Ultrasensitive Raman Gas Spectroscopy for Dinitrogen Sensing at the Parts-per-Billion Level

Sensing small changes in the concentration of dinitrogen (N₂) is a difficult analytical task. As N₂-sensing is crucial for nitrogen cycle research in general and studies of denitrification in particular, researchers went to great lengths to develop techniques like the gas-flow-soil-core method, which achieves a precision of 200 ppb at 20 ppm of N₂. Here, we present a Raman gas spectroscopic technique based on high pressure, high laser power, and high-NA signal collection, which achieves a limit of detection (LoD) of 59 ppb N₂ and a precision of 27 ppb at 10 ppm of N₂. This improves the lowest LoD for N₂ reported for Raman gas spectroscopy by 2 orders of magnitude. Furthermore, this constitutes an improvement in precision by 1 order of magnitude compared to the GC-MS-based gas-flow-soil-core method currently established in denitrification research. We show that the presented setup is both stable and tight enough to ensure highly sensitive, precise, and repeatable measurements of N₂. As Raman gas spectroscopy is a versatile and comprehensive method, the described technique could be easily expanded to other relevant gases like nitrous oxide or to simultaneous multigas sensing. In summary, our method offers possibilities for N₂-sensing and could eventually enable denitrification studies with increased sensitivity and a larger scope.