A non-contact quartz-enhanced photoacoustic spectroscopy gas detection device

Abstract

This invention provides a non-contact quartz-enhanced photoacoustic spectroscopy gas detection device, comprising: a gas chamber and a quartz tuning fork; the gas chamber includes: two gas buffer zones and an acoustic resonant cavity; a slit opening is formed in the middle of the acoustic resonant cavity, and the slit opening is filled by an elastic membrane; the quartz tuning fork is placed outside the gas chamber, and its vibrating arm is coupled to the elastic membrane on the acoustic resonant cavity, the two can be in contact or not; when a laser passes through the acoustic resonant cavity, under the action of photoacoustic effect, the sound wave excited by the laser beam forms a standing wave in the acoustic resonant cavity, and the sound wave signal corresponding to the standing wave passes through the elastic membrane to form a vibration wave, which drives the vibrating arm of the quartz tuning fork to vibrate; the vibration signal intensity of the quartz tuning fork vibrating arm is positively correlated with the gas concentration in the gas chamber, and the gas detection device can detect the gas concentration without the quartz tuning fork coming into contact with the gas. The device provided by this invention is of great significance for the detection of corrosive gases and dusty gases.

Description

Technical Field

[0001] This invention belongs to the field of gas sensing, and more specifically, relates to a non-contact quartz-enhanced photoacoustic spectroscopy gas detection device. Background Technology

[0002] Photoacoustic spectroscopy is a trace gas detection technique widely used in various applications. Its advantage over other spectroscopic methods is that its detection sensitivity is independent of absorption length. Photoacoustic spectroscopy allows for highly sensitive detection of small amounts of gas samples. The detection principle of photoacoustic spectroscopy involves a modulated laser beam driving gas molecules from their ground state to an excited state. The excited molecules release energy through non-radiative relaxation and return to their ground state. This released energy causes a local increase in temperature and pressure, generating sound waves—the photoacoustic effect. A microphone then converts these sound waves into an electrical signal, which is used to determine the concentration of the detected gas. Photoacoustic gas sensors are widely used in atmospheric environmental monitoring, space station air monitoring, human exhaled breath diagnostics, oilfield natural gas analysis, and early fire prediction. In 2002, A. Kosterev et al. at Rice University proposed quartz-enhanced photoacoustic spectroscopy, which uses a tuning fork quartz crystal instead of a microphone as a transducer, converting the acoustic signal into an electrical signal through the piezoelectric effect. Tuning fork quartz crystals are excellent piezoelectric devices with a high resonant frequency (32.7 kHz) and a high Q-factor (10,000) in the atmosphere. In quartz-enhanced photoacoustic spectroscopy, highly sensitive analysis can be performed using only a few cubic centimeters of gas sample. Quartz-enhanced photoacoustic spectroscopy has been successfully used for the detection of greenhouse gases such as carbon dioxide, nitrous oxide, and methane.

[0003] In quartz-enhanced photoacoustic spectroscopy, a laser beam is typically focused between the two arms of a tuning fork quartz crystal, generating acoustic waves that drive the crystal to vibrate symmetrically. To improve the performance of quartz-enhanced photoacoustic spectroscopy, a pair of acoustic microresonators composed of stainless steel tubes are placed on either side of the crystal to enhance sound pressure through longitudinal resonance; this configuration is known in the industry as coaxial quartz-enhanced photoacoustic spectroscopy. Ren Wei et al. from the Chinese University of Hong Kong used coaxial quartz-enhanced photoacoustic spectroscopy based on a quantum cascade laser operating at ~7.73 μm to achieve ppb-level detection of hydrogen peroxide. Liu Kun et al. from the Anhui Institute of Optics and Fine Mechanics demonstrated an off-axis quartz-enhanced photoacoustic spectroscopy sensor where the laser beam no longer needs to be focused in the middle of the tuning fork quartz crystal's arms. The tuning fork quartz crystal is positioned close to the resonator slit to collect the acoustic signal, thus reducing the difficulty of laser collimation. Dong Lei et al. from Shanxi University used an elliptical tube as an off-axis acoustic resonator to match a high-power multimode laser diode. Zheng Huadan and colleagues from Jinan University demonstrated a radial cavity quartz-enhanced photoacoustic spectroscopy sensor to improve sensitivity through radial acoustic resonance. M. Duquesnoy from Paris-Saclay University in France developed a radial cavity quartz-enhanced photoacoustic spectroscopy sensor based on a custom tuning fork quartz crystal oscillator.

