Isotopomeric Peak Assignment for N₂O in Cross-Labeling Experiments by Fiber-Enhanced Raman Multigas Spectroscopy

Human intervention in nature, especially fertilization, greatly increased the amount of N2O emission. While nitrogen fertilizer is used to improve nitrogen availability and thus plant growth, one negative side effect is the increased emission of N2O. Successful regulation and optimization strategies require detailed knowledge of the processes producing N2O in soil. Nitrification and denitrification, the main processes responsible for N2O emissions, can be differentiated using isotopic analysis of N2O. The interplay between these processes is complex, and studies to unravel the different contributions require isotopic cross-labeling and analytical techniques that enable tracking of the labeled compounds. Fiber-enhanced Raman spectroscopy (FERS) was exploited for sensitive quantification of N2O isotopomers alongside N-2, O-2, and CO2 in multigas compositions and in cross-labeling experiments. FERS enabled the selective and sensitive detection of specific molecular vibrations that could be assigned to various isotopomer peaks. The isotopomers (NNO)-N-14-N-15-O-16 (2177 cm(-1)) and (NNO)-N-15-N-14-O-16 (2202 cm(-1)) could be clearly distinguished, allowing site-specific measurements. Also, isotopomers containing different oxygen isotopes, such as (NNO)-N-14-N-14-O-17, (NNO)-N-14-N-14-O-18, (NNO)-N-15-N-15-O-16, and (NNO)-N-15-N-14-O-18 could be identified. A cross-labeling showed the capability of FERS to disentangle the contributions of nitrification and denitrification to the total N2O fluxes while quantifying the total sample headspace composition. Overall, the presented results indicate the potential of FERS for isotopic studies of N2O, which could provide a deeper understanding of the different pathways of the nitrogen cycle.