Highly sensitive photoacoustic multi-gas analyzer combined with mid-infrared broadband source and near-infrared laser

A highly sensitive photoacoustic (PA) multi-gas analyzer was developed in combination with a mid-infrared broadband source and a near-infrared laser. A multi-pass cylindrical PA cell was designed to efficiently couple the two PA excitation sources. The mid-infrared thermal radiation light was transmitted into the longitudinal direction of the cell for detecting CO, CO2, CH4, C2H6, C2H4 and H2O gas. The fundamental-frequency intensity modulation (1f-IM) technique was used to measure the generated PA signal. The collimated near-infrared laser beam was incident from the side hole of the cell for detecting C2H2 gas. The second-harmonic wavelength modulation spectroscopy (2f-WMS) technique was used to extract the laser-excited PA signal. The noise equivalent detection limits (1 sigma) were achieved to be 27 ppb, 10 ppb, 94 ppb, 24 ppb, 20 ppb and 37 ppb for C2H2, CO, CO2, CH4, C2H6 and C2H4 with an averaging time of 60s, respectively. The proposed method takes full advantage of the multi-component gas measurement capability of mid-infrared broadband source based PA spectroscopy (PAS) and the high precision of laser-based PAS.