Formaldehyde measurement method based on absorption spectrum

Abstract

The invention relates to a formaldehyde measuring method based on an absorption spectrum, and belongs to the technical field of infrared absorption spectrums.The formaldehyde measuring method comprises the steps that laser is injected into formaldehyde to be measured, and the light intensity after absorption of the formaldehyde to be measured is detected; moving the harmonic signal to a direct current part in a frequency domain; sequentially performing low-pass filtering, background deduction and normalization to obtain a background-deducted signal, and obtaining a peak value after peak searching; laser is injected into formaldehyde with the standard concentration, and a formaldehyde concentration-peak value curve is obtained; and comparing to obtain the concentration of the formaldehyde to be detected. The problems that a traditional formaldehyde measurement method is easy to drift and slow in response are solved, the formaldehyde measurement method based on the absorption spectrum is applied to the fields of environmental protection, safety and the like, a new thought is provided for formaldehyde measurement, and the application of the absorption spectrum technology in the aspect of toxic and harmful gas measurement is further expanded.

Description

Technical Field

[0001] This invention belongs to the field of infrared absorption spectroscopy technology, specifically relating to a method for measuring formaldehyde based on absorption spectroscopy. Background Technology

[0002] Formaldehyde (CH2O) is a common indoor pollutant gas. It is colorless, flammable, toxic, and carcinogenic. It is readily soluble in water; formalin, a 35-40% formaldehyde aqueous solution, is a common preservative. Furthermore, formaldehyde is an important chemical raw material used to manufacture synthetic resins. Formaldehyde escaping from these materials leads to poor indoor air quality and adversely affects human health. At 0.5 ppm, formaldehyde gas in the air can irritate the eyes; higher concentrations can cause nausea, coughing, chest tightness, and shortness of breath. According to the newly promulgated "Indoor Air Quality Standard" (GB / T 18883-2022), the minimum standard for formaldehyde content in human living environments is ≤0.08 mg / m³. 3 .

[0003] Tunable Semiconductor Absorption Spectroscopy (TDLAS) utilizes a tunable, narrow-linewidth semiconductor laser. By injecting a repetitive current, it continuously scans the absorption line of the target gas, obtaining the absorption characteristics of the gas. Formaldehyde concentration is then measured by utilizing the correlation between the peak value of the normalized harmonic signal (after background subtraction) and the concentration. This technology features fast response and in-situ measurement capabilities, and is widely used in environmental monitoring, industrial process control, and combustion diagnostics and optimization. TDLAS is categorized into direct absorption spectroscopy and wavelength modulation spectroscopy based on the type of injected electrical signal. Direct absorption spectroscopy uses a sawtooth or triangular wave as the injected current, while wavelength modulation spectroscopy superimposes a higher-frequency sine wave onto the direct absorption signal.

[0004] Traditional methods for measuring formaldehyde include spectrophotometry, chromatography, and electrochemical methods. Spectrophotometry utilizes the reaction of formaldehyde with specific reagents to generate a colored compound, and the formaldehyde concentration is measured by measuring absorbance; however, this method is time-consuming. Chromatography separates formaldehyde from a mixture and measures it quantitatively; its operation is complex and expensive. Electrochemical methods utilize the redox reaction of formaldehyde at electrodes to generate a current signal, and the formaldehyde concentration is obtained by measuring the current value; however, this method is greatly affected by other components and requires regular calibration. Utilizing the technological advantages of TDLAS provides new ideas for the engineering application of formaldehyde measurement. In practical applications, issues such as absorption line selection and laser operating mode need to be comprehensively considered. Summary of the Invention

[0005] The purpose of this invention is to provide a formaldehyde measurement method based on absorption spectroscopy. After a laser outputs a laser beam into the formaldehyde to be measured, the light intensity signal is collected. The collected light intensity signal is then subjected to phase-sensitive detection and low-pass filtering to obtain the peak value of the normalized harmonic signal after background subtraction. Based on the concentration-peak curve, the formaldehyde gas concentration is obtained by combining the difference algorithm.