[0004] To date, all reported quartz-enhanced photoacoustic spectroscopy sensors place a tuning fork quartz crystal within a gas chamber. These sensors suffer from an insurmountable problem: the metal film electrodes on the surface of the tuning fork quartz crystal are corroded by corrosive gases, making them unsuitable for detecting high concentrations of corrosive gases. Another issue is their inability to detect dusty gases, as particles adhering to the tuning fork quartz crystal significantly affect its vibration. To address this problem, Ma Yufei et al. from Harbin Institute of Technology proposed photoinduced thermoelastic spectroscopy, placing the tuning fork quartz crystal as a photodetector behind the absorption cell. However, photoinduced thermoelastic spectroscopy is essentially absorption spectroscopy, not photoacoustic spectroscopy, and its detection sensitivity depends on the optical absorption length. Compared to quartz-enhanced photoacoustic spectroscopy, photoinduced thermoelastic spectroscopy sacrifices the advantages of compact sensor structure and small gas sampling volume. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a non-contact quartz-enhanced photoacoustic spectroscopy gas detection device, which aims to solve the problems of poor corrosion resistance and system instability when detecting high-concentration corrosive gases or dusty gases based on quartz-enhanced photoacoustic spectroscopy sensors.

[0006] To achieve the above objectives, the present invention provides a non-contact quartz-enhanced photoacoustic spectroscopy gas detection device, comprising: a gas chamber and a quartz tuning fork;

[0007] The gas chamber includes: two gas buffer zones and an acoustic resonant cavity; the acoustic resonant cavity is placed between the two gas buffer zones, connecting the two gas buffer zones; each of the two gas buffer zones is provided with a vent, one as an air inlet and the other as an air outlet; the outer sides of the two gas buffer zones are both light-transmitting windows; a slit notch is opened in the middle of the acoustic resonant cavity, and the slit notch is filled by an elastic membrane;

[0008] The quartz tuning fork is placed outside the air chamber and does not come into contact with the gas inside the air chamber. The distance between its vibrating arm and the elastic film on the acoustic resonant cavity is within a preset distance range. The maximum value of the preset distance range is a positive number, and the minimum value is a negative number.

[0009] When a laser beam enters from one gas buffer zone, passes through the acoustic resonant cavity, and exits from another gas buffer zone, the sound waves excited by the laser beam form a standing wave in the acoustic resonant cavity under the action of photoacoustic effect. The sound wave signal corresponding to the standing wave is transmitted through the elastic membrane and forms a vibration wave in the air. The vibration wave drives the quartz tuning fork arm to vibrate. The vibration signal intensity of the quartz tuning fork arm is positively correlated with the gas concentration in the gas chamber. The gas detection device can detect the gas concentration when the quartz tuning fork does not come into contact with the gas and the distance between it and the elastic membrane is within a preset distance range.

[0010] In an optional example, the elastic film is a thin, non-breathable, elastic film made of a corrosion-resistant material.

[0011] In an optional example, the two gas buffers and the acoustic resonant cavity are made of corrosion-resistant materials.

[0012] In an optional example, the elastic film is a perylene film or a polyethylene film.

[0013] In one optional example, the two gas buffers and the acoustic resonant cavity are made of stainless steel.

[0014] In an optional example, the light-transmitting windows on the outer sides of the two gas buffers are calcium fluoride or quartz windows.

[0015] In an optional example, the distance between the coupling point of the elastic film and the quartz tuning fork arm and the top of the arm is between 0 mm and 2 mm.

[0016] In one optional example, the length of the elastic film is less than or equal to half the perimeter of the acoustic resonant cavity, the width is between 0.6 mm and 1 mm, and the thickness is less than 10 μm.

[0017] In one alternative example, the length of the acoustic resonant cavity is more than four times its diameter, making its length much greater than its diameter.

[0018] In one optional example, the distance between the quartz tuning fork arm and the elastic diaphragm ranges from 0.4 mm to -0.07 mm.

