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Abstract

The amount of dissolved hydrogen sulfide is an important parameter in many environmental applications, Conventional methods for H2S detection depend on iodometric titration or spectroscopic measurements. Unfortunatley these methods are not applicable for direct measurements in natural systems. A recently described method for the on-line detection of H2S is based on quenching of fluorescence of thionine (1,2).The reaction between H2S and thionine was described as reversible photoreduction. This reaction was tested in order to design an optical microsensor for the measurement of H2S in sediments and other biological systems. We immobilised thionine in several matrices and investigated these materials with respect to response time, mechanical stability, the influence of the excitation light and the reversibility. The sensing materials were deposited on the tip of optical fibers. The measuring system for the excitation and detection of the fluorescence consisted of a yellow light emitting diode, a fiber-optic coupler and a photomultiplier. The excitation light was intensity modulated to enable measurements in ambient light. Our results indicate that the thionine based reaction scheme for H2S detection is not very suitable for use in a H2S optode due to lack of reversibility, long response times, and the need for regeneration of the sensor chemistry.