A Compact Multiplexed Hazardous Gas Sensing Platform for Real-Time Monitoring in Mining Scenarios

The determination of hazardous gases in confined environments, such as underground mines, is essential for ensuring suitable occupational health conditions. Although gas chromatography coupled with mass spectrometry (GC-MS) is the gold-standard analytical technique for gas analysis, its application is not feasible in this context due to the bulkiness of the instrumentation and the inability to provide real-time responses. Alternative low-cost sensing techniques, such as metal oxide semiconductors (MOXs), are limited by low molecular selectivity, susceptibility to humidity interference, and long-term instability. Herein, we propose an integrated and compact analytical platform that combines multiple quantum-cascade lasers (QCLs) with substrate-integrated hollow waveguides (iHWGs) acting as miniaturized gas cells for the real-time quantification of hazardous gases, including SO₂, H₂S, CO₂, CH₄, NO, and NO₂, in confined workspaces such as underground mines. Each QCL was selected and tuned to match the absorbance profile of a particular target analyte, with the exception of H₂S, which was converted to SO₂ via UV irradiation prior to quantification. An Arduino-based system facilitates signal acquisition and processing, enabling rapid data interpretation and wireless communication with external devices. At optimized conditions, the system enables the quantification of all target gases near their permissible exposure limits (PELs) in underground mines, with limits of detection of 0.3, 7, 100, 100, 5, and 3 ppmv for SO₂, H₂S, CO₂, CH₄, NO, and NO₂, respectively. The entire system has a footprint of 470 × 320 × 100 mm, which is compatible with real-world deployment scenarios in underground mines. This advanced sensing platform provides real-time monitoring of toxic gases in confined environments with high molecular selectivity and suitable sensitivity. Designed for harsh conditions, it can operate reliably at high humidity, dust, vibrations, and electromagnetic interferences.