Rapid detection method for high-altitude chemical cloud cluster

Abstract

The invention provides a rapid detection method for high-altitude chemical clouds, which comprises the following steps: sampling by using a small unmanned aerial vehicle carrying a solid-phase microextraction device, taking back a sample, and detecting by using a surface acoustic wave gas chromatograph. According to the method, rapid detection is carried out through GC-SAW, the separation analysis time of a sample is shorter than 20 s, and the complete analysis time of a single sample is shorter than or equal to 90 s. According to the design of the technical scheme, the particularity and requirements of application scenes are considered, the surface acoustic wave gas chromatograph is fully optimized, the sampling and detection speed is high, multiple substance types can be recognized, and the selected SPME extraction head has hydrophobicity and does not enrich water molecules; the plant smoke is mainly particulate matters and cannot be enriched by the SPME extraction head, so that the influence of water vapor and the plant smoke can be avoided, the single SPME can be selected to be less than 40g, and the requirement on low load of the unmanned aerial vehicle is met. A large number of experiments prove that the target component can be normally detected.

Description

Technical Field

[0001] The present application relates to the technical field of atmospheric VOCs detection, and in particular to a method for rapid detection of high-altitude chemical clouds. Background Art

[0002] At present, UAVs (unmanned aerial vehicles) are widely used in atmospheric detection. Some researchers have used UAVs equipped with Suma cans to conduct high-altitude atmospheric sampling to study the vertical distribution of atmospheric VOCs. However, the UAV has a limited load capacity and can only carry 1 to 2 Suma cans per flight, making it impossible to continuously collect multiple atmospheric samples. If VOCs adsorption tubes are used, they are light in weight and small in size, and are lighter than Suma cans. UAVs can carry multiple (>10) adsorption tubes to achieve a single flight and collect multiple high-altitude atmospheric VOCs samples. However, the adsorption tubes need to be thermally desorbed, and a thermal desorber needs to be additionally equipped.

[0003] In the existing technology, there are three main gas sampling and detection technologies:

[0004] ① UAV carries Suma can sampling, GC-MS detection: Currently, it is a common method for atmospheric VOCs detection, which places high requirements on the load of UAVs; air samples are collected using pre-vacuumed Suma cans, and then the samples are enriched by condensation method, and GC-MS is used for qualitative and quantitative analysis. ② UAV carries adsorption tube sampling, GC-MS detection: The adsorbent enrichment sampling method has the advantages of low price, easy transportation and cleaning, and reusable (at least 100 times). In addition, the adsorbent enrichment sampling method is applicable to more types of compounds and collects sample bodies. However, this method is not suitable for collecting highly volatile compounds (such as C2 hydrocarbons), and the adsorption tube needs to be thermally desorbed, and a thermal desorber needs to be additionally equipped; ③ UAV carries solid phase microextraction, GC-MS detection: Limited by the detection sensitivity of GC-MS, it takes a long time to enrich samples using solid phase microextraction, and the GC-MS analysis time is also long.

[0005] Especially for the detection scenario of a combustion and explosion in a building in the city, a chemical cloud is formed above the building, the gas composition on site is complex, and the overall situation is unclear. It is necessary to preliminarily determine the type of chemical cloud, and take samples back for further analysis and testing to confirm the composition of the chemical cloud, so as to provide a basis for personnel evacuation and rescue command. The above detection scenario has the following detection requirements: 1. It is necessary to determine the type of chemical cloud; 2. Long-distance sampling to ensure personnel safety; 3. The faster the speed, the better.

[0006] The existing detection technology takes a long time. Without considering the sampling time, a single analysis time of GC-MS detection requires 30 minutes, which is far from meeting the requirements of rapid detection. Summary of the invention

[0007] The purpose of this application is to provide a rapid detection method for high-altitude chemical clouds, which uses a small drone equipped with a solid phase microextraction device to collect samples, and after retrieving the samples, they are detected by a surface acoustic wave gas chromatograph (hereinafter referred to as GC-SAW). This method uses GC-SAW for rapid detection, and the separation and analysis time of the samples is less than 20s, and the complete analysis time of a single sample is ≤90s.

