Isotopomeric Peak Assignment for NO in Cross-Labeling Experiments by Fiber-Enhanced Raman Multigas Spectroscopy.

Human intervention in nature, especially fertilization, greatly increased the amount of NO emission. While nitrogen fertilizer is used to improve nitrogen availability and thus plant growth, one negative side effect is the increased emission of NO. Successful regulation and optimization strategies require detailed knowledge of the processes producing NO in soil. Nitrification and denitrification, the main processes responsible for NO emissions, can be differentiated using isotopic analysis of NO. 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 NO isotopomers alongside N, O, and CO 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 (2177 cm) and NNO (2202 cm) could be clearly distinguished, allowing site-specific measurements. Also, isotopomers containing different oxygen isotopes, such as NNO, NNO, NNO, and NNO could be identified. A cross-labeling showed the capability of FERS to disentangle the contributions of nitrification and denitrification to the total NO fluxes while quantifying the total sample headspace composition. Overall, the presented results indicate the potential of FERS for isotopic studies of NO, which could provide a deeper understanding of the different pathways of the nitrogen cycle.