[0019] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art:

[0020] This invention provides a non-contact quartz-enhanced photoacoustic spectroscopy gas detection device. In this device, a synthetic elastic thin film is used for sound wave conduction. The elastic thin film and acoustic resonant cavity are resonantly coupled with a tuning fork-type quartz crystal oscillator, achieving non-contact detection between the quartz tuning fork and the gas to be detected, as well as the acoustic resonant cavity, while significantly improving the signal-to-noise ratio. Therefore, the non-contact quartz-enhanced photoacoustic spectroscopy provided by this invention is of great significance for the detection of corrosive and dust-laden gases, breaking through the current bottlenecks of traditional quartz-enhanced photoacoustic spectroscopy and possessing broad practical prospects. Attached Figure Description

[0021] Figure 1(a) is a schematic diagram of the non-contact quartz-enhanced photoacoustic spectroscopy meter provided in an embodiment of the present invention.

[0022] Figure 1(b) is a schematic diagram of the non-contact quartz-enhanced photoacoustic spectroscopy sensor device provided in an embodiment of the present invention.

[0023] Figure 2(a) is a schematic diagram of the acoustic resonant cavity composed of an elastic film and a stainless steel tube provided in an embodiment of the present invention.

[0024] Figure 2(b) is a schematic diagram showing the relationship between the vibration frequency and the width of the elastic film provided in the embodiment of the present invention.

[0025] Figure 2(c) is a schematic diagram showing the relationship between the sound pressure on the elastic film and the length of the acoustic resonant cavity provided in the embodiment of the present invention.

[0026] Figure 3(a) is a schematic diagram showing the relationship between different coupling distances and the frequency and Q factor of tuning fork quartz crystals provided in the embodiments of the present invention.

[0027] Figure 3(b) is a schematic diagram showing the relationship between different coupling distances and normalized signal amplitudes provided in the embodiments of the present invention.

[0028] Figure 4(a) is a schematic diagram of the 2f signal of the conventional contact quartz-enhanced photoacoustic spectrum and the non-contact quartz-enhanced photoacoustic spectrum provided in the embodiments of the present invention.

[0029] Figure 4(b) is a noise diagram of the conventional contact quartz-enhanced photoacoustic spectrum and the non-contact quartz-enhanced photoacoustic spectrum provided in the embodiments of the present invention.

[0030] Figure 5This is a schematic diagram of the Allan variance of the non-contact quartz-enhanced photoacoustic spectroscopy sensor provided in an embodiment of the present invention.

[0031] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is a function generator, 2 is a sine wave signal, 3 is a sawtooth wave signal, 4 is an adder, 5 is a laser driver, 6 is a semiconductor laser, 7 is a fiber optic focusing lens, 8 is a spectral acoustic meter, 9 is an impedance preamplifier, 10 is a lock-in amplifier, and 11 is a computer. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] In the description of this invention, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is generally based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this invention. The directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0034] In the description of this invention, "several" means one or more, "multiple" means two or more, and "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, the number of indicated technical features, or the order of the indicated technical features. In the description of this invention, unless otherwise explicitly defined, terms such as "set," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of these terms in this invention based on the specific content of the technical solution.

[0035] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0036] To address the instability issue of quartz-enhanced photoacoustic spectroscopy sensors when detecting high-concentration corrosive or dusty gases, this invention proposes a method using an elastic thin film to separate a tuning fork quartz crystal oscillator from the detection gas. This technique is named non-contact quartz-enhanced photoacoustic spectroscopy. Unlike previously reported quartz-enhanced photoacoustic spectroscopy sensors, the tuning fork quartz crystal oscillator is no longer placed in the gas chamber in contact with the detection gas. A thin, elastic thin film is synthesized and then attached to the notch of the quartz-enhanced photoacoustic spectroscopy acoustic resonator, forming a closed photoacoustic gas chamber. The tuning fork quartz crystal oscillator is placed outside the gas chamber, close to the elastic thin film for coupling, forming the entire spectroscopic acoustic transducer, as shown in Figure 1(a).

