Multi-gas quartz-enhanced photoacoustic sensor for environmental monitoring exploiting a Vernier effect-based quantum cascade laser

Abstract

We report on a gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) able to detect multiple gas species for environmental monitoring applications, by exploiting a Vernier effect-based quantum cascade laser as the excitation source. The device emission spectrum consists of ten separated emission clusters covering the range from 2100 up to 2250 cm^-1. Four clusters were selected to detect the absorption features of carbon monoxide (CO), nitrous oxide (N₂O), carbon dioxide (CO₂), and water vapor (H₂O), respectively. The sensor was calibrated with certified concentrations of CO, N₂O and CO₂ in a wet nitrogen matrix. The H₂O absorption feature was used to monitor the water vapor within the gas line during the calibration. Minimum detection limits of 6 ppb, 7 ppb, and 70 ppm were achieved for CO, N₂O and CO₂, respectively, at 100 ms of integration time. As proof of concept, the QEPAS sensor was tested by continuously sampling indoor laboratory air and monitoring the analytes concentrations.

Keywords: Environmental monitoring; Multi-gas sensing; QEPAS; Widely tunable QCL.

Fig. 1

(a) Laser emission at I_L = 840 mA and different values of I_B: 450 mA (red curve), 850 mA (blue curve), 1100 mA (black curve), while I_F = 0; (b) Laser emission at I_L = 840 mA and different values of I_F: 450 mA (red curve), 750 mA (blue curve), 1050 mA (black curve), while I_B = 0; (c) Optical emission power as a function of the laser peak emission wavenumber for configurations employing the Front Section (black dots), the Back Section (red dots) and the QCL section (green dots).

Fig. 2

Schematic of the employed experimental setup. QTF, quartz tuning fork; mR, resonator tube; ADM, acoustic detection module.

Fig. 3

Absorption cross section of target analytes at atmospheric concentration simulated using the HITRAN database (solid curve) and laser optical power within the simulation spectral ranges (green squares). a) 150 ppb of CO (black curve) and 1.19 % of H₂O (red curve) in N₂; b) 310 ppb of N₂O (black curve) and 1.19 % of H₂O (red curve) in N₂; c) 330 ppm of CO₂ (black curve) and 1.19 % of H₂O (red curve) in N₂; and d) 1.19 % of H₂O (red curve) in N₂. All the spectra were simulated at atmospheric pressure.

Fig. 4

(a) 2f-QEPAS spectral scan of the H₂O absorption feature at 2 % concentration in N₂; (b) peak values from a 75-min-long acquisition of QEPAS signal for 2 % H₂O in N₂.

Fig. 5

(a) 2f-QEPAS spectral scans of the CO absorption feature at eight CO concentrations; (b) 2f-QEPAS spectral scans of the N₂O absorption features at eight N₂O concentrations; (c) 2f-QEPAS spectral scans of the CO₂ absorption peaks at eight CO₂ concentrations.

Fig. 6

(a) QEPAS peak signals as a function of the CO concentration (black squares) and the corresponding best linear fit (black dashed line); (b) QEPAS peak signals as a function of the N₂O concentration (red triangles) and the corresponding best linear fit (red dashed line); (c) QEPAS peak signals as a function of the CO₂ concentration (blue dots) and the corresponding best linear fit (blue dashed line).

Fig. 7

(a) 2f-QEPAS spectral scan of CO absorption feature at atmospheric concentration in indoor environment; (b) 2f-QEPAS spectral scan of N₂O absorption feature at atmospheric concentration in indoor environment; (c) 2f-QEPAS spectral scan of CO₂ absorption feature at atmospheric concentration in indoor environment. 2f-QEPAS spectral scan of H₂O at a concentration of 2 % can be observed in Figs. (a) and (b).

Fig. 8

Target gases concentrations estimated during the long-term monitoring of indoor laboratory air. (a) Estimated CO concentration (black squares); (b) Estimated N₂O concentration (red triangles); (c) Estimated CO₂ concentration (blue dots).