Photoacoustic simultaneous detection of multiple trace gases for industrial park application

Abstract

The determination of toxic or harmful gases in industrial parks is a challenge to monitoring exhaust contaminants due to the features of complex compositions and ubiquity. Blackbody sources play an important role in simultaneously detecting the multiple gas species in the presence of cross-interfering absorption lines due to their effective ultra-wide wavelength range. Nevertheless, the problem of lower intensity per wavelength and less stability persists as an obstacle for highly sensitive trace gas detection. In this study, a dual optical path (DOP) enhanced differential photoacoustic and spectral detection mode is developed for simultaneously detecting the multiple toxic or harmful gas through augmenting the weak effective absorption signals and suppressing the spurious coherent background noise. Two identical T-type photoacoustic resonators are introduced to enable the differential mode. Neverthelss, the pure optical approach cannot distinguish the absorption characteristics of acetylene (C<sub>2</sub>H<sub>2</sub>) with volume fraction 5 × 10<sup>–5</sup> even with the DOP enhancement, whereas emerging peaks in the differential photoacoustic (PA) mode reveal the capability of PA spectroscopy to suppress coherent noise. The results demonstrate that the differential PA signal is improved by 1.91 times that obtained by the DOP design. Methane (NH<sub>3</sub>), acetylene (C<sub>2</sub>H<sub>2</sub>) and carbon dioxide (CO<sub>2</sub>) are used to verify the performance of this DOP enhanced differential PA gas sensor, and the volume fraction of the sensitivity is found to be 7.25 × 10<sup>–7</sup> for CO<sub>2</sub>, 1.84 × 10<sup>–6</sup> for C<sub>2</sub>H<sub>2</sub>, and 1.43 × 10<sup>–6</sup> for NH<sub>3</sub> at standard temperature and pressure, which is an order of magnitude higher than the original single mode PA value. Linear PA amplitude responses ranging from 0 to 3 × 10<sup>–3</sup> in volume fraction with respect to the three target gases are observed, and the correction coefficients are all greater than 0.995. The DOP enhanced differential PA detection mode compensates for the weakness of the limited sensitivity associated with broadband spectroscopic methods based on blackbody radiator. Thus, the broadband DOP enhanced differential photoacoustic modality is demonstrated to be an effective approach to simultaneous, highly sensitive and selective detection of multiple trace gases.