A cavity-enhanced optical gas detection method based on rayleigh scattering cross-section

Abstract

This invention relates to the field of gas detection technology, specifically disclosing a cavity-enhanced optical gas identification method based on Rayleigh scattering cross-sections. The method includes: calibrating a broadband cavity detection system to obtain the reflectivity of the optical resonant cavity; introducing a reference gas with a known Rayleigh scattering cross-section into the optical resonant cavity and measuring its transmitted light intensity; introducing a test gas into the optical resonant cavity and determining whether absorption exists based on the transmission spectrum of the test gas; if not, then: recording the transmitted light intensity of the test gas, calculating the Rayleigh scattering extinction coefficient of the test gas, and retrieving the composition and concentration of the test gas based on its Rayleigh scattering cross-section. This invention utilizes the intrinsic differences in the Rayleigh scattering cross-sections of different gas molecules as the detection basis, extending the detection principle of traditional cavity-enhanced spectroscopy from characteristic absorption to molecular Rayleigh scattering, thus realizing a novel method for quantitative gas optical analysis.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, and in particular to a cavity-enhanced optical gas identification method based on Rayleigh scattering cross section. Background Technology

[0002] Broadband cavity-enhanced absorption spectroscopy (BBCEAS) is a technique that primarily relies on the characteristic absorption of the analyte gas in the ultraviolet-visible band for qualitative and quantitative analysis. Due to its advantages such as high sensitivity, fast response, and simple optical path, it has been widely used for measuring trace atmospheric absorption gases (such as NO2, HONO, and CHOCHO). However, for some gases (such as N2, Ar, SF6, and He), which lack characteristic absorption in the ultraviolet-visible band and cannot generate measurable absorption signals, traditional broadband cavity-enhanced spectroscopy methods cannot be used for the detection of these gases.

[0003] Existing methods for detecting such gases mainly rely on techniques such as gas chromatography (GC) or mass spectrometry (MS). These methods have the following shortcomings: (1) they require carrier gases (such as He, N2, H2), resulting in high operating costs; (2) they have long analysis cycles (30–60 minutes), making rapid on-site detection impossible; (3) they are complex instruments that require professional personnel to operate; and (4) they require separation and pretreatment of mixed gases. Summary of the Invention

[0004] Based on this, the purpose of this invention is to provide a cavity-enhanced optical gas identification method based on Rayleigh scattering cross section, which solves the problems of existing methods that lack absorption feature gas detection, rely on gas chromatography, are complicated to operate, and cannot be quickly detected on-site. The invention extends the detection principle of cavity-enhanced spectroscopy from feature absorption to molecular Rayleigh scattering.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a cavity-enhanced optical gas identification method based on Rayleigh scattering cross section, which includes the following steps: S1. Calibrate the broadband cavity detection system to obtain the reflectivity of the optical resonant cavity. ; S2. Introduce a reference gas with a known Rayleigh scattering cross section into the optical resonant cavity and measure its transmitted light intensity. ; S3. Introduce the gas to be tested into the optical resonant cavity, and determine whether there is an absorbing component in the gas to be tested based on the transmission spectrum characteristics of the gas to be tested; if not, proceed to S4. S4. Record the transmitted light intensity of the gas to be tested. , and then combine and Calculate the Rayleigh scattering extinction coefficient of the gas to be tested. Based on the intrinsic differences in the Rayleigh scattering cross sections of different gas molecules, and taking advantage of the linear superposition of Rayleigh scattering extinction across the entire wavelength range, the Rayleigh scattering coefficient of the gas to be tested is fitted with a database of known Rayleigh scattering cross sections using least squares fitting, thereby retrieving the composition and concentration of the gas to be tested, and realizing the qualitative identification and quantitative analysis of the gas to be tested.

[0006] As a further improvement to the above-described solution of the present invention, in step S1, the calibration includes: Standard reference gas A and standard reference gas B, with known Rayleigh scattering cross sections, are sequentially introduced into the optical resonant cavity of a broadband cavity detection system. The transmitted light intensities of standard reference gas A and standard reference gas B are then obtained. I A (λ), I B (λ); Calculate the reflectivity of the optical resonant cavity based on the transmitted light intensity of the standard reference gas A and the standard reference gas B, and their known Rayleigh scattering coefficients. :

[0007] In the formula, , These represent the Rayleigh scattering coefficients of the standard reference gas A and the standard reference gas B, respectively. d This represents the distance between the two highly reflective mirrors in the broadband cavity detection system.

