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 N₂, O₂, 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 ¹⁴N¹⁵N¹⁶O (2177 cm^-1) and ¹⁵N¹⁴N¹⁶O (2202 cm^-1) could be clearly distinguished, allowing site-specific measurements. Also, isotopomers containing different oxygen isotopes, such as ¹⁴N¹⁴N¹⁷O, ¹⁴N¹⁴N¹⁸O, ¹⁵N¹⁵N¹⁶O, and ¹⁵N¹⁴N¹⁸O 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.