Cavity-enhanced raman gas spectrometry of biogenic gases
The application of an innovative design of a miniaturized and extremely robust gas sensor based on cavity enhanced Raman spectrometry is introduced. The versatility of the new sensor allows online, simultaneous identification and quantification of various biogenic gases and volatiles in a wide range of concentrations and without cross-sensitivity. The different experiments in which this instrument has been applied demonstrate its stability for a range of important environmental research questions. In particular, the ability to simultaneously measure O2 and N2 besides other gases like CO2 and the ability of real-time, in-situ measurements using isotope labels with a comparable inexpensive sensor is unique. The application of stable isotope tracers, such as used in the examples with 13C, provides a powerful tool for understanding metabolic pathways, using low flow rates and small sample volumes in closed systems. All this is possible because Raman spectra are based on the characteristic inherent molecular vibrations of the measured gases such that the measurement does not influence or disturb the experimental site by the consumption of gases, labels or transducers. Fast analysis of gas exchange processes is possible, since time consuming sampling and sample preparation steps are not necessary. Also long-term gas observations can be performed due to stable linear calibration with no saturation and aging effects. It was shown that Raman gas spectrometry provides versatility, while at the same time portability is maintained by miniaturized instrument components and low power consumption. The comparably simple calibration and the straightforward handling allow focusing on the actual experiment rather than on the gas quantification. The experiments demonstrated that Raman spectrometry enables the simultaneous investigation of various gases produced and/or consumed by plants and microbes in controlled laboratory studies and in complex ecosystems. Miniaturization and low power consumption constitute a high potential for on-field measurement campaigns. This method is urgently needed and will contribute in the future to the elucidation of complex and strongly interdependent environmental processes.