Mid-infrared (MIR; 3-20 μm) sensor technology is increasingly adopted in environmental analysis, atmospheric sensing, process monitoring, and biodiagnostics due to its inherent molecular specificity enabling the discrimination of molecular constituents - and in particular of volatile organics - at ppm-ppb concentration levels. A new generation of hollow waveguide (HWG) gas cells of unprecedented compact dimensions facilitating low sample volumes and high throughput suitable for broad- and narrow-band mid-infrared sensing applications is discussed along with the potential and challenges for urban chemical cartography applications. Substrate-integrated hollow waveguides (iHWGs) are layered structures providing light guiding channels integrated into a solid-state substrate material, which simultaneously serve as miniaturized gas cells. iHWGs are proven competitive – and in part superior - in performance compared to conventional leaky-mode fiber optic silica HWGs with similar absorption path length. The flexibility in device and optical design, and the wide variety of manufacturing strategies, substrate materials, access to the optical channel, and optical coating options highlight the advantages of iHWGs in terms of robustness, compactness, and cost-effectiveness. Finally, the unmatched modularity of this novel waveguide approach facilitates tailoring iHWGs to almost any kind of gas sensor technology, thereby providing adaptability to the specific demands of a wide range of application scenarios. The analytical capabilities of such devices for advanced MIR gas sensing applications are discussed for a variety of gaseous constituents including carbon dioxide (e.g., simultaneous 12CO2/13CO2 detection), cyclopropane, isobutylene, and methane, as well as constituents relevant in breath diagnostics (e.g., isoprene).[3,4] Furthermore, environmental monitoring applications (e.g., detecting ozone), and hazardous gas sensing (e.g., detecting H2S)[6,7] are highlighted. The potential for miniaturization of IR sensor technology based on advanced light sources such as tunable quantum cascade lasers (QCLs) in combination with innovative waveguide structures such as iHWGs simultaneously serving as ultra-compact gas cells paves the way for this measurement technology toward applications in urban chemical cartography scenarios using e.g., drone-mounted systems for plume tracking and source targeting/identification.[8-10]
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