Bridging chromatographic and photoacoustic domains: A hybrid system for compositional and isotopic analysis of natural gas

Natural gas (NG) analysis plays a pivotal role in the global energy landscape, and accurate quantification of its molecular and isotopic composition is essential for extraction process optimization, emission control, and environmental monitoring. Conventional detection methods based on gas chromatography (GC) coupled with flame ionization detector (FID) or mass spectrometry (MS) offer high sensitivity but either lack of specificity or are bulky and costly. This work presents a hybrid sensing platform that integrates GC with Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) combining the temporal separation capabilities of GC with the high sensitivity and molecular selectivity of mid-infrared laser spectroscopy. This dual-domain approach enables: (i) selective identification and quantification of light alkanes in complex natural gas mixtures; (ii) sub-ppm detection of propane using a single interband cascade laser emitting at 3.367 µm; and (iii) isotopic discrimination between methane isotopologues (12CH₄ and 13CH₄) employing a quantum cascade laser operating near 7.81 µm. By overcoming spectral overlap through chromatographic separation and leveraging QEPAS’s intrinsic sensitivity, the GC–QEPAS configuration offers a high-performance alternative to GC–FID/MS for in situ compositional and isotopic gas analysis.