Modulated photoionization detector and trace gas detection method

Abstract

The invention discloses a modulation type photoionization detector and a trace gas detection method, and relates to the technical field of gas detection. According to the modulation type photoionization detector provided by the invention, carrier gas and sample gas can alternately enter the ionization chamber at a preset frequency through switching of the three-way electromagnetic valve, ion current signals generated by the carrier gas and the sample gas are respectively detected, and a signal response value of gas to be detected can be obtained by subtracting the ion current signals measured in two adjacent times; based on the modulation type photoionization detector, the gas is detected by utilizing a differential noise reduction principle, interference of a small amount of impurity gas and water vapor in carrier gas on a signal response value of the gas to be detected is avoided, and the modulation type photoionization detector is suitable for photoionization detection of trace gas; noise caused by factors such as ultraviolet lamp power fluctuation, environment temperature and humidity changes and circuit thermal changes is effectively filtered out, and the detection signal-to-noise ratio is increased.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, and in particular to a modulated photoionization detector and a method for detecting trace gases. Background Technology

[0002] Photoionization detectors (PIDs) use ultraviolet light emitted by a vacuum ultraviolet lamp (VUV) to photoionize volatile organic compounds (VOCs), and then measure the ion current generated during the ionization process to achieve trace detection (ppb~ppm level). PIDs have advantages such as broad spectrum response (covering 2000+ VOCs), non-destructive detection, and fast response (second level), and are therefore widely used in environmental monitoring, industrial safety, and medical breath analysis.

[0003] However, traditional PID controllers use a single gas path to continuously supply sample gas. Because the ionization chamber is in direct contact with the gas to be measured for a long time, factors such as UV lamp power fluctuations, changes in ambient temperature and humidity, and circuit thermal noise can cause significant baseline drift (±5%FS) and a signal-to-noise ratio (SNR) of less than 20dB. Moreover, the background current generated by the ionization of a small amount of impurity gas and water vapor in the carrier gas can mask the trace VOCs signal, making traditional PID controllers significantly in ppb-level trace detection. Some improved PID controllers usually only optimize the sealing structure to prevent gas contamination, without solving the problem of background current interference, or rely on post-processing algorithms to compensate and correct the response signal, which cannot eliminate the noise of the signal source. Summary of the Invention

[0004] This invention provides a modulated photoionization detector and a method for detecting trace gases, aiming to achieve effective detection of trace gases and improve the signal-to-noise ratio of gas photoionization detection.

[0005] The modulated photoionization detector provided by this invention includes a carrier gas inlet, a sample gas inlet, an exhaust port, and:

[0006] Ionization chamber;

[0007] A three-way solenoid valve has a first port, a second port, and a third port;

[0008] The first flow channel includes a first confluence section, an ionization section, and a first venting section connected in sequence. The ionization section is located inside the ionization chamber, and the first venting section is connected to the venting port.

[0009] The second flow channel includes a first air intake section and a second air intake section; the first air intake section connects the carrier gas inlet and the first port; the upstream of the second air intake section is connected to the second port, and the downstream is connected to the first confluence section;

[0010] The third flow channel includes a third air inlet section and a fourth air inlet section; the third air inlet section connects the sample gas inlet and the upstream of the fourth air inlet section, and the downstream of the fourth air inlet section intersects with the second air inlet section and the first confluence section at a first confluence node;

[0011] The fourth flow channel includes a second venting section and a bypass section; the second venting section connects the third port and the venting outlet; one end of the bypass section intersects with the third intake section and the fourth intake section at a second junction node, and the other end is connected to the third junction node on the second venting section;

[0012] The ionization chamber is configured to ionize the gas in the ionization section; the three-way solenoid valve is configured to alternately open the first port and the second port, and the first port and the third port at a preset frequency.

[0013] Optionally, the modulated photoionization detector further includes a second busbar; the first drain section, the second drain section and the second busbar converge at a fourth junction node; the upstream of the second busbar is connected to the fourth junction node, and the downstream is connected to the drain port.

[0014] Optionally, the preset frequency is 10~50Hz.

[0015] Optionally, the volume of the ionization section is ≤5μL.

[0016] Optionally, the ionization chamber includes electrodes; the electrodes are used to detect the ion current signal of the ionization section, and the surface of the electrodes is coated with a polytetrafluoroethylene film and / or a silicon dioxide film.

