Accurate measurement of trace H<sub>2</sub>S based on Cavity Ring-Down Spectroscopy under CO<sub>2</sub>/CO

Since H₂S is a corrosive and toxic gas pollutant, the accurate measurement of its concentration is significant. However, in the practical industrial processes, it is difficult to implement because of the disturbance caused by other emissions such as CO₂ and CO. Therefore, in this paper, the concentrations of H₂S, CO₂ and CO were measured simultaneously based on cavity ring-down spectroscopy (CRDS) as a viable alternative to measure the concentration of H₂S accurately when CO₂ and CO exist. First, the wavelength of mixed gas within the range of 6336~6339 cm^-1 was selected as the target region where the spectral line intensity of H₂S was stronger than 10 times of that of CO₂ or CO and the water absorption was extremely weak. Second, the influence of the sampling length (Tm) on the accuracy of the ring-down time was analyzed by evaluating average (accuracy), standard deviation (precision) and consumption time (speed). Third, the experiments were carried out at different pressures in order to obtain the optimal pressure condition. Fourth, the concentration of trace H₂S was measured when the disturbances caused by CO₂ or CO were added, and the error analysis of the measured concentration was carried out. Finally, the detection limit of CRDS-based system was calculated as 6.9 ppb by analyzing the SNR of four groups of low concentration H₂S spectra, while the lower limit of detection of CRDSbased system was calculated as 2 ppb by analyzing the Allan variance of long-term data. The measured concentration and its desired value show a great linearity at different dilution ratios. Both the high linearity and the low detection limit of H₂S indicate the effectiveness of the CRDS-based measurement system to measure H₂S when CO₂ and CO exist. The successful application of the CRDS-based system to the measurement of H₂S shows its promising prospect in gas concentration measurement for practical industrial processes.