[0008] As a further improvement to the above-mentioned scheme of the present invention, in step S1, the standard reference gas A and the standard reference gas B are gases with accurate theoretical values ​​of Rayleigh scattering cross sections and no absorption characteristics in the measurement band, such as high-purity nitrogen (purity ≥99.999%), high-purity helium (purity ≥99.999%), etc.

[0009] As a further improvement to the above-mentioned solution of the present invention, in step S2, the standard reference gas B mentioned in step S1 is used as the reference gas, such as high-purity helium (purity ≥99.999%). = I B (λ) .

[0010] This invention eliminates the need for repeated calibration after system calibration, and the reference baseline can utilize the gas data measured during the calibration process. Subsequent measurements only require the introduction of the gas to be tested to directly obtain results, eliminating the need to recalibrate the system before each measurement. This significantly simplifies the on-site operation process and improves detection efficiency.

[0011] As a further improvement to the above-mentioned solution of the present invention, in step S3, determining whether there is absorption based on the transmission spectrum characteristics of the gas to be tested specifically involves: determining whether the transmission spectrum of the gas to be tested is smooth overall; if the transmission spectrum has a narrow band absorption depression that causes the overall spectrum to be unsmooth, then it is determined that there is an absorbing component; otherwise, it is determined that there is no absorbing component.

[0012] As a further improvement to the above-described solution of the present invention, in step S4, the Rayleigh scattering coefficient is calculated according to the following formula. :

[0013] In the formula, The Rayleigh scattering coefficient represents the reference gas.

[0014] As a further improvement to the above-mentioned scheme of the present invention, in step S4, the least-squares fitting of the Rayleigh scattering coefficient of the gas to be tested with the known cross-section database is specifically as follows:

[0015]

[0016] In the formula, m This represents the quantity of components in the gas being tested. i Representing the i Components, Representing the i Rayleigh scattering cross section of each component; N This represents the number density of molecules in the gas being tested. , P , T These represent the actual pressure and temperature at the time of measurement, respectively. Represents the Boltzmann constant. k B =1.380649×10 23 J / K; Determine the volume fraction of each component in the gas to be tested. It can invert the gas types and concentrations of each component in the gas to be tested, thereby realizing the qualitative identification and quantitative analysis of the gas to be tested.

[0017] As a further improvement to the above-mentioned solution of the present invention, the broadband light source of the broadband cavity detection system is an LED or a xenon lamp with a wavelength range of ultraviolet-visible band, with preference given to the ultraviolet band with greater Rayleigh scattering cross-section; the cavity length of the optical resonant cavity is 30-100cm, and the reflectivity of the high-reflection mirror is greater than 0.999.

[0018] As a further improvement to the above-mentioned scheme of the present invention, the database of known Rayleigh scattering cross sections in step S4 can be the Rayleigh scattering cross sections of different gas molecules derived from known formulas in the literature, or it can be a cross section database composed of experimentally measured Rayleigh scattering cross sections of different gas molecules.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes the intrinsic differences in the Rayleigh scattering cross-sections of different gas molecules as the basis for detection, extending the detection basis of cavity enhancement spectroscopy from characteristic absorption to molecular Rayleigh scattering, enabling optical quantitative analysis of gases that were previously undetectable by absorption spectroscopy.

[0020] Existing detection methods for non-absorbing gases mainly rely on gas chromatography or mass spectrometry, which has its drawbacks. This invention employs a cavity-enhanced optical structure, allowing the gas to be measured to be directly introduced into the detection cavity without the need for a carrier gas or a vacuum system. The optical path is simple, enabling rapid measurement under normal pressure conditions. It is easy to operate, significantly reducing detection costs and operational barriers. No repeated calibration is required, making it suitable for rapid qualitative identification and quantitative analysis of gases such as Ar, CO2, SF6, H2, Ne, He, and CH4 in the ultraviolet band.

[0021] This invention eliminates the need for pretreatment such as chromatographic separation of the mixed gas. The gas to be tested can be directly introduced into the optical resonant cavity to collect the transmission spectrum and perform data processing. The measurement speed is extremely fast, achieving a response time in seconds, which meets the actual needs of rapid on-site detection.