[0037] It should be noted that the embodiment of the present invention uses a Piriton film as an example of an elastic film for illustration, and will not be specifically described thereafter. Furthermore, those skilled in the art can select other types of elastic films to implement the technical solution of the present invention as needed. It is understood that any film capable of transmitting sound wave vibrations should fall within the protection scope of the present invention.

[0038] Similarly, this embodiment of the invention uses stainless steel as the main structure of the gas chamber for illustrative purposes. It is understood that those skilled in the art can select other suitable materials as the main structure of the gas chamber based on corrosion resistance requirements, and this invention does not impose any particular limitations on this.

[0039] Accordingly, the light-transmitting window of the gas chamber in this invention can be a calcium fluoride window. Of course, those skilled in the art can also select other light-transmitting windows with better laser light transmission performance according to actual needs. The following embodiments only use a calcium fluoride window as an example for illustration and do not limit the embodiments of this invention in any way.

[0040] First, the geometry of the resonator and the thin film was studied in detail to achieve resonance enhancement with the tuning fork quartz crystal. A stainless steel tube with two gas buffers was used as the acoustic micro-resonator to contain the gas, in which the sound waves would form standing waves. The inner and outer diameters of the tube were 0.8 mm and 1.2 mm, respectively. Both ends were gas buffers with a diameter of 10 mm and sealed with calcium fluoride windows. A thin film with excellent elasticity was synthesized using a vacuum thermal evaporation system. The coating process was carried out at a pressure of 0.1 Torr. The material was placed in a vacuum evaporation chamber and evaporated into gaseous molecules at 180 °C, finally forming a thin film. The thickness of the elastic film was measured to be 1 μm by spectroelastic ellipsometry. As shown in the inset of Figure 1(a), there is a slit in the middle of the acoustic resonator, and the elastic film is attached to the slit to form a closed photoacoustic unit. The tuning fork quartz crystal is located outside the gas chamber and coupled to the elastic film. Due to the elasticity of the elastic membrane, sound wave energy can be transferred into the air to form vibration waves, which are then transmitted to the tuning fork quartz crystal oscillator. During this process, gas molecules do not come into contact with the tuning fork quartz crystal oscillator.

[0041] Figure 1(b) is a schematic diagram of the non-contact quartz-enhanced photoacoustic spectral sensor device provided in an embodiment of the present invention. As shown in Figure 1(b), one channel of the function generator 1 generates a sine wave signal 2 with a frequency of f0 / 2, where f0 is the resonant frequency of the tuning fork. The other channel generates a sawtooth wave signal 3 with a frequency of 4 MHz. The two signals are superimposed by the adder 4 and then sent to the laser driver 5. The laser driver 5 can adjust the power and wavelength of the output light of the semiconductor laser 6 by adjusting the injection current and temperature of the semiconductor laser 6. The laser beam output by the semiconductor laser 6 is transmitted through an optical fiber to the fiber optic focusing lens 7, and the focused laser beam is incident from free space into the spectrophotometer 8.

[0042] The spectrometer 8 is shown in Figure 1(a). The gas chamber in the spectrometer consists of two gas buffers and an acoustic resonant cavity, resembling a dumbbell shape. Each gas buffer has an air hole, serving as both an outlet and an inlet. The outer sides of both gas buffers are sealed with windows made of calcium fluoride, and the gap in the acoustic resonant cavity is filled with an elastic membrane, thus forming a sealed gas chamber. The laser beam enters the gas chamber through one of the windows, passes through one of the gas buffers, enters the acoustic resonant cavity, and is focused on the elastic membrane. Finally, it exits the gas chamber through the other gas buffer. The sound wave generated by the photoacoustic effect is conducted through the elastic membrane to the tuning fork quartz crystal oscillator. The sound wave drives the lever arm of the tuning fork quartz crystal oscillator to vibrate, exciting the piezoelectric effect to generate an electrical signal. The electrical signal is transmitted through the pins of the tuning fork quartz crystal oscillator to the transimpedance preamplifier 9 for amplification, and then transmitted to the lock-in amplifier 10 for second harmonic demodulation. The synchronization signal required for demodulation is generated by function generator 1, with a frequency of f0 / 2. The signal demodulated by the lock-in amplifier is transmitted to computer 11 and recorded.