[0006] The above-mentioned objectives of the present invention are mainly achieved through the following technical solutions: A formaldehyde measurement method based on absorption spectroscopy includes the following steps: (1) The laser is directed into the formaldehyde to be tested at a set center wavelength, and the intensity of the light after absorption by the formaldehyde is measured. ; (2) The light intensity obtained in step (1) The harmonic signals are shifted to the DC portion of the frequency domain; (3) The harmonic signal obtained in step (2) is subjected to low-pass filtering, background subtraction and normalization in sequence to obtain the signal with background subtraction, and the peak value is obtained after peak finding; (4) Repeat steps (1) to (3) multiple times to inject the laser into the standard concentration of formaldehyde and obtain the concentration-peak curve of formaldehyde. (5) Compare the peak value obtained in step (3) with the concentration-peak curve of formaldehyde obtained in step (4) to obtain the concentration of formaldehyde to be tested.

[0007] In step (1), the laser transmittance for:

[0008] in, The laser output frequency, For input light intensity, , Each is a single gas molecule The absorption line intensity and line shape function of the transition. , , , The values ​​are the temperature, pressure, absorption path length, and mole fraction of the formaldehyde to be tested.

[0009] Using wavelength modulation technology, the laser's frequency-time response is:

[0010] in, The center wavelength of the laser. Let be the angular frequency of the sine wave. , For modulation frequency, Modulation depth; We can obtain:

[0011] Laser transmittance Expanding the series using Fourier series, we get:

[0012] in, The order of harmonics, for Fourier coefficients.

[0013] The Fourier coefficients are expressed as:

[0014]

[0015] Where θ is the frequency Time phase, For the first The mole fraction of a substance.

[0016] In step (2), the light intensity is... Demodulation is performed, light intensity Multiply by respectively and This enables the transfer of harmonic signals to the DC portion in the frequency domain.

[0017] In step (2), the second harmonic signals of the dual channels are represented as follows:

[0018]

[0019] in This represents the gain coefficient of the photoelectric conversion system.

[0020] In step (3), the second harmonic is normalized using the first harmonic, and the normalized harmonic signal after removing the background is:

[0021] in, , They represent The second harmonic signal and background signal of the channel, , They represent The second harmonic signal and background signal of the channel, , These represent the first harmonic signal and the background signal after orthogonal calculation, respectively.

[0022] In step (1), the laser passes through the formaldehyde to be tested and hits the photodetector, which converts the light signal into an electrical signal and collects it.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In this invention, the light intensity signal containing absorption information is sequentially subjected to phase-sensitive detection and low-pass filtering, and the background signal is subtracted and normalized by channel. Combined with the concentration-peak curve obtained by standard concentration formaldehyde gas calibration, the formaldehyde concentration is rapidly measured using the difference method based on the measured harmonic signal peak value. This solves the problems of easy drift and slow response in existing traditional formaldehyde measurement methods.

[0024] (2) In the preferred embodiment of the present invention, the light intensity signal containing absorption information is demodulated in two channels to effectively suppress the influence of the initial phase drift of the laser on the measurement results; the working wavelength of the laser is further improved by line selection analysis to improve the accuracy and repeatability of formaldehyde measurement.

[0025] (3) The present invention provides a formaldehyde measurement method based on absorption spectroscopy, which can be used for formaldehyde measurement needs in various environments, further broadening the application scope of absorption spectroscopy technology. Attached Figure Description

[0026] Figure 1 This is a flowchart of a formaldehyde measurement method based on absorption spectroscopy disclosed in this invention; Figure 2 This describes the molecular absorption near the center wavelength of 5793.3 nm in Example 1 of the present invention. Figure 3 The formaldehyde harmonic signal obtained by the absorption model accurate construction method in Embodiment 1 of the present invention; Figure 4 This is the formaldehyde concentration-peak curve for Example 1 of the present invention. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments: like Figure 1As shown, a formaldehyde measurement method based on absorption spectroscopy utilizes tunable semiconductor absorption spectroscopy technology and an effective spectral line selection mechanism. Relying on the hardware environment, a laser of a specific wavelength continuously scans the target absorption line. The laser operates in a temperature-fixed current scanning mode. The laser's output light is collimated by a lens and enters a multi-reflection cell, which serves as the measurement medium. The output light strikes a photodetector, converting the optical signal into an electrical signal, which is then acquired by the signal acquisition unit and transmitted to the data processing unit. The data processing includes two steps: first, phase-sensitive detection to transfer the first and second harmonic information to the DC portion of the frequency domain; and second, low-pass filtering to extract the transferred first and second harmonic signals from the DC portion using a low-pass filter, which is used to characterize the formaldehyde concentration.