[0022] This invention, based on the intrinsic differences in Rayleigh scattering cross-sections of different gas molecules, significantly improves the optical path length through cavity enhancement technology. Utilizing the linear superposition of Rayleigh scattering extinction across the entire wavelength range, it directly compares experimentally measured Rayleigh scattering coefficients with a known cross-section database using least-squares fitting, thereby retrieving the types and concentrations of each gas component. This represents a novel method for quantitative gas optical analysis. This method eliminates the need for pre-establishing standard working curves for each analyte gas, making it particularly suitable for rapid analysis of mixed gases with unknown or frequently changing components. Attached Figure Description

[0023] Figure 1 A flowchart of a cavity-enhanced optical gas identification method based on Rayleigh scattering cross section is provided for embodiments of the present invention; Figure 2 This is a comparison chart of the experimental and theoretical calculation values ​​of the Rayleigh scattering cross section of argon gas in an embodiment of the present invention; Figure 3 The transmitted light intensity of different mixed gases in the embodiments of the present invention; Figure 4 These are the CO2 volume ratios in different gas mixtures measured in embodiments of the present invention. Detailed Implementation

[0024] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0026] Reference Figure 1 This embodiment proposes a cavity-enhanced optical gas identification method based on Rayleigh scattering cross section, including the following steps: S1. Calibrate the broadband cavity detection system to obtain the reflectivity of the optical resonant cavity. Specifically: Standard reference gas A and standard reference gas B, with known Rayleigh scattering cross sections, are sequentially introduced into the optical resonant cavity of a broadband cavity detection system. The transmitted light intensities of standard reference gas A and standard reference gas B are then obtained. I A (λ), I B (λ); Calculate the reflectivity of the optical resonant cavity based on the transmitted light intensity of standard reference gas A and standard reference gas B and their known Rayleigh scattering coefficients. :

[0027] In the formula, , These represent the Rayleigh scattering coefficients of standard reference gas A and standard reference gas B, respectively. d This represents the distance between the two highly reflective mirrors in the broadband cavity detection system.

[0028] Standard reference gas A and standard reference gas B are gases with accurate theoretical Rayleigh scattering cross sections and no absorption characteristics within the measurement band. When selecting standard reference gases, the following criteria must be met: 1. The theoretical value of the Rayleigh scattering cross section is highly accurate and supported by specific references; 2. There is absolutely no absorption within the measurement band; 3. Chemically stable and will not react with the cavity material; 4. Widely available and easily accessible, with a purity of up to 99.99%. In this embodiment, standard reference gas A and standard reference gas B are high-purity nitrogen (purity ≥99.999%) and high-purity helium (purity ≥99.999%), respectively.

[0029] It should be noted that the broadband cavity detection system in this embodiment uses a conventional broadband cavity enhanced spectral measurement device, mainly including: a broadband light source for generating incident light covering the measurement band; an optical resonant cavity composed of two high-reflectivity mirrors to increase the interaction path between light and gas; a spectral detection system for acquiring the transmitted light intensity spectrum; a gas path control system, including a mass flow controller, a pressure sensor, and a temperature sensor, for controlling gas introduction, displacement, and monitoring environmental parameters; and a data processing unit for data recording and post-processing. In this embodiment, the broadband light source is a 375 nm LED, and the measurement band is 370-395 nm; the cavity length of the optical resonant cavity is 94 cm, and the reflectivity of the mirrors in the measurement band is greater than 0.9997.

[0030] S2. Introduce a reference gas with a known Rayleigh scattering cross section into the optical resonant cavity and measure its transmitted light intensity. This serves as a reference baseline for subsequent measurements.

[0031] The reference gas in this step can be either standard reference gas A or standard reference gas B from step S1, using the transmitted light intensity of high-purity nitrogen or high-purity helium as the reference. No repeated measurements are required. In this embodiment, the transmitted light intensity of high-purity helium is used as... , = I B (λ) .

[0032] S3. Introduce the gas to be tested into the optical resonant cavity. Based on the transmission spectrum characteristics of the gas to be tested, determine whether there is an absorbing component in the gas to be tested; if not, proceed to S4.

[0033] Specifically, the presence of absorption is determined based on the transmission spectrum characteristics of the gas to be tested. Specifically, it is determined whether the transmission spectrum of the gas to be tested is smooth overall. If the transmission spectrum has narrow absorption depressions that cause the overall spectrum to be uneven, it is determined that there are absorbing components in the gas to be tested. Otherwise, it is determined that there are no absorbing components in the gas to be tested.

[0034] If an absorbing component is present, the absorption coefficient is calculated using the following formula. Then, the gas is quantified using a traditional broadband cavity enhanced absorption spectroscopy algorithm, and the concentration of the absorbed gas is directly output:

[0035] S4. Record the transmitted light intensity of the gas to be tested. , and then combine and Calculate the Rayleigh scattering extinction coefficient of the gas to be tested. Based on the intrinsic differences in Rayleigh scattering cross sections of different gas molecules, and taking advantage of the linear superposition of Rayleigh scattering extinction of different gas molecules across the entire wavelength range, the Rayleigh scattering coefficient of the gas to be tested is fitted with a database of known Rayleigh scattering cross sections using least squares, thereby retrieving the composition and concentration of the gas to be tested, and realizing the qualitative identification and quantitative analysis of the gas to be tested.