[0008] The rapid detection method comprises the following steps:

[0009] 1) A small drone equipped with a solid phase microextraction device flies in the suspicious direction and uses the drone camera to observe and determine the location of the cloud;

[0010] 2) After reaching the cloud, hover for 10s to 1min, and the solid phase microextraction device performs enrichment sampling;

[0011] 3) After sampling, the drone returns and the solid phase microextraction device is removed;

[0012] 4) Rapidly test the enriched samples using a surface acoustic wave gas chromatograph to determine the composition of the chemical substances leaked in the cloud.

[0013] For multiple mixed components with large differences in sensitivity, the detection sensitivity is adjusted by adjusting the temperature and sampling time of the detector, and two tests are performed to complete the identification of the components. The two tests include high-sensitivity condition detection and low-sensitivity condition detection, and the analysis time is ≤3min.

[0014] As an improvement of the above technical solution, the solid phase microextraction device is composed of 2 to 4 extraction needles, which are installed directly above or on the side of the small drone to achieve enrichment sampling of different components. A commercial SPME (solid-phase micro extraction) extraction device can also be used. After removing its casing, the extraction fiber head is pushed out and directly installed directly above or on the side of the small drone, which can further reduce the weight and reduce the load requirements for the drone. Preferably, the extraction needle uses a 75μm CAR / PDMS extraction head as the extraction head for acrylonitrile sampling.

[0015] As another improvement of the above technical solution, the surface acoustic wave gas chromatograph includes: a gas chromatography column and a surface acoustic wave detector; the gas chromatography column uses chromatographic separation technology to separate the collected gas sample to be tested, so that different gas components flow out of the chromatography column in sequence and condense on the chip surface of the surface acoustic wave detector; the surface acoustic wave detector measures the amount corresponding to each component according to the frequency change, thereby achieving quantification of different VOCs components, and at the same time, qualitative analysis of different components through retention time.

[0016] Furthermore, the surface acoustic wave detector includes a 500MHz SAW resonator, an oscillation circuit, a reference oscillator and a semiconductor refrigerator; the oscillator is mixed with the reference oscillator to convert the oscillation frequency from about 500MHz to about 1MHz.

[0017] The gas chromatography column adopts a metal capillary column. Conventional gas chromatography instruments use quartz capillary columns, and the present technology GC-SAW uses metal capillary columns. Because the chromatographic column needs to be heated during separation and analysis; programmed temperature heating is also required when analyzing complex samples; traditional gas chromatography uses a heating box to heat the chromatographic column, that is, the entire box in which the chromatographic column is located is heated. However, the present application can adopt on-column heating technology, treating the chromatographic column as a resistance wire (therefore a metal column is required), and directly heating it after power is turned on at both ends, which not only achieves heating, but also avoids the use of a large heating box, and the instrument can be miniaturized. In addition, the on-column heating technology is used, and the heating rate of the chromatographic column is faster, and the fastest can reach 20℃ / s (the traditional GC is tens of℃ / min). In addition, the length of the chromatographic column used makes the separation and analysis time of the sample short, <20s.

[0018] The gas chromatographic column adopts a polar column DB-WAX column to detect pollutants with a wide boiling range. Chromatographic columns are classified into non-polar columns, weakly polar columns, and polar columns according to the polarity of the stationary phase, and the DB-WAX involved here is a polar column. The present application can achieve simultaneous separation and detection of four components, namely, xylene, DMMP (dimethyl methyl phosphate), acrylonitrile, and dichloromethane, by selecting a polar column DB-WAX column, and the retention indexes of the four components are 930, 1010, 1170, and 1530, respectively.