[0028] The transmittance of the laser after passing through the absorbing medium can be expressed as:

[0029] in Light transmittance The laser output frequency (usually expressed in wavenumbers per centimeter) -1 Formal representation, with wavelength The relationship is: ), and These are the input and output light intensities, respectively. , Each is a single gas molecule The absorption line intensity and line shape function of the transition. , , , These represent the temperature, pressure, absorption optical path length, and mole fraction of the absorbing gas, respectively.

[0030] Tunable semiconductor absorption spectroscopy (TDLAS) utilizes the narrow linewidth and wavelength of tunable semiconductor lasers, which change with the injected current, to measure single or several closely spaced, indistinguishable absorption lines of molecules. Based on the type of injected current, it is divided into direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). Direct absorption spectroscopy involves injecting only a triangular or sawtooth wave of a certain frequency, while wavelength modulation involves simultaneously superimposing a sinusoidal current of a higher frequency. This invention employs wavelength modulation, where the light intensity is modulated along with the wavelength. The laser frequency-time response of wavelength modulation can be expressed as:

[0031] in, The center wavelength of the laser corresponding to the sawtooth wave. The angular frequency of the sine wave ( ), For modulation frequency, The modulation depth.

[0032] The intensity of the transmitted light from a wavelength-modulated laser after passing through the absorbing medium is expressed as:

[0033] Regarding transmittance Expanding the series using Fourier series, we get:

[0034] The order of the harmonics, where for Fourier coefficients.

[0035] The Fourier coefficients are expressed as:

[0036]

[0037] θ is the frequency Time phase, For the first The mole fraction of each substance is given, where the summation represents the contribution of adjacent absorption lines. When absorption lines overlap, their combined impact on the signal needs to be considered. Based on this, the intensity of the absorbed light is demodulated and multiplied by [the appropriate factor]. and This allows the harmonic signal to be transferred to the DC portion in the frequency domain. After low-pass filtering, the DC signal can be extracted to obtain the harmonic signal.

[0038] The second harmonic signals of the dual channels can be represented as follows:

[0039]

[0040] in Here, represents the gain coefficient of the photoelectric conversion system. To eliminate the effects of detector gain and laser output jitter, the second harmonic is normalized using the first harmonic during formaldehyde measurement. This also eliminates the effects of common-mode noise from the laser and detector, as well as other non-resonant transmission losses. The normalized harmonic signal after background subtraction can be expressed as:

[0041] in, , They represent The second harmonic signal and background signal of the channel, , They represent The second harmonic signal and background signal of the channel, , These represent the first harmonic signal and the background signal after orthogonal calculation, respectively.

[0042] Normalized harmonic signal after background subtraction As the measurement target, its peak value is recorded. Standard formaldehyde gas of discrete concentrations is introduced into the absorption cell, and the signal peak values ​​are recorded to form a concentration-peak curve. Gas of unknown concentration is then introduced into the absorption cell. Based on the measured signal peak values ​​and the concentration-peak curves, combined with a difference algorithm, the formaldehyde gas concentration can be obtained.

[0043] The technical solution adopted in this invention is as follows: relying on a hardware environment consisting of four parts—laser control, optical path transmission, signal acquisition, and data processing—the center wavelength of the laser is determined based on line selection analysis. Wavelength modulation spectroscopy is used to collimate and couple the stably output laser beam into an absorption cell. The absorbed beam then strikes a photodetector, where the intensity signal after photoelectric conversion is acquired. and non-absorbed light intensity Following the Beer-Lambert law, it can be expressed as:

[0044] in For transmittance, Indicates absorbance. , , These represent the total pressure of the absorbing medium, the volume fraction of the absorbing substance, and the effective optical path, respectively. For line strength.