[0036] Calculate the Rayleigh scattering coefficient using the following formula :

[0037] In the formula, The Rayleigh scattering coefficient represents the reference gas (i.e., helium).

[0038] The Rayleigh scattering coefficient of the gas to be tested is fitted with a database of known Rayleigh scattering cross sections using least squares fitting.

[0039]

[0040] In the formula, m This represents the quantity of components in the gas being tested. i Representing the i Components, Representing the i Rayleigh scattering cross section of each component; N This represents the number density of molecules in the gas being tested. , P , T These represent the actual pressure and temperature at the time of measurement, respectively. Represents the Boltzmann constant. k B =1.380649×10 23 J / K.

[0041] Determine the volume fraction of each component in the gas to be tested. It can invert the gas types and concentrations of each component in the gas to be tested, thereby realizing the qualitative identification and quantitative analysis of the gas to be tested.

[0042] For binary gas mixtures, it can be simplified to:

[0043] The method described in this embodiment can be applied to any measurement band. However, since the Rayleigh scattering cross section of a gas is inversely proportional to the fourth power of the wavelength, the scattering signal is stronger in the short band than in the long band. This method works better in the ultraviolet band.

[0044] In this embodiment, gases with no absorption characteristics or negligible absorption within the measurement band include, but are not limited to: inert gases: He, Ne, Ar, etc.; diatomic homonuclear molecules: N2, O2, H2, etc.; polyatomic non-absorbing molecules: CO2, SF6, CH4, and other fluorocarbon compounds; and mixed gases: any binary or multi-component mixture of the above gases.

[0045] Next, the method of this embodiment will be described using a single sample as an example. The light source of the broadband cavity detection system is an ultraviolet LED (center length at 380 nm), and the distance between the two highly reflective mirrors is d = 94 cm.

[0046] 1. System Calibration: Introduce high-purity N2 (purity ≥ 99.999%) and record the system pressure P and temperature T. After stabilizing for 10 minutes, acquire the transmission spectrum. The integration time was 0.6 s, and the average was taken after 100 repetitions. High-purity helium gas (purity ≥ 99.999%) was introduced, and transmission spectra were collected after the spectra stabilized under the same conditions. The effective reflectivity of the two highly reflective mirrors is calculated using the following formula:

[0047] 2. Establish a reference baseline. Directly use the measurements taken in step 1. As a reference baseline .

[0048] 3. Measurement of a single gas sample. Maintain the same pressure P and temperature T as during calibration. Introduce argon (purity ≥99.999%) into the optical resonant cavity of the broadband cavity detector system and purge at a flow rate of 0.5 L / min for 3-4 minutes to ensure complete gas replacement within the optical resonant cavity. After the spectrum stabilizes under the same conditions, acquire the transmission spectrum to obtain the transmitted light intensity. Calculate the Rayleigh scattering extinction coefficient. :

[0049] 4. Data Processing and Comparison. The measured data for the single gas sample argon... The experimental values ​​were compared with the theoretical values ​​of the Rayleigh scattering cross section of argon gas calculated based on literature, and the relative deviations were calculated. The comparison results are as follows: Figure 2 As shown.

[0050] Figure 2 The results show that the Rayleigh scattering cross section of argon obtained by using helium as a reference baseline has a very small relative error compared with the theoretical value, which is within the error range of the theoretical value, thus verifying the reliability of the method of the present invention.

[0051] Next, the method of this embodiment will be described using a two-component sample as an example. The light source of the broadband cavity detection system is an ultraviolet LED (center wavelength at 380 nm), and the distance between the two highly reflective mirrors is d = 94 cm.

[0052] 1. System Calibration: Introduce high-purity N2 (purity ≥ 99.999%) and record the system pressure and temperature. After stabilizing for 10 minutes, acquire the transmission spectrum. The integration time was 0.6 s, and the average was taken after 100 repetitions. High-purity helium gas (purity ≥ 99.999%) was introduced, and transmission spectra were collected after the spectra stabilized under the same conditions. The effective reflectivity of the two highly reflective mirrors is calculated using the following formula:

[0053] 2. Establish a reference baseline. Directly use the measurements taken in step 1. As a reference baseline .

