A Fiber-Optic FT-NIR Evanescent Field Absorbance Sensor

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Abstract
A polymer-clad fiber optic is used as an evanescent field absorbance sensor for solvents that penetrate into the polymer cladding. The sensor is coupled to an FT-NIR spectrometer for spectral measurements from 1.0 to 2.2 μm. A 400- μm core fiber optic, 1.5 m in length, was coiled with a 1.5-cm radius on a Teflon® support. The coiled sensor was then immersed in various nonpolar organic liquids which partitioned into the hydrophobic polysiloxane cladding. The evanescent-field near-infrared spectra of pure hexane, chloroform, and carbon tetrachloride are shown along with ethanol in chloroform. Various amounts of chloroform in carbon tetrachloride and toluene in cyclohexane were used to test the quantitative response. This sensor data are compared with data from a conventional NIR spectrometer using principal component regression.