Development of a universal measuring instrument for quality monitoring of ultrapure water

This thesis presents the design, development, and analytical validation of two novel sensor systems for ultrapure water (UPW). In particular, it addresses the challenges involved in measuring total organic carbon (TOC) and dissolved ozone. The research is driven by the growing need for decentralized, reagent-free, and highly sensitive instruments for UPW analysis, as conventional analytical methods are often too bulky, slow, or lack the robustness required for continuous online measurement. The first part of the thesis addresses the development of a miniaturized TOC sensor based on a combined UV/O3 advanced oxidation process (AOP). In this system, a compact electrolytic ozone generator and UV-LED irradiation are integrated to directly oxidize organic contaminants within the measurement cell. The TOC concentration is determined by correlation with the change in conductivity before and after oxidation, and systematic, concentration-dependent experiments demonstrate that both the absolute increase in conductivity after 100 s of oxidation and the initial slope of the conductivity signal yield reproducible, quantifiable correlations with TOC levels ranging from 10 to 1000 ppb. These findings validate the applicability of UV/O3-based AOPs for TOC monitoring, particularly for readily oxidizable organic compounds. To evaluate oxidation efficiency, a critical parameter for the reliability of AOP-based TOC measurements, an LC-MS/MS method was developed and validated for two structurally distinct model analytes: sucrose, which is rapidly and almost completely oxidized, and caffeine, which represents slower and more complex oxidation pathways. Comparative measurements using a commercial TOC analyzer and the miniaturized system demonstrated high oxidation efficiencies for both devices (≥ 98 %), while also revealing difficulties in determination due to differences in the oxidizability of the analytes. These results highlight the challenge of TOC as a sum parameter and demonstrate that great attention must be paid to the different oxidation behaviors of substances in UPW. The second part of the thesis describes the development of an advanced potentiometric ozone sensor. This sensor was developed to overcome the challenges associated with the low conductivity of UPW and uses a three-electrode configuration, comoprising a measuring electrode made of gold or platinum, an iridium dioxide reference electrode, and a reset electrode. The integration of a Nafion® 117 membrane ensures defined proton conductivity and enables stable electrochemical measurement conditions. The reset function enables dynamic signal evaluation and restores the electrode surface in a controlled manner. Concentration-dependent measurements yielded two distinct concentration ranges. The sensor with the gold electrode shows reliable results in the low concentration range (< 50 ppb), achieving correlation coefficients up to 95 %, whereas the sensor with the platinum electrode shows superior performance at higher ozone concentrations (> 50 ppb). The obtained results from these measurements showed that accurately and continuously monitoring dissolved ozone across relevant concentrations for UPW sanitization is possible. It is conceivable that the parameters monitored here with the two sensor systems could be combined into a single measuring device. After further research and optimizations to improve the robustness of the systems, online monitoring in the semiconductor or pharmaceutical industry, and other applications using UPW could be further revolutionized.