brittle fiber core against fracture induced by mechanical stress. Moreover, as a lower-refractive-index medium, it causes total reflection in 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 originating from contaminated areas. In this study aqueous solutions of typical drainage-water contaminants like dichloromethane, chloroform, and trichloroethylene were measured in the 900–2100 nm spectral range. The influence 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. Kinetics measurements are presented, which allow explanation of the diffusion mechanism of CHC13 enrichment in the polysiloxane cladding. The data show that the rate-determining step for penetration of this substance into the sensor polymer layer can be described mainly by film diffusion through the aqueous boundary layer. In most cases no remarkable influence of gel diffusion in the polysiloxane membrane was observed.