Detection of hydrocarbons in water by MIR evanescent-wave spectroscopy with flattened silver halide fibers

IR transmitting AgClxBr1-x fibers were used as internal reflection elements for the spectroscopic detection of hydrocarbons that are dissolved in wate r in an attenuated total reflection (ATR) type of setup. The fibers were pa rtly flattened by pressing and then coated with an analyte-enriching organi c polymer in order to increase the sensor sensitivity. A portable tunable d iode laser spectrometer served as a spectrometric unit. The dependence of t he sensitivity on coupling conditions was investigated and lead to an optim ized coupling. It is shown that the sensitivity of the flattened waveguide with a thickness of 170 mum can be enhanced by a factor of 5 compared to a fiber with 900 mum diameter. Aqueous toluene solutions with concentrations in the parts-per-million range were used to characterize the sensor system that showed a linear response with regard to analyte concentration. The noi se equivalent concentration (NEC) was on the order of 1 ppm.