[0017] Optionally, the modulated photoionization detector further includes a lock-in amplifier; the lock-in amplifier is used to extract the alternating ion current signal that has the same frequency as the preset frequency from the ion current signal.

[0018] Based on the above-mentioned modulated photoionization detector, the present invention provides a method for detecting trace gases, comprising the following steps:

[0019] S1, Carrier gas is introduced into the carrier gas inlet and sample gas is introduced into the sample gas inlet; the inlet flow rate of the carrier gas inlet is greater than the inlet flow rate of the sample gas inlet.

[0020] S2, control the three-way solenoid valve to open the first port and the second port, the carrier gas flows through the ionization chamber, and the corresponding ion current signal is recorded, which is denoted as the first ion current signal;

[0021] S3, control the three-way solenoid valve to open the first port and the third port, the sample gas flows through the ionization chamber, and the corresponding ion current signal is recorded, which is recorded as the second ion current signal;

[0022] S4, the response signal of the gas to be measured is obtained by subtracting the second ion current signal from the first ion current signal.

[0023] S5. Repeat steps S2 to S4 at a preset frequency to fit the response signal of the gas to be tested into a response curve.

[0024] Optionally, the inlet flow rate of the carrier gas inlet is 5~10 ml / min, and the inlet flow rate of the sample gas inlet is 1~2 ml / min.

[0025] Optionally, the carrier gas is nitrogen.

[0026] Optionally, the sample gas is a mixture of carrier gas and analyte gas, wherein the analyte gas includes volatile organic compounds.

[0027] The present invention has the following beneficial effects:

[0028] The modulated photoionization detector provided by this invention allows carrier gas and sample gas to alternately enter the ionization chamber through the switching of a three-way solenoid valve. The specific implementation principle is as follows: the inlet flow rate of the carrier gas is greater than the inlet flow rate of the sample gas. When the three-way solenoid valve is open at the first and second ports, the carrier gas flows through the second and first flow channels into the ionization chamber. At this time, the sample gas can only be discharged along the bypass section and the second venting section at the second junction due to the pressure difference. When the three-way solenoid valve is open at the first and third ports, the carrier gas is discharged through the second venting section. At the second junction, the sample gas flows towards the fourth inlet section and the first flow channel due to the pressure difference, thus flowing through the ionization chamber.

[0029] This invention is based on the aforementioned modulated photoionization detector. Carrier gas and sample gas are driven alternately into the ionization chamber at a preset frequency. The ion current signals generated by the carrier gas and sample gas are detected separately. The signal response value of the gas to be tested is obtained by subtracting the ion current signals from two adjacent measurements. Therefore, this invention utilizes the principle of differential noise reduction to avoid interference from small amounts of impurity gases and water vapor in the carrier gas on the signal response value of the gas to be tested. This enables trace detection of gases and effectively filters out noise caused by factors such as ultraviolet lamp power fluctuations, changes in ambient temperature and humidity, and circuit thermal changes, thereby improving the signal-to-noise ratio of gas photoionization detection. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1These are schematic diagrams of some embodiments of the modulated photoionization detector of the present invention;

[0032] Figure 2 This is a schematic diagram illustrating the working principle of some embodiments of the modulated photoionization detector of the present invention;

[0033] Figure 3 This is a flowchart of some embodiments of the trace gas detection method of the present invention.

[0034] Explanation of reference numerals in the attached diagram: 1. Ionization chamber; 2. Three-way solenoid valve. Detailed Implementation

[0035] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification and drawings of this invention are used to distinguish similar objects and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features, nor are they describing a particular order or sequence. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus. In the description of this invention, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0037] See Figure 1 In some embodiments, the modulated photoionization detector provided by the present invention includes a carrier gas inlet, a sample gas inlet, an exhaust port, an ionization chamber 1, a three-way solenoid valve 2, and a gas flow channel, wherein the gas flow channel includes a first flow channel, a second flow channel, a third flow channel, and a fourth flow channel.

[0038] Specifically, the first flow channel includes a first confluence section (ab), an ionization section (bc), and a first exhaust section (cf) connected in sequence; the first confluence section (ab) is used to introduce gas into the ionization chamber 1; the ionization section (bc) is located inside the ionization chamber; the first exhaust section (cf) is connected to the exhaust port; the ionization chamber 1 typically includes at least a light source and electrodes, the gas flowing through the ionization section (bc) is ionized under the irradiation of a light source of a specific wavelength (e.g., ultraviolet), and the electrodes are configured to detect the ion current signal generated by the ionization section (bc).