[0019] The advantage of this application is that the rapid detection method of high-altitude chemical clouds uses GC-SAW to achieve rapid detection of complex gases based on the combination of drones and SPME sampling. The design of the technical solution of this application takes into account the particularity and needs of the application scenario. It not only fully optimizes the surface acoustic wave gas chromatograph, but also achieves fast sampling and detection speeds, can identify a wide variety of substances, and the selected SPME extraction head is hydrophobic and will not enrich water molecules; grass smoke is mainly particulate matter and will not be enriched by the SPME extraction head, so the influence of water vapor and grass smoke can be avoided, and a single SPME of less than 40g can be selected, which meets the requirements for low drone load. In addition, we have also verified through a large number of experiments that the target components can be detected normally.

[0020] Compared with the existing technology, its advantages are specifically reflected in:

[0021] 1) This application has low requirements for drones and does not require additional devices. The overall system is simpler and more suitable for use in emergency scenarios.

[0022] 2) The portable surface acoustic wave gas chromatograph used in this application has a shorter separation and analysis time. The separation and analysis time of most VOCs is within 20 seconds, which can provide test results faster in emergency scenarios.

[0023] 3) The single detection time is relatively shorter: Based on the shorter sampling time (5s), chromatographic separation time (20s) and other processes, when SPME sampling is combined with GC-SAW technology for detection, the single detection process time is shorter and can be as low as 90s. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The technical roadmap of the rapid detection method of high-altitude chemical clouds of the present application is shown;

[0025] Figure 2 A schematic diagram showing the structure of a surface acoustic wave gas chromatograph in a sampling state;

[0026] Figure 3 A schematic diagram showing the structure of the surface acoustic wave gas chromatograph in the analysis state;

[0027] Figure 4 A schematic diagram of the detection chromatogram of normal alkanes (C6-C18) in the embodiment of the present application is shown;

[0028] Figure 5 This is the comparison of the chromatograms after enriching acrylonitrile with different solid phase microextraction heads (compared with the blank);

[0029] Figure 6 This is the test chromatogram after DMMP gas enrichment for 1 min (detector 120°C, sampling 5 s);

[0030] Figure 7 This is the test chromatogram after acrylonitrile gas enrichment for 1 min (detector 40°C, sampling 5 s);

[0031] Figure 8 This is a comparison of the chromatograms after SPME direct enrichment detection and SPME detection on a drone in this application. DETAILED DESCRIPTION

[0032] The technical solution of the present application is described in detail below in conjunction with the accompanying drawings and embodiments. In these drawings, the same or similar components are marked with the same or similar reference numerals.

[0033] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

[0034] It should be noted that, in the description of the present invention, terms such as "up", "down", "left", "right", "inside" and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings. This is merely for the convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be understood as a limitation on the present invention.

[0035] The surface acoustic wave gas chromatograph uses a method combining rapid gas chromatography (GC) technology with surface acoustic wave (SAW) technology to quickly perform qualitative and quantitative detection of volatile organic compounds (VOCs) in ambient air. In this embodiment, the gas chromatographic column uses a metal capillary column. Figure 1 Shown is the technical roadmap for the rapid detection method of high-altitude chemical clouds in this application.

[0036] First, the collected mixed gas is separated using chromatographic separation technology, allowing different components of the mixed gas to flow out of the chromatographic column in sequence; then, they are condensed on the surface of the surface acoustic wave detector chip, causing frequency changes to measure the amount corresponding to each component, thereby achieving quantification of different VOCs components. At the same time, different components are qualitatively identified through retention time.

[0037] The whole detection process is completed in three steps. The first step is the sampling stage, such as Figure 2 As shown in the figure, in the sampling state, after the sampling pump samples through the six-way valve hole No. 6 port, the sample to be tested enters the flash tube from the injection port through the six-way valve hole No. 1 and No. 2 ports, and the target component is adsorbed by the filler in the flash tube. At the same time as sampling, the carrier gas enters the chromatographic column (stainless steel capillary column) through the six-way valve hole No. 3 and No. 4 ports, and then to the detector.

