Hahn, Tobias (2014) Untersuchungen zur Kalibrierung und Charakterisierung einer neuartigen Sauerstoffoptode in Labor und Feld. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 76 pp.

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Abstract

The formation of oxygen minimum zones (OMZ) in the ocean causes extensive changes of biogeochemical processes. Oxygen determines many chemical and biological processes in the ocean, hence, it can be considered as a characteristic marker for tracing those changes. As the result, it is necessary to quantify the concentration of dissolved oxygen. In the past, various new sensors have been developed in order to obtain data in the ocean in a spatial and continuous manner which cannot be achieved via discrete sampling or selective profiling during research cruises.
Optical oxygen sensors (optodes) based on dynamic fluorescence quenching show enormous potential for autonomous application in the ocean. A major advantage of optodes towards other sensors is their robustness, long-term stability and low power consumption. Primarily, requirements for the optimization of their time constant for the oxygen measurement are still needed. In this context, CONTROS Systems & Solution GmbH has developed a novel oxygen optode.
The main aim of this thesis was to conduct a comprehensive study about the novel optode towards its characterization and lab-based calibration as well as its performance during field experiments. The fastest response time of the optode was 3.5±0.6 s as an average over the temperature range between 2°C and 32°C. Despite an optical isolation layer, the time response is faster compared to other manufacturers’ models by a factor of three. Storage experiments over a time period of 4.5 months indicate that extreme temperatures of 4°C and 50°C, respectively, did not cause a visible drift of the novel optode. On the contrary, continuous exposure to light results in a signal loss of ϕ0 in an order of magnitude by 0.08%.
Accuracies of ± 1.1 μmol · L-1 could be achieved by two complete lab-based oxygen calibrations which were conducted within a time period of around 4 months. By
comparing both lab calibrations, a significant drift occurred over this time period. A pressure dependence of the response signal of the optode could be assumed based on fragmentary data from 8 CTD-profiles during field experiments. Differences of < 1 μmol · L-1 were observed between the optode and the reference. Moreover, two other optodes were used for measuring sea surface oxygen. Average deviations of - 0.2 μmol · L -1 between the optode and reference were found, while a drift rate of around 0.02 μmol · L-1 per day was quantified based on a total amount of around 80.000 measurements.

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