[0043] The device of this invention can monitor target gases containing high concentrations of corrosive gases and dust-containing gases in real time for extended periods.

[0044] Figure 2(a) shows a schematic diagram of the acoustic resonator after the elastic film is attached. The vibration frequency of the elastic film was analyzed using the finite element method. The length of the elastic film is approximately 1.88 mm, which is the same as half the perimeter of the acoustic resonator. The relationship between the vibration frequency of the elastic film and its width is shown in Figure 2(b). As the film width increases from 0.6 mm to 1 mm, the vibration frequency decreases monotonically. When the film width increases to 0.98 mm, the vibration frequency of the film decreases to 32.7 kHz, which matches the frequency of the tuning fork quartz crystal oscillator. Since the length of the acoustic resonator is much larger than its diameter, the acoustic resonator can be regarded as a one-dimensional longitudinal acoustic resonator. Therefore, the acoustic modes in the acoustic resonator are mainly determined by the length of the resonator. Therefore, the acoustic pressure on the elastic film with different acoustic resonator lengths was analyzed. The frequency of the sound wave was set to 32.7 kHz. As shown in Figure 2(c), the pressure on the elastic film shows a peak when the length of the acoustic resonator is 5 mm. Therefore, the optimal length of 5 mm obtained from the simulation matches half the wavelength of the 32.7 kHz sound wave. The final fabricated film has a width of 0.98 mm and an acoustic resonant cavity length of 5 mm. The elastic film is expected to resonate with the tuning fork quartz crystal oscillator, thereby enhancing the non-contact quartz-enhanced photoacoustic spectroscopy sensor.

[0045] It should be noted that the parameters of each component of the above-mentioned gas detection device can be adjusted according to actual needs. The embodiments of the present invention are only illustrated by individual examples and are not intended to limit the specific parameters of the device.

[0046] To experimentally evaluate the performance of the non-contact quartz-enhanced photoacoustic spectroscopy (GAPS) sensor, a gas sensing system based on GAPS was established, as shown in Figure 1(b). As a proof of concept, a 1.3 μm near-infrared distributed feedback laser was used to detect water vapor (H2O). A 6 mHz ramp signal and a sinusoidal signal with a frequency of f0 / 2, generated by a function generator, were added to the laser driver. The laser beam, focused by an optical fiber focuser, passed through the entire gas chamber. Pure nitrogen gas was humidified by a gas humidifier and introduced into the GAPS gas chamber. The acoustic waves generated by the photoacoustic effect were transmitted through an elastic thin film to a tuning fork quartz crystal oscillator, causing resonance. Finally, the tuning fork quartz crystal oscillator generated an electrical signal through its piezoelectric effect. The electrical signal was amplified by a custom transimpedance preamplifier with a 10 MΩ feedback resistor. A lock-in amplifier demodulated the electrical signal using its second harmonic.

[0047] The coupling between the elastic thin film and the tuning fork quartz crystal determines the performance of the non-contact quartz-enhanced photoacoustic spectroscopy sensor. Therefore, the positional effect between the elastic thin film and the tuning fork quartz crystal was investigated. To obtain the maximum torque, the acoustic resonant cavity is positioned to contact the top side of the tuning fork quartz crystal arm. The elastic thin film is aligned with one side of the tuning fork quartz crystal arm, as shown in Figure 1(a).

[0048] Figure 3(a) shows the relationship between the Q factor and resonant frequency of the tuning fork crystal oscillator and the distance between the elastic diaphragm and the tuning fork crystal pins. A distance of 0 mm means the tuning fork crystal is just touching the acoustic resonant cavity. A negative distance indicates that the tuning fork crystal is squeezing the elastic diaphragm, causing it to deform elastically. A positive distance indicates that the tuning fork crystal is moving away from the elastic diaphragm. The distance between the elastic diaphragm and the tuning fork crystal changes from 3.9 mm to -0.07 mm. As the elastic diaphragm gradually approaches the tuning fork crystal from 3.9 mm, the Q factor of the tuning fork crystal gradually decreases due to air damping.