[0038] The second step is the injection stage. Figure 3 As shown in the figure, in the sample analysis state, the six-way valve is turned to another position, connecting the carrier gas cylinder, filter tube, flash tube, chromatographic column and SAW detector into a channel. The carrier gas enters the flash tube through ports 2 and 3 of the six-way valve hole. At the same time, a large current short pulse is applied to the flash tube. The flash tube is quickly heated to about 250°C to desorb the sample adsorbed on the filler and enter the chromatographic column through ports 4 and 5 of the six-way valve with the carrier gas.

[0039] In the initial stage, the chromatographic column is in a low temperature state, and the components to be measured gather at the entrance of the chromatographic column. Then, the temperature of the chromatographic column is programmed to separate the components of the injected mixed gas. The separation of compounds by the chromatographic column is achieved through the interaction between the stationary phase and the substance. The solubility of the compound and the stationary phase determines the time for the analyte to flow out of the chromatographic column, and the flow time is called the retention time. Different substances will flow out of the chromatographic column in turn, and condense on the surface of the detector chip by using the temperature difference between the outflowing gas itself and the surface of the SAW detector chip, causing the propagation speed of the surface acoustic wave to decrease, thereby causing the oscillation frequency of the SAW detector to change.

[0040] The third step is the analysis phase. Different substances will flow out of the chromatographic column in turn, and condense on the surface of the detector chip using the temperature difference between the outflowing gas itself and the surface of the SAW detector chip, causing the propagation speed of the surface acoustic wave to decrease, thereby causing the oscillation frequency of the SAW detector to change.

[0041] The SAW detector is composed of a SAW resonator of about 500MHz, an oscillation circuit and a semiconductor refrigerator. In this embodiment, the SAW resonator plus the circuit constitutes an oscillator. In order to improve the signal-to-noise ratio, the oscillator is mixed with a reference oscillator to change the oscillation frequency from about 500MHz to about 1MHz. The semiconductor refrigerator cools the detector during detection to facilitate gas adsorption on the detector surface; after the detection is completed, the detector is heated to remove the adsorbed gas. The desorbed gas is removed with a purge gas.

[0042] The frequency signal output by the SAW detector is sampled at high speed and digitized as a function of discrete time intervals. The derivative of frequency with respect to time gives the traditional chromatogram, see Figure 4 This change signal is compared with the calibration data stored in the database to determine the nature and concentration of the gas being measured.

[0043] The present application proposes a scheme for sampling by drone equipped with solid phase microextraction (SPME) and GC-SAW detection after retrieving the samples, including the following steps:

[0044] 1) Small drone equipped with SPME extraction device;

[0045] The drone is equipped with 2 to 3 SPME extraction needles (including SPME extraction fiber heads);

[0046] 2) Fly in the suspected (approximate) direction and use the drone camera to observe and determine the location of the cloud;

[0047] 3) After reaching the cloud cluster, hover for about 1 minute to conduct enrichment sampling;

[0048] 4) After sampling, the drone returns;

[0049] 5) Remove the SPME extraction needle and perform rapid detection by GC-SAW.

[0050] GC-SAW can quickly detect the samples enriched by SPME, with a detection time of ≤90s, and can quickly determine the composition of the chemical substances leaked in the cloud;

[0051] 6) For several limited organic substances, they can be quickly distinguished (detected once), and the analysis time is ≤90s;

[0052] 7) For multiple mixed components with large differences in sensitivity, the components can be identified by two tests (high sensitivity and low sensitivity conditions), and the analysis time is ≤3min.

[0053] in,

[0054] (1) Selection of SPME extraction heads. SPME extraction heads have different enrichment effects on different substances. For different detection targets, it is necessary to select SPME extraction heads in a targeted manner.

