IR transmitting AgCl<sub>x</sub>Br<sub>1-x</sub> fibers were used as internal reflection elements for the spectroscopic detection of hydrocarbons that are dissolved in water in an attenuated total reflection (ATR) type of setup. The fibers were partly flattened by pressing and then coated with an analyte-enriching organic polymer in order to increase the sensor sensitivity. A portable tunable diode laser spectrometer served as a spectrometric unit. The dependence of the sensitivity on coupling conditions was investigated and lead to an optimized coupling. It is shown that the sensitivity of the flattened waveguide with a thickness of 170 μm can be enhanced by a factor of 5 compared to a fiber with 900 μm 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 noise equivalent concentration (NEC) was on the order of 1 ppm.
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