Study on the Detection Mechanism of Sulfur Dioxide Gas Concentration

1 Principle of Monochromatic Sulfur Dioxide Fluorescence Detection

Ultraviolet fluorescence method for measuring sulfur dioxide concentration is a better method proposed in recent years to determine sulfur dioxide. According to the energy level transition mechanism of the absorption spectrum and fluorescence spectrum of the substance molecule, when the substance with the ability to absorb photons is irradiated with light of a specific wavelength (such as ultraviolet light), the molecule is excited to transition to a high energy level (excited state), and emits light with a longer wavelength than the excitation light, namely fluorescence, when it returns to the ground state. Sulfur dioxide molecules have this characteristic, and the process equation is as follows:

From the above formula, we can see that the fluorescence intensity has a certain relationship with the number of SO2 molecules. The SO2 concentration can be calculated by measuring the fluorescence intensity.

According to the Lambert-Beer law, the expression of the intensity of ultraviolet light absorbed by sulfur dioxide in the photoreaction chamber is:

In the formula: I0 is the incident ultraviolet light intensity, α is the absorption coefficient of SO2 molecules to ultraviolet light, l is the optical path, and c is the concentration of SO2 gas. The fluorescence intensity received by the photomultiplier tube is expressed as:

Where: G represents the geometric coefficient of the photoreaction cavity, φ represents the fluorescence quantum efficiency. Expanding equation (4) at the zero point Taylor series, we get

This is the fluorescence detection principle of measuring low-concentration sulfur dioxide with a small beam of monochromatic light. It can be seen from the formula that when the incident intensity of monochromatic light remains unchanged, the fluorescence intensity of low-concentration sulfur dioxide gas is proportional to its concentration, which provides a theoretical basis for the quantitative analysis of sulfur dioxide concentration.

2 Mathematical model of time-based dual-light-path sulfur dioxide fluorescence detection

This paper gives a time-based dual-light-path sulfur dioxide fluorescence detection method. It rotates the motor to make two filters with central wavelengths of λ1 and λ2 work alternately, just like the sulfur dioxide fluorescence passes through two different light paths before and after, generating two fluorescence signals in a very short time interval. By processing these two fluorescence signals, the purpose of removing interference and noise is achieved, and the measurement accuracy is improved. The mathematical model of time-based dual-light-path detection is as follows:

The other interfering light signals are completely eliminated. This method can overcome the deviation caused by background noise and changes in gas composition.

3 Experimental study

3.1 Instrument structure

The structural principle of the analyzer for measuring sulfur dioxide concentration by dual-path ultraviolet fluorescence method is shown in Figure 1. First, the gas to be measured is sent into the measuring chamber. The ultraviolet light emitted by the excitation light source on the left side of the measuring chamber enters the chamber through an interference filter with a central wavelength of 214nm and a half-width of 12nm. When the ultraviolet light passes through the gas to be measured, the SO2 molecules with a very low concentration in the gas are excited by the ultraviolet light and become excited. The molecules emit fluorescence in the process of returning to the ground state. Above the measuring chamber, the fluorescence is collected by a quartz convex lens and passed through a narrow-band interference filter to be received by a photomultiplier tube. The rotation of the motor causes the two filters to transmit fluorescence alternately, generating two fluorescence electrical signals with different sampling bands at a short interval. These two signals are amplified and calculated by the signal processing system, and finally converted into the concentration of sulfur dioxide for display. The components used in the experiment are: the excitation light source - zinc lamp, the main spectrum of the ultraviolet light excited by it is 213.8nm; the central wavelength of filter 1 is 340nm, the half-wave width is 100nm, and it can pass through all the spectra excited by sulfur dioxide; the central wavelength of filter 2 is 350nm, the half-wave width is 30nm, and it can pass through part of the spectrum excited by sulfur dioxide. [page]

3.2 Experimental steps

Preheat the light source for 30 minutes. After the light intensity stabilizes, pass the sample gas prepared by using a SO2 permeation tube and air that has been dusted, desulfurized, and dehumidified, and measure the output voltage when filter 1 and filter 2 are working respectively. Change the sulfur dioxide concentration in the sample gas and measure the output voltage again. Repeat this 10 times. The measured data are shown in Table 1.

3.3 Data Processing

Based on the data in the table, we use recursive least squares parameter identification to determine the parameters c = 3.89 and d = 1.03, substitute them into formula (7), and calculate the voltage signal of the SO2 molecule excited to produce fluorescence after dual light path correction for each measurement as shown in Table 2.

Linear fitting was performed on the three groups of voltage signals, as shown in Figure 2. Obviously, the output voltage signal is linearly related to the sulfur dioxide concentration at low concentrations, which shows that it is feasible to detect sulfur dioxide concentration using ultraviolet fluorescence method.

The correlation coefficients of the above three fitting lines are:

By comparison, it can be seen that the correlation coefficient R double between the voltage signal and the sulfur dioxide concentration after dual optical path correction is closer to 1, indicating that the dual optical path ultraviolet fluorescence measurement method is superior to the single optical path measurement method.

4 Conclusion

The dual-path sulfur dioxide fluorescence detection method theoretically eliminates the measurement error of sulfur dioxide caused by the change of gas composition. Through experimental analysis, this method is obviously better than the single-path detection method, which greatly improves its selectivity, sensitivity, anti-interference ability, etc., and has a strong promotion value.

References
[1] H. Okabe, et al. Ambient and Source SO2 Detector Based on a Fluorescence Method [J]. Journal of the Air Pollution Control Association, 1998, (23): 514-516.
[2] Chen Jiujiang, et al. Dual-path UV absorption measurement of sulfur dioxide concentration [J]. Optical Technology, 2000, 5 (26).
[3] Xiong Jianwen, Yang Chuping, He Zhenjiang, Yang Guanling. Experimental study on multi-wavelength ultraviolet fluorescence sulfur dioxide detection [J]. Optoelectronics·Laser, 2002, (8). [4] Gong Ruikun. Research on sulfur dioxide sensor
based on ultraviolet fluorescence difference method [J]. Sensor World, 2001, (8).