[0055] It is very important for the technical solution of this application to select a suitable SPME extraction head to achieve enrichment sampling of different components such as xylene, DMMP and acrylonitrile. The data show that most SPME extraction heads have good enrichment effects on DMMP and xylene; there are reports that 100μm PDMS, 75μm CAR / PDMS or 85μm PA can be used to enrich acrylonitrile. Through a lot of theoretical research and experimental verification, we found that only the 75μm CAR / PDMS extraction head showed a good enrichment effect. After 1 minute of enrichment, the obvious target peak can be detected, while the test results of other extraction heads after enrichment are close to the blank test. Therefore, the 75μm CAR / PDMS extraction head was finally selected as the extraction head for acrylonitrile sampling, such as Figure 4 shown.

[0056] (2) The GC-SAW detection effect is not ideal for some light components (low boiling point components) of key concern. By selecting chromatographic columns with different polarities, the detection of pollutants with a wide boiling range can be achieved.

[0057] GC-SAW uses retention time (or retention index) for qualitative analysis. Generally speaking, different substances have different retention effects on the chromatographic column, that is, different retention times. The components that are focused on in this detection scenario are xylene, DMMP, acrylonitrile, and dichloromethane. When GC-SAW uses the general-purpose chromatographic column DB-5 (weak polarity column) for detection, the retention indexes of xylene and DMMP are 850 and 900, respectively. The difference between the two is large, and separation and analysis can be achieved; however, acrylonitrile (boiling point 77.3°C) and dichloromethane (boiling point 39.8°C) have low boiling points and poor retention effects on the chromatographic column. The measured retention indexes of the two are in the range of 590 to 600, which are very close. The two cannot be separated, so qualitative analysis cannot be performed.

[0058] We conducted theoretical analysis and experimental verification by combining instrument usage experience and available database data, and finally selected the polar column DB-WAX column, which can achieve simultaneous separation and detection of four components: xylene, DMMP, acrylonitrile and dichloromethane. The retention indices of the four components are 930, 1010, 1170 and 1530, respectively, as shown in the table below.

[0059] Table 1 Retention index information of four components, xylene, DMMP, acrylonitrile and dichloromethane, detected on DB-WAX column

[0060]

[0061]

[0062] In addition, the measured results of the DB-WAX column showed that the retention indices of acrylonitrile and dichloromethane were consistent with the database query results and could be completely separated.

[0063] (3) GC-SAW detection sensitivity can be flexibly adjusted to detect components with different boiling points

[0064] The detection sensitivity of GC-SAW is inversely proportional to the volatility of the substance being tested, and has a higher detection sensitivity for components with low volatility (high boiling point). For specific test substances, the detection sensitivity can be adjusted by adjusting the temperature of the detector and the sampling time (thermal desorption time).

[0065] For example, for DMMP and xylene with slightly higher boiling points, the detector temperature is recommended to be set at 100℃~140℃, and the sampling time is 2~5s. If the detector temperature is set too low, high-concentration samples will cause the detector to overload, affecting the accurate determination of the substance.

[0066] For acrylonitrile and dichloromethane with low boiling points, it is recommended to set the detector temperature at 30-50°C and increase the sampling time to 5-10s.

[0067] (4) The solid phase microextraction device further reduces weight, significantly reducing the payload requirements of the UAV.

[0068] In this embodiment, a commercial SPME handle and extraction head are selected. When mounted on a drone, the sleeve is further removed to reduce the overall weight of the SPME handle. The weight of a single SPME handle is reduced by 1 / 2 (only about 20 g).

[0069] (5) When the drone is close to the pollutant cloud for sampling, the wind speed is high when the wings rotate, which may affect the enrichment effect of SPME. Through experimental verification, the SPME enrichment effect is less affected by using a small drone and installing the SPME directly above or on the side of the drone.

[0070] The experimental method verified the influence of wind on the SPME enrichment effect when the wings rotated after the SPME was loaded on the drone. The results showed that after the drone was started, the target component response was reduced to 82.5% of the original. Affected by the wind, the SPME enrichment effect was reduced, but the response was still high, and there was no significant effect on the overall collection and detection of the target components.

