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An optical fiber chemical sensor for detecting/monitoring trace ammonia in high-temperature gas streams has been developed. This sensor uses a CuCl2-doped porous silica optical fiber, prepared via a previously reported sol-gel process, as a transducer. Trace ammonia in a gas sample diffuses into the porous fiber to react with the doped agent to form a Cu2+-ammonia complex. The concentration of the Cu2+ -ammonia complex inside the porous silica optical fiber is proportional to ammonia concentration in the gas sample, to which the sensing porous silica fiber is exposed. Therefore, ammonia concentration in the gas sample can be detected through detecting the optical absorption signal of the formed Cu2+-ammonia complex inside the fiber by using a fiber-optic UV/V is absorption spectrometric method. This sensor can be used to reversibly monitor trace ammonia in a gas sample at an elevated temperature up to 450degC in the tested range. A detection limit of 0.24 ppmv ammonia in an air gas sample was achieved when the sensor was tested at a temperature of 450degC.