[0049] When the elastic film comes into contact with the tuning fork quartz crystal, the Q factor and frequency of the tuning fork quartz crystal change dramatically. The Q factor decreases, while the resonant frequency increases. As the distance continues to decrease, the damping effect continues to increase, the Q factor gradually approaches 0, and the resonant frequency increases to >33kHz. The relationship between the normalized 2f signal amplitude and the coupling distance between the elastic film and the tuning fork quartz crystal is shown in Figure 3(b). Similarly, the distance between the tuning fork quartz crystal and the elastic film decreases from 3.9mm to -0.07mm. When the distance is >0mm, coupling is mainly by acoustic waves. As the distance decreases, the amplitude of the 2f signal in the quartz-enhanced photoacoustic spectrum increases monotonically. When the distance is <0mm, the tuning fork quartz crystal contacts the elastic film, and strong coupling occurs through elastic waves. The amplitude of the non-contact quartz-enhanced photoacoustic spectrum signal reaches its maximum at a distance of ~-30μm, as shown by the pentagram in Figure 3(b). After a distance of approximately -30μm, the damping effect begins to dominate, and the signal amplitude drops sharply. Finally, an optimal coupling distance of -30 μm was obtained in the non-contact quartz-enhanced photoacoustic spectroscopy sensor.

[0050] Figure 4(a) shows a comparison of the 2f signals obtained by conventional contact photoacoustic spectroscopy (CAS) and non-contact CAS (non-contact CAS). For CAS, an exposed tuning fork quartz crystal was placed in a CAS chamber filled with water vapor. For non-contact CAS, the exposed tuning fork quartz crystal was located outside the CAS chamber. The 2f signals and background noise of the two CAS sensors were measured separately. The experiment was conducted at ambient temperature and pressure. A humidifier was used to control the water vapor concentration to 2.8%. The laser temperature was set to 18.5 °C, and the input current was adjusted from 38 mA to 52.5 mA to cover the area at 7194.8 cm⁻¹. -1 The H2O absorption line has an absorption intensity of 3.07 × 10⁻⁶. -21 cm / mol. The modulation depth of the laser is 1.01 cm. -1 The amplitude of the quartz-enhanced photoacoustic spectral signal is determined by the peak value of the 2f signal. The signal amplitude obtained by non-contact quartz-enhanced photoacoustic spectroscopy is 1.52 × 10⁻⁶. -3 V is 5 times that of traditional contact quartz-enhanced photoacoustic spectrum.

[0051] The background noise of conventional contact quartz-enhanced photoacoustic spectroscopy and non-contact quartz-enhanced photoacoustic spectroscopy are shown in Figure 4(b). When measuring the background noise, the laser wavelength was modulated to be far from the absorption line of H₂O. The noise value was obtained by calculating the 1σ standard deviation of the noise point. The 1σ noise of the non-contact quartz-enhanced photoacoustic spectroscopy is 0.38 times lower than that of the contact quartz-enhanced photoacoustic spectroscopy. The final results show that the signal-to-noise ratio of the non-contact quartz-enhanced photoacoustic spectroscopy is 13 times higher than that of the conventional contact quartz-enhanced photoacoustic spectroscopy.

[0052] Finally, the long-term stability of the non-contact quartz-enhanced photoacoustic spectroscopy sensor was demonstrated by calculating the Allen variance. Figure 5 As shown. A total of over 10,000 points were measured for evaluation. With an integration time of 1 second, the detection limit for H2O was 2.8 ppm. With a longer integration time of 1,000 seconds, the detection limit was improved to 0.4 ppm. The normalized noise equivalent absorption coefficient of the non-contact quartz-enhanced photoacoustic spectroscopy sensor is 8.8 × 10⁻⁶. -9 cm -1 W Hz -1 / 2 .

[0053] There are four types of spectroscopic gas detection techniques based on tuning fork quartz crystals: direct absorption spectroscopy, wavelength modulation spectroscopy, photothermoelastic spectroscopy, and photoacoustic spectroscopy. In the first three, tuning fork quartz crystals are used as photodetectors, and the spectral detection sensitivity depends on their light absorption length. In photoacoustic spectroscopy, tuning fork quartz crystals are used as acoustic detectors. The detection sensitivity is independent of the absorption length, the sensor structure is compact, and the sensitivity is high. However, traditional tuning fork quartz crystal acoustic detectors suffer from corrosion and contamination issues in the gas chamber. Non-contact quartz-enhanced photoacoustic spectroscopy combines the advantages of the above technologies, providing a corrosion-resistant, unaffected by the detected gas, and compact sensor structure.

