Ultrasensitive\nRaman Gas Spectroscopy for Dinitrogen\nSensing at the Parts-per-Billion Level

Sensing small changes in the concentration of dinitrogen\n(N<sub>2</sub>) is a difficult analytical task. As N<sub>2</sub>-sensing\nis crucial for nitrogen cycle research in general and studies of denitrification\nin particular, researchers went to great lengths to develop techniques\nlike the gas-flow-soil-core method, which achieves a precision of\n200 ppb at 20 ppm of N<sub>2</sub>. Here, we present a Raman gas spectroscopic\ntechnique based on high pressure, high laser power, and high-NA signal\ncollection, which achieves a limit of detection (LoD) of 59 ppb N<sub>2</sub> and a precision of 27 ppb at 10 ppm of N<sub>2</sub>. This\nimproves the lowest LoD for N<sub>2</sub> reported for Raman gas spectroscopy\nby 2 orders of magnitude. Furthermore, this constitutes an improvement\nin precision by 1 order of magnitude compared to the GC-MS-based gas-flow-soil-core\nmethod currently established in denitrification research. We show\nthat the presented setup is both stable and tight enough to ensure\nhighly sensitive, precise, and repeatable measurements of N<sub>2</sub>. As Raman gas spectroscopy is a versatile and comprehensive method,\nthe described technique could be easily expanded to other relevant\ngases like nitrous oxide or to simultaneous multigas sensing. In summary,\nour method offers possibilities for N<sub>2</sub>-sensing and could\neventually enable denitrification studies with increased sensitivity\nand a larger scope.