[0045] Additionally, linear functions It has normalization properties, meaning that the integral over the entire frequency domain equals 1.

[0046] By analyzing the absorbed light intensity signal The harmonic signals contained therein can be represented as:

[0047] in The gain coefficient of the photoelectric conversion system. To eliminate the phase difference between the reference signal and the laser's output frequency, a dual-channel orthogonal demodulation method is used to determine the phase difference. and The square root of can be expressed as:

[0048] Due to the change in the laser's output frequency over time For time It is an even function of time, therefore transmittance is also a function of time. The even function is passed through phase-sensitive detection and low-pass filtering in sequence, i.e., multiplied by respectively. and The harmonic signal containing absorption information is transferred to the DC section, and then passed through a low-pass filter to obtain the harmonic signals of each order. The second harmonic is normalized point-to-point using the first harmonic and the background is subtracted in a channel-by-channel manner to obtain the background-subtracted normalized second harmonic signal. Using the concentration-peak curve obtained in claim 6, the formaldehyde concentration value can be obtained by inversion using the difference method based on the measured peak value.

[0049] The formaldehyde measurement method based on absorption spectroscopy consists of five steps: Step 1: Using a measurement system consisting of four parts—laser control, optical path transmission, signal acquisition, and data processing—and combined with line selection analysis, determine the center wavelength of the laser and obtain the intensity of the absorbed light. .

[0050] In step one, the laser center wavelength is determined by accurately constructing an absorption spectral model based on absorption spectroscopy theory. The influence of water vapor, carbon dioxide, and other elements in the measured environment on formaldehyde absorption is considered. Based on the strong absorption of formaldehyde, the laser center wavelength that can be used for formaldehyde measurement is obtained.

[0051] Step 2: Analyze the light intensity containing absorption information. Harmonic signal shifting is performed, based on discrete signal processing theory, to... Multiply by respectively , The orthogonal reference signal can be used to shift the frequency of the harmonic signal and transfer it to the DC part.

[0052] The initial phase of the orthogonal reference signal in step two can be any value, but it is generally set to 0.

[0053] Step 3: Obtain the peak value of the normalized harmonic signal after background subtraction, and adjust the light intensity. After harmonic signal shifting is completed, the harmonic signals of each order are obtained through a low-pass filter. Components and The components are processed using a channel-specific background subtraction algorithm and first harmonic normalization to obtain the background-subtracted signal. The peak value is obtained after finding the peak.

[0054] Step 4: Complete the standard concentration formaldehyde calibration and obtain the formaldehyde concentration-peak curve. Using standard formaldehyde gas of discrete concentration, repeat steps 1 to 3 in order of increasing concentration to obtain the normalized harmonic signal peak value under different concentrations after background subtraction, obtain the concentration-peak curve, and complete the repeatability test.

[0055] Step 5: Based on the peak value of the harmonic signal obtained from the actual measurement environment, combine it with the formaldehyde concentration-peak curve to obtain the formaldehyde concentration in the measurement environment.

[0056] Example 1 Based on the infrared properties of molecules, taking the absorption of formaldehyde in the mid-infrared band as an example, the measurement environment is at normal temperature and pressure. First, the selection of absorption lines is analyzed. According to the principles of absorption spectroscopy, in addition to formaldehyde, the measurement environment also contains other gases such as oxygen, water vapor, and carbon dioxide. The absorption of light intensity is the result of the combined effect of all substances. According to the infrared activity law of molecules, oxygen only absorbs near 760 nm, which can eliminate its influence on the absorption of the target molecule. Besides oxygen, the potential molecules affecting formaldehyde absorption are mainly water vapor and carbon dioxide. Based on the fundamental vibration and line selection analysis of formaldehyde, the center wavelength of the laser is 5793.3 nm. This band can effectively suppress the interference of other gas absorption, such as… Figure 2 As shown, it is clear that the absorption of water vapor and carbon dioxide in this band is at least two orders of magnitude weaker than that of formaldehyde, and their influence on the absorption characteristics of formaldehyde is negligible.