[0039] The specific structure and working principle of the ionization chamber can be found in the patent application number "202410985530.X" entitled "A High-Sensitivity Microchannel Photoionization Detector". It should be noted that the patent application number "202410985530.X" is only an example. Other structures of ionization chambers can also be used in this invention. The specific structure of the ionization chamber only needs to meet the requirements of photoionization detection. In some specific embodiments, the modulation photoionization detector provided by this invention can be fabricated using MEMS technology to process silicon wafers and create microchannels as micro ionization chambers.

[0040] In some preferred embodiments, to avoid signal attenuation caused by the deposition of high-boiling-point components (e.g., siloxanes) in the sample gas, the surface of the electrode is coated with a polytetrafluoroethylene (PTFE) film and / or a silica film. The PTFE film has low surface energy and non-stick properties, making it difficult for droplets or solid particles of high-boiling-point components to adhere. Even if a small amount appears, it is easily carried away by the gas flow and cannot be stably deposited on the electrode. The silica film has high density and strong chemical inertness, which can isolate the electrode from high-boiling-point components, thereby avoiding chemical adsorption and reducing the probability of deposition. The surface of the electrode can be coated with either a PTFE film or a silica film, or a combination of coatings or mixed coatings. In some specific embodiments, the coating thickness is about 10 nm.

[0041] In some preferred embodiments, the volume of the ionization section in the ionization chamber is ≤5μL. The smaller volume can improve the gas ionization efficiency, enhance the response speed and detection sensitivity of photoionization detection, ensure that the gas replacement speed in the ionization chamber is extremely fast, reduce the residual influence of the previous ventilation on the subsequent ventilation, and also meet the miniaturization and portability requirements of the photoionization detector.

[0042] A three-way solenoid valve is a valve that controls the flow direction of fluid via electromagnetic signals and has three fluid ports; such as Figure 1 As shown, in this embodiment of the invention, the three-way solenoid valve 2 includes a first port (COM), a second port (NO), and a third port (NC). The first port (COM) is a common air inlet, and the second port (NO) and the third port (NC) are two different air outlets.

[0043] The second flow channel is used to introduce carrier gas into the ionization chamber, including a first inlet section and a second inlet section (NO-a); the first inlet section connects the carrier gas inlet and the first port (COM); the upstream of the second inlet section (NO-a) is connected to the second port (NO), and the downstream is connected to the first confluence section (ab).

[0044] The third flow channel is used to introduce sample gas into the ionization chamber, including the third inlet section and the fourth inlet section (da); the third inlet section connects the sample gas inlet and the upstream of the fourth inlet section (da), and the downstream of the fourth inlet section (da) intersects with the second inlet section (NO-a) and the first confluence section (ab) at the first confluence node (a).

[0045] To enable the carrier gas and sample gas to alternately enter the ionization chamber, this embodiment of the invention designs a fourth flow channel; the fourth flow channel includes a second vent section (NC-f) and a bypass section (de); the second vent section (NC-f) connects the third port (NC) and the vent outlet; one end of the bypass section (de) intersects with the third inlet section and the fourth inlet section (da) at a second junction node (d), and the other end is connected to the third junction node (e) on the second vent section (NC-f).

[0046] In some specific embodiments, the gas flow channel further includes a second confluence section; the first vent section (cf), the second vent section (NC-f) and the second confluence section intersect at the fourth confluence node (f), the upstream of the second confluence section is connected to the fourth confluence node (f), and the downstream is connected to the vent outlet.

