Development and application of analytical methods based on enhanced Raman gas spectroscopy for biogeochemical process monitoring
The present work reports on novel analytical approaches and instrumentation for several biogeochemical gas monitoring applications. Exploiting Raman gas spectroscopy in combination with two signal enhancement techniques either fiber or cavity enhancement we developed quantitative methods for estimating the gas composition and exchange during soil biodegradation, biological nitrogen fixation, fruit ripening and physical leakage processes in environmental chambers. The first part of this thesis describes the fruit ripening analysis and the developed gas sensor based on fiber enhanced Raman spectroscopy for fast and non-destructive gas monitoring throughout the complete postharvest production chain of tropical produce. Analytical solutions for the other applications rely on the use of a cavity enhanced Raman gas analyzer. Linking gas diffusion theory, the tracer sulfur hexafluoride and a developed experimental protocol, we demonstrate the influence of physical gas leakage on determined gross exchange rates of biological systems and provide an analytical correction method to quantify the underlying biological signal. Within the scope of a soil biodegradation study, we developed an analytical method to follow the fate of xenobiotics after a contamination. The non-invasive gas monitoring solution we present is capable of quantifying the fraction of degraded hydrocarbons as contaminants, as well as identifying changes in respiration. Our Raman spectroscopic approach indicates the potential to elucidate the dynamics of specific enzymatic reactions and the occurrence of concomitant processes such as changes in the substrate for soil bacterial metabolism. In the last part of this thesis, we report on a novel analytical approach, which enables determination of biological nitrogen fixation rates without requiring a proxy, isotopes or an exchange of the natural ecosystem atmosphere. Common standard techniques do not support such a simple and most natural experimental design and we report on the first biological nitrogen fixation rate estimates derived by optical spectroscopy of N2. Our proposed method indicates the potential to reduce existing uncertainties in nitrogen fixation measurements and might open up a new avenue of biological nitrogen fixation research.