[0057] Based on the precise construction method of the absorption model, the laser with a center wavelength of 5793.3nm, after being absorbed by formaldehyde, sequentially passes through phase-sensitive detection and low-pass filtering. The harmonic signal, after background subtraction and normalization using a dual-channel background subtraction method, is as follows: Figure 3 As shown in the figure, the processed harmonic signal exhibits a clear peak characteristic at the center of the target absorption line, which can be used as a basis for concentration measurement. The formaldehyde concentration is discretized, and the peak values ​​of the harmonic signals are obtained sequentially from low to high, resulting in a concentration-peak curve as shown in the figure. Figure 4 As shown, the curve is a monotonic curve within the set formaldehyde concentration measurement range, which meets the requirements for formaldehyde concentration measurement.

[0058] The above description is only the best specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

[0059] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A method for measuring formaldehyde based on absorption spectroscopy, characterized in that: Includes the following steps: (1) The laser is directed into the formaldehyde to be tested at a set center wavelength, and the intensity of the light after absorption by the formaldehyde is measured. ; (2) The light intensity obtained in step (1) The harmonic signals are shifted to the DC portion of the frequency domain; (3) The harmonic signal obtained in step (2) is subjected to low-pass filtering, background subtraction and normalization in sequence to obtain the signal with background subtraction, and the peak value is obtained after peak finding; (4) Repeat steps (1) to (3) multiple times to inject the laser into the standard concentration of formaldehyde and obtain the concentration-peak curve of formaldehyde. (5) Compare the peak value obtained in step (3) with the concentration-peak curve of formaldehyde obtained in step (4) to obtain the concentration of formaldehyde to be tested.

2. The formaldehyde measurement method based on absorption spectroscopy according to claim 1, characterized in that: In step (1), the laser transmittance for: in, The laser output frequency, For input light intensity, , Each is a single gas molecule The absorption line intensity and line shape function of the transition. , , , The values ​​are the temperature, pressure, absorption path length, and mole fraction of the formaldehyde to be tested.

3. The formaldehyde measurement method based on absorption spectroscopy according to claim 2, characterized in that: Using wavelength modulation technology, the laser's frequency-time response is: in, The center wavelength of the laser. Let be the angular frequency of the sine wave. , For modulation frequency, Modulation depth; We can obtain: 。 4. The formaldehyde measurement method based on absorption spectroscopy according to claim 3, characterized in that: Laser transmittance Expanding the series using Fourier series, we get: in, The order of harmonics, for Fourier coefficients.

5. The formaldehyde measurement method based on absorption spectroscopy according to claim 4, characterized in that: The Fourier coefficients are expressed as: Where θ is the frequency Time phase, For the first The mole fraction of a substance.

6. The formaldehyde measurement method based on absorption spectroscopy according to claim 1, characterized in that: In step (2), the light intensity is... Demodulation is performed, light intensity Multiply by respectively and This enables the transfer of harmonic signals to the DC portion in the frequency domain.

7. The formaldehyde measurement method based on absorption spectroscopy according to claim 6, characterized in that: In step (2), the second harmonic signals of the dual channels are represented as follows: in This represents the gain coefficient of the photoelectric conversion system.

8. The formaldehyde measurement method based on absorption spectroscopy according to claim 7, characterized in that: In step (3), the second harmonic is normalized using the first harmonic, and the normalized harmonic signal after removing the background is: in, , They represent The second harmonic signal and background signal of the channel, , They represent The second harmonic signal and background signal of the channel, , These represent the first harmonic signal and the background signal after orthogonal calculation, respectively.

9. The formaldehyde measurement method based on absorption spectroscopy according to claim 1, characterized in that: In step (1), the laser passes through the formaldehyde to be tested and hits the photodetector, which converts the light signal into an electrical signal and collects it.

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