[0047] See Figure 2 In this embodiment of the invention, the principle of the modulated photoionization detector in alternately introducing carrier gas and sample gas into the ionization chamber is as follows: carrier gas is introduced from the carrier gas inlet ( Figure 2 (Represented in blue), sample gas is introduced through the sample gas inlet. Figure 2 (Indicated in red), the carrier gas inlet flow rate must be greater than the sample gas inlet flow rate; see reference. Figure 2 In section A, the three-way solenoid valve is open at the first port (COM) and the second port (NO). At the second junction (d), the pressure of the carrier gas is greater than that of the sample gas. Due to the pressure difference, the sample gas can only flow along the bypass section (de) to the third junction (e), and finally is vented through the second venting section (NC-f). At this point, only the carrier gas flows through the ionization chamber. (See also...) Figure 2In section B, the three-way solenoid valve is open at the first port (COM) and the third port (NC). At this time, at the second junction node (d), the pressure of the carrier gas is greater than the pressure of the sample gas. Due to the pressure difference, the sample gas moves along the fourth inlet section (da) to the first junction node (a) and then flows into the first flow channel. At this time, only the sample gas flows through the ionization chamber. By switching the opening state of the three-way solenoid valve at a preset frequency, that is, alternately opening the first port (COM) and the second port (NO), and the first port (COM) and the third port (NC), the carrier gas and the sample gas can be driven to alternately enter the ionization chamber 1.

[0048] Based on the modulated photoionization detector provided in this embodiment of the invention, the carrier gas and sample gas are driven to alternately enter the ionization chamber, and the ion current signals generated by the carrier gas and sample gas are detected respectively. The signal response value of the gas to be tested can be obtained by subtracting the two measured ion current signals. This avoids the interference of a small amount of impurity gas and water vapor in the carrier gas on the signal response value of the gas to be tested, and can realize trace gas detection. In actual testing, the detection limit of the modulated photoionization detector for volatile organic compounds (VOCs) can reach 0.1 ppb (volatile organic compounds are benzene).

[0049] In some specific embodiments, the modulated photoionization detector also includes a lock-in amplifier, which is used to extract the alternating ion current signal with the same frequency as the preset frequency in the ion current signal. This allows the response value of the gas signal generated during the detection time to be fitted and plotted as a response curve. This effectively filters out noise caused by factors such as ultraviolet lamp power fluctuations, changes in ambient temperature and humidity, and circuit thermal changes, thereby improving the signal-to-noise ratio of gas photoionization detection.

[0050] In some preferred embodiments, the preset frequency for the three-way solenoid valve to switch the flow direction is 10~50Hz. If the switching frequency is too low, it may result in insufficient data points to be collected and the response curve cannot be plotted. If the switching frequency is too high, it will lead to an increase in system noise.

[0051] Specifically, see Figure 3 In some embodiments, the method for detecting trace gases using a modulated photoionization detector includes steps S1 to S5:

[0052] S1, introduce carrier gas into the carrier gas inlet and sample gas into the sample gas inlet; the sample gas is a mixture of carrier gas and the gas to be tested, and the inlet flow rate of the carrier gas inlet is greater than the inlet flow rate of the sample gas inlet.

[0053] S2 controls the three-way solenoid valve to open the first port (COM) and the second port (NO), and the carrier gas flows through the ionization chamber 1. The corresponding ion current signal is recorded and denoted as the first ion current signal Sig1.

[0054] S3 controls the three-way solenoid valve to open the first port (COM) and the third port (NC), and the sample gas flows through the ionization chamber 1. The corresponding ion current signal is recorded and denoted as the second ion current signal Sig2.

[0055] S4, the difference between the second ion current signal and the first ion current signal is used to obtain the response signal of the gas to be measured (Sig2-Sig1).

[0056] S5. Repeat steps S2 to S4 at a preset frequency to fit the response signals (Sig2-Sig1, Sig4-Sig3, Sig6-Sig5, Sig8-Sig7, ...) of the gas to be tested into a response curve.

[0057] Furthermore, based on the measured response curve, the concentration value or concentration change of the gas to be tested can be obtained according to the preset correspondence between the signal response value of the gas to be tested and the concentration of the gas to be tested (which can be obtained through calibration experiments).

[0058] It should be noted that in the embodiments of the present invention, the carrier gas is a pure gas that does not contain the gas to be tested (but may generally contain trace amounts of impurity gases and water vapor). Common types of carrier gases include nitrogen, dry and clean air, helium, etc.; the sample gas is a mixture of the carrier gas and the gas to be tested, which is the actual detection object of the modulated photoionization detector.

[0059] In some preferred embodiments, the inlet flow rate of the carrier gas inlet is 5~10 ml / min, and the inlet flow rate of the sample gas inlet is 1~2 ml / min. This inlet flow rate design can meet the pressure difference requirements of the carrier gas and the sample gas, and is also compatible with the design of miniaturized and portable modulated photoionization detectors, balancing ionization efficiency, gas replacement efficiency in the ionization chamber, practicality and detection cost.

