Simultaneous dual-gas measurement using hybrid photoacoustic spectroscopy

In this study, we have developed a hybrid photoacoustic spectroscopy (PAS) system for simultaneous measurement of nitrogen dioxide (NO2) and methane (CH4) based on a dual-photoacoustic (PA) generation mechanism. The system employs two PA generation mechanisms, specifically direct absorption spectroscopy (DAS) based on a fixed wavelength light source and wavelength modulation spectroscopy (WMS) using a tunable laser source, to enable simultaneous detection of NO2 and CH4, respectively. A semiconductor laser emitting at 453 nm with a bandwidth of 8.8 nm allowed the detection of NO2 with broadband absorption, while the wavelength of a distributed feedback (DFB) laser was scanned in the range from 1653.3 nm to 1654.0 nm to cover the narrow absorption line of CH4 at 1653.7 nm. During a scanning cycle, the PA signal of NO2 generated from DAS provides a background PA signal, while the PA signal of CH4 from WMS gives a featured PA signal. The hybrid PAS system exhibited low detection limit of 8 ppb and 0.24 ppm for NO2 and CH4 (integrating time of 10 s), respectively. The developed PAS system principally investigated photoacoustic signal as background and featured signal for simultaneous measurement of multi-gas with different types of absorption lines. This approach provides a new scheme to combine different PA generation mechanisms for simultaneous multi-gas detection.