| Summary: | Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015. === Cataloged from PDF version of thesis. === Includes bibliographical references. === The development of three optical instruments for the chemical exploration and characterization of natural waters is described. The first instrument (called LEDIF) employs a novel flowcell, 6 UV LEDs as excitation sources, a wideband lamp, and a spectrometer to measure steady state chemical fluorescence and absorbance. The instrument is packaged aboard an autonomous underwater vehicle (AUV) and demonstrates the ability to map chemical concentrations in three dimensions. The second instrument repackages the sensor components to study dissolved organic matter (DOM) in tropical Southeast Asia peatland rainforests. This instrument is optimized for low power consumption over long deployments to remote locations. Two field trials in Pontianak Indonesia with durations of two and six weeks captured peatland river fluorescence measurements at 20 minute intervals. The results show changes in DOM linked to tidally induced water level fluctuations and provide insight into the complex biogeochemical dynamics of the system. The third instrument increases the chemical sensitivity and specificity of LEDIF with the addition of fluorescence lifetime sensing capabilities. The development of this sensor for AUV deployment required the engineering of a compact, low power, high speed (GHz) data acquisition circuit board. The resulting circuit digitizes data at a rate of 1 gigasample/second and performs user customizable digital signal processing. This board is used along with a 266 nm Q-switch laser, fast photomultiplier tube (PMT), and computer controlled monochromator to build a small fluorescence lifetime instrument. The instrument is tested with solutions of low concentration pyrene to demonstrate its ability to identify small, long-lived fluorescence signals in the presence of large background fluorescence. Results indicate a pyrene limit of detection below environmentally relevant levels. The final overall instrument dimensions allows it to be packaged for future AUV deployments === by Schuyler Senft-Grupp. === Ph. D.
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