The design and fabrication of fiber optic sensors and photometers and their application in measuring aqueous ammonia

Three low-cost, solid-state fiber optic instrument designs for the analysis of aqueous ammonia in natural and process waters are presented. Two separate fiber optic chemical sensor (FOCS) designs were developed and tested. One was constructed by placing the chemical phase at the distal end of the fiber (optode FOCS), and the other was constructed by placing the chemical phase in a gap between two fibers (in-line FOCS). The dynamic responses of the two designs were evaluated relative to the chemical phase chemistry, the chemical phase volume, and the chemical phase depth (cell depth). Sensor sensitivity and response time were determined to be a function of the three chemical phase parameters. Improvements in sensitivity that increased either volume or cell depth were found to result in an increase in response time. The optode FOCS had a typical sensor response range between 0.1 mg/L and 5.0mg/L ammonia-nitrogen, and the in-line sensor had a sensor response range between 0.02 mg/L and 0.20 mg/L ammonia-nitrogen. The response rate increased with increased aqueous ammonia concentration; however, the time for the sensor to reach an equilibrium condition (response time) also increased with increased concentration. The response time was typically between 5 and 20 minutes.; The third instrument design, the fiber optic photometer (FOP), evolved from the in-line FOCS and, was shown to be an important finding of this research. The FOP was developed to have performance characteristics comparable to field spectrometers and filter colorimeters. This included a detection range between 0.01 mg/L and 1.0 mg/L ammonia-nitrogen. Additionally, the FOP was demonstrated to be capable of measuring orthophosphate in surface water with a detection range between 0.01 mg/L and 1.0 mg/L as phosphate. The FOP had the advantage of reduced size, weight and system cost relative to commercially available field spectrometers and colorimeters.; The results of this study illustrated the potential application of fiber optic sensors and photometers to the analysis and monitoring of aqueous ammonia in wastewater, surface water, and aquaculture process water. The study also demonstrated the FOP's application to the measurement of orthophosphate in surface water. A potential expansion of the measured parameters by the addition of LEDs with differing peak wavelength is recommended.