New IR Fiber-Optic Chemical Sensor for in Situ Measurements of Chlorinated Hydrocarbons in Water

In this work the development and validation of a new MIR fiber-optic physicochemical sensor system for the continuous in situ analysis of chlorinated hydrocarbons (CHCs) in water is described. This study took advantage of the selectivity and sensitivity of fiber evanescent wave spectroscopy (FEWS) and the recent development of polycrystalline silver halide fibers. Since these fibers are transparent up to 20 μm, it was possible for the first time to develop a fiber-optic sensing system for CHCs, which have their strongest absorption bands > 10 μm. The silver halide fibers were coated with low-density polyethylene (LDPE) to enrich the CHC within the evanescent wave and to exclude the IR absorbing water from the measurement. For the quantitative in situ FEWS measurements, the coated silver halide fibers were coupled to a Fourier transform infrared (FT-IR) spectrometer using an off-axis parabolic mirror and a fiber-detector coupling system. This setup enabled the simultaneous in situ detection of the most common chlorinated hydrocarbons in concentrations between 1 to 50 mg/L in water by employing a fiber sensing part only 10 cm in length. A comparative analysis of waste water samples under participation of two experienced head space-gas chromatography (HSGC) laboratories showed good agreement of this continuous sensor system with the established standard techniques. The resulting working curve for tetrachloroethylene showed a correlation coefficient of r2 = 0.968 and a relative standard deviation of 17% in the range from 1 to 10 ppm.