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.