[0054] 3. Prepare mixed gases with different volume ratios. Use high-purity CO2 and high-purity N2 (purity ≥99.999%) as sample gases, and prepare four sets of mixed gases with different volume ratios using a gas mixing system: 20% CO2 + 80% N2, 40% CO2 + 60% N2, 60% CO2 + 40% N2, and 80% CO2 + 20% N2. After uniform mixing, introduce them sequentially into the broadband cavity enhanced spectral detection system.

[0055] 4. Measure the gas mixture. Under the same conditions, wait for the spectrum to stabilize before collecting the transmission spectrum. Calculate the Rayleigh scattering extinction coefficient .

[0056] 5. Concentration inversion. Based on the linear superposition model: The volume ratio was determined by least squares fitting within the experimental band. The result is as follows Figure 3 , Figure 4 As shown.

[0057] The results show that the method of this invention can achieve quantitative analysis of gases using the Rayleigh scattering cross section.

[0058] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0059] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A cavity-enhanced optical gas identification method based on Rayleigh scattering cross section, characterized in that, It includes the following steps: S1. Calibrate the broadband cavity detection system to obtain the reflectivity of the optical resonant cavity. ; S2. Introduce a reference gas with a known Rayleigh scattering cross section into the optical resonant cavity and measure its transmitted light intensity. ; S3. Introduce the gas to be tested into the optical resonant cavity, and determine whether there is an absorbing component in the gas to be tested based on the transmission spectrum characteristics of the gas to be tested; if not, proceed to S4. S4. Record the transmitted light intensity of the gas to be tested. , and then combine and Calculate the Rayleigh scattering extinction coefficient of the gas to be tested. Based on the intrinsic differences in the Rayleigh scattering cross sections of different gas molecules, and taking advantage of the linear superposition of Rayleigh scattering extinction across the entire wavelength range, the Rayleigh scattering coefficient of the gas to be tested is fitted with a database of known Rayleigh scattering cross sections using least squares fitting, thereby retrieving the composition and concentration of the gas to be tested, and realizing the qualitative identification and quantitative analysis of the gas to be tested.

2. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S1, the calibration includes: Standard reference gas A and standard reference gas B, with known Rayleigh scattering cross sections, are sequentially introduced into the optical resonant cavity of a broadband cavity detection system. The transmitted light intensities of standard reference gas A and standard reference gas B are then obtained. I A (λ), I B (λ); Calculate the reflectivity of the optical resonant cavity based on the transmitted light intensity of the standard reference gas A and the standard reference gas B, and their known Rayleigh scattering coefficients. : In the formula, , These represent the Rayleigh scattering coefficients of the standard reference gas A and the standard reference gas B, respectively. d This represents the distance between the two highly reflective mirrors in the broadband cavity detection system.

3. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S1, the standard reference gas A and standard reference gas B are gases with accurate theoretical values ​​for Rayleigh scattering cross sections and no absorption characteristics in the measurement band.

4. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S2, the standard reference gas B mentioned in step S1 is used as the reference gas. = I B (λ) .

5. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S3, the presence of absorption is determined based on the transmission spectrum characteristics of the gas to be tested. Specifically, it is determined whether the transmission spectrum of the gas to be tested is smooth overall. If the transmission spectrum has a narrow band absorption depression that causes the overall spectrum to be uneven, it is determined that there is an absorbing component; otherwise, it is determined that there is no absorbing component.

6. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S4, the Rayleigh scattering coefficient is calculated using the following formula. : In the formula, The Rayleigh scattering coefficient represents the reference gas.

7. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, In step S4, the Rayleigh scattering coefficient of the gas to be tested is fitted with the known cross-section database using least squares method as follows: In the formula, m This represents the quantity of components in the gas being tested. i Representing the i Components, Representing the i Rayleigh scattering cross section of each component; N This represents the number density of molecules in the gas being tested. , P , T These represent the actual pressure and temperature at the time of measurement, respectively. Represents the Boltzmann constant. k B =1.380649×10 23 J / K; Determine the volume fraction of each component in the gas to be tested. It can invert the types and concentrations of each component in the gas to be tested, thereby realizing the qualitative identification and quantitative analysis of the gas to be tested.

8. The cavity-enhanced optical gas identification method based on Rayleigh scattering cross section according to claim 1, characterized in that, The broadband light source of the broadband cavity detection system is an LED or a xenon lamp, and the measurement wavelength is in the ultraviolet-visible band; the cavity length of the optical resonant cavity is 30-100cm, and the reflectivity of the high-reflection mirror is greater than 0.999.

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