[0054] This invention proposes a non-contact quartz-enhanced photoacoustic spectroscopy (GAPS) for trace gas analysis, aiming to address the limitation of traditional GAPS in detecting corrosive and dusty gases due to the placement of the tuning fork quartz crystal oscillator within the gas chamber. In this non-contact GAPS, a 1 μm thick elastic film is synthesized for acoustic wave conduction. The resonant behavior of the elastic film and the acoustic resonant cavity is analyzed using FEM. Experimentally, the geometries of the elastic film and the acoustic resonant cavity are carefully designed to resonantly couple with the tuning fork quartz crystal oscillator. The positional effect during coupling is investigated in detail to optimize the performance of the non-contact GAPS. With optimized resonant behavior and positional effect, the non-contact GAPS not only demonstrates non-contact detection capabilities but also achieves a 13-fold improvement in signal-to-noise ratio. Calculations of the Allen variance indicate that the developed non-contact GAPS sensor possesses long-term stability of >1,000 seconds. Non-contact quartz-enhanced photoacoustic spectroscopy is of great significance for the detection of corrosive and dusty gases, which is precisely the current bottleneck of traditional quartz-enhanced photoacoustic spectroscopy.

[0055] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A non-contact quartz-enhanced photoacoustic spectroscopy gas detection device, characterized by, The utility model relates to a gas detection device, including: a gas chamber and a quartz tuning fork; the gas chamber includes two gas buffer zones and an acoustic resonance cavity, the acoustic resonance cavity is arranged between the two gas buffer zones and connects the two gas buffer zones, each of the two gas buffer zones is provided with a gas hole, one of which is an air inlet and the other is an air outlet, the outer side of each of the two gas buffer zones is a light-transmitting window, a slit gap is formed in the middle of the acoustic resonance cavity and is filled with an elastic film; the quartz tuning fork is arranged outside the gas chamber and does not contact the gas in the gas chamber, the distance between the tuning fork arm and the elastic film in the acoustic resonance cavity is within a preset distance range, the maximum value of the preset distance range is a positive number and the minimum value is a negative number; when laser is incident from one of the gas buffer zones, passes through the acoustic resonance cavity and is emitted from the other gas buffer zone, under the effect of the photoacoustic effect, the acoustic wave excited by the laser beam forms a standing wave in the acoustic resonance cavity, the acoustic wave signal corresponding to the standing wave is conducted through the elastic film to form a vibration wave in the air, and the vibration wave pushes the tuning fork arm to vibrate; the intensity of the vibration signal of the tuning fork arm is positively correlated with the concentration of the gas in the gas chamber, and the gas detection device can detect the concentration of the gas when the quartz tuning fork does not contact the gas and the distance between the quartz tuning fork and the elastic film is within the preset distance range.

2. The apparatus of claim 1, wherein, The elastic film is a thin and elastic film that is not air-permeable and is made of a corrosion-resistant material.

3. The apparatus of claim 1, wherein, The material of the two gas buffer zones and the acoustic resonance cavity is a corrosion-resistant material.

4. The apparatus of claim 2, wherein, The elastic film is a Paraylin film or a polyethylene film.

5. The device of any one of claims 1 to 3, wherein, The light-transmitting window on the outer side of each of the two gas buffer zones is a calcium fluoride window or a quartz window.

6. The device of any one of claims 1 to 3, wherein, The distance between the coupling point of the elastic film and the top end of the tuning fork arm is between 0mm and 2mm.

7. The device of any one of claims 1 to 3, wherein, The length of the elastic film is less than or equal to half the circumference of the acoustic resonance cavity, the width is between 0.6mm and 1mm, and the thickness is less than 10um.

8. The device of any one of claims 1 to 3, wherein, The cavity length of the acoustic resonance cavity is more than 4 times the diameter.

9. The device of any one of claims 1 to 3, wherein, The distance between the tuning fork arm and the elastic film is between 0.4mm and -0.07mm.

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