[0060] As can be seen from the above embodiments, the present invention is based on a modulated photoionization detector. The carrier gas and sample gas are driven alternately into the ionization chamber at a preset frequency, and the ion current signals generated by the carrier gas and sample gas are detected respectively. The signal response value of the gas to be tested can be obtained by subtracting the ion current signals of two adjacent measurements. Thus, the present invention utilizes the principle of differential noise reduction to avoid interference from a small amount of impurity gas and water vapor in the carrier gas on the signal response value of the gas to be tested. It can realize trace detection of gas and effectively filter out noise caused by factors such as ultraviolet lamp power fluctuations, changes in ambient temperature and humidity, and circuit thermal changes, thereby improving the signal-to-noise ratio of gas photoionization detection.

[0061] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A modulated photoionization detector, characterized in that, Includes carrier gas inlet, sample gas inlet, vent, and: Ionization chamber; A three-way solenoid valve has a first port, a second port, and a third port; The first flow channel includes a first confluence section, an ionization section, and a first venting section connected in sequence. The ionization section is located inside the ionization chamber, and the first venting section is connected to the venting port. The second flow channel includes a first air intake section and a second air intake section; the first air intake section connects the carrier gas inlet and the first port; the upstream of the second air intake section is connected to the second port, and the downstream is connected to the first confluence section; The third flow channel includes a third air inlet section and a fourth air inlet section; the third air inlet section connects the sample gas inlet and the upstream of the fourth air inlet section, and the downstream of the fourth air inlet section intersects with the second air inlet section and the first confluence section at a first confluence node; The fourth flow channel includes a second venting section and a bypass section; the second venting section connects the third port and the venting outlet; one end of the bypass section intersects with the third intake section and the fourth intake section at a second junction node, and the other end is connected to the third junction node on the second venting section; The ionization chamber is configured to ionize the gas in the ionization section; the three-way solenoid valve is configured to alternately open the first port and the second port, and the first port and the third port at a preset frequency.

2. The modulated photoionization detector according to claim 1, characterized in that, It also includes the second busbar; The first drain section, the second drain section, and the second confluence section converge at the fourth confluence node; the upstream of the second confluence section is connected to the fourth confluence node, and the downstream is connected to the drain outlet.

3. The modulated photoionization detector according to claim 1, characterized in that, The preset frequency is 10~50Hz.

4. The modulated photoionization detector according to claim 1, characterized in that, The volume of the ionization section is ≤5μL.

5. The modulated photoionization detector according to claim 1, characterized in that, The ionization chamber includes electrodes; The electrode is used to detect the ion current signal of the ionization section, and the surface of the electrode is coated with a polytetrafluoroethylene film and / or a silicon dioxide film.

6. The modulated photoionization detector according to claim 5, characterized in that, It also includes lock-in amplifiers; The lock-in amplifier is used to extract the alternating ion current signal that has the same frequency as the preset frequency from the ion current signal.

7. A method for detecting trace gases, characterized in that, The trace gas detection method using the modulated photoionization detector according to any one of claims 1-6 includes the following steps: S1, Carrier gas is introduced into the carrier gas inlet and sample gas is introduced into the sample gas inlet; the inlet flow rate of the carrier gas inlet is greater than the inlet flow rate of the sample gas inlet. S2, control the three-way solenoid valve to open the first port and the second port, the carrier gas flows through the ionization chamber, and the corresponding ion current signal is recorded, which is denoted as the first ion current signal; S3, control the three-way solenoid valve to open the first port and the third port, the sample gas flows through the ionization chamber, and the corresponding ion current signal is recorded, which is recorded as the second ion current signal; S4, the response signal of the gas to be measured is obtained by subtracting the second ion current signal from the first ion current signal; S5. Repeat steps S2 to S4 at a preset frequency to fit the response signal of the gas to be tested into a response curve.

8. The trace gas detection method according to claim 7, characterized in that, The inlet flow rate of the carrier gas is 5~10 ml / min, and the inlet flow rate of the sample gas is 1~2 ml / min.

9. The trace gas detection method according to claim 7, characterized in that, The carrier gas is nitrogen.

10. The trace gas detection method according to claim 7, characterized in that, The sample gas is a mixture of carrier gas and test gas, and the test gas includes volatile organic compounds.

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