[0071] Figure 4 A schematic diagram of the chromatogram for detecting normal alkanes (C6 to C18) in an embodiment of the present application is shown.

[0072] Figure 5 This is a comparison of the test chromatograms after enriching acrylonitrile with different solid phase microextraction heads (compared with blank). The effects of three solid phase microextraction fiber heads, 100μm PDMS, 75μm CAR / PDMS, and 85μm PA, on enriching acrylonitrile were compared. After the two extraction heads, 100μm PDMS and 85μm PA, enriched acrylonitrile gas, the GC-SAW test results were no different from the blank test results, indicating that these two extraction heads had poor acrylonitrile enrichment effects; and after 75μm CAR / PDMS enriched acrylonitrile, the target peak was detected at the acrylonitrile peak position, indicating that this extraction head is suitable for enriching acrylonitrile. The above experimental comparison results provide a basis for the selection of extraction heads for acrylonitrile sampling.

[0073] Figure 6 This is the test chromatogram after DMMP gas enrichment for 1 min (detector 120°C, sampling 5 s); Figure 7 This is the chromatogram after acrylonitrile gas enrichment for 1 minute (detector 40°C, sampling 5 seconds); In summary, due to the characteristics of the SAW detector, the detection sensitivity of GC-SAW is inversely proportional to the volatility of the substance, that is, it is highly sensitive to substances with poor volatility and low sensitive to substances with good volatility. But at the same time, the temperature of the detector directly affects its detection sensitivity. The lower the detector temperature, the higher the sensitivity. Therefore, the detection of components with different boiling points can be achieved by adjusting the detector temperature for different components.

[0074] If the scene is a chemical emergency, the concentration of leaked chemical gases is generally high. Therefore, for components with higher boiling points, use a high detector temperature to detect, such as 120°C for DMMP; for components with lower boiling points, use a low detector temperature to detect, such as 40°C or lower for acrylonitrile (corresponding to Figure 7 ).

[0075] like Figure 8 As shown, the comparison of the chromatograms after direct SPME enrichment detection and SPME detection on a drone in the present application verifies the feasibility of the technical solution of the present application.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present application.

Claims

1. A rapid detection method for high-altitude chemical clouds, which uses a small drone equipped with a solid phase microextraction device to collect samples. After the samples are retrieved, they are quickly detected using a surface acoustic wave gas chromatograph.

2. The rapid detection method of high-altitude chemical clouds according to claim 1, characterized in that: The steps include: 1) A small drone carrying a solid phase microextraction device flies towards the suspicious direction, and the drone camera is used to observe and determine the location of the cloud; 2) After reaching the cloud, hover for 10s to 1min, and the solid phase microextraction device performs enrichment sampling; 3) After sampling, the drone returns and the solid phase microextraction device is removed; 4) Rapidly test the enriched samples using a surface acoustic wave gas chromatograph to determine the composition of the chemical substances leaked in the cloud.

3. The rapid detection method of high-altitude chemical clouds according to claim 1 or 2, characterized in that: For multiple mixed components with large differences in sensitivity, the detection sensitivity is adjusted by adjusting the temperature and sampling time of the detector, and two tests are performed to complete the identification of the components. The two tests include high-sensitivity condition detection and low-sensitivity condition detection.

4. The rapid detection method of high-altitude chemical clouds according to claim 1 or 2, characterized in that: The solid phase microextraction device comprises 2 to 4 extraction needles, which are installed just above or on the side of the small UAV.

5. The rapid detection method of high-altitude chemical clouds according to claim 4, characterized in that: The extraction needle adopts a 75 μm CAR / PDMS extraction head as the extraction head for acrylonitrile sampling.

6. The rapid detection method of high-altitude chemical clouds according to claim 1, characterized in that: The surface acoustic wave gas chromatograph comprises: a gas chromatographic column and a surface acoustic wave detector; The gas chromatography column uses chromatographic separation technology to separate the collected gas samples to be tested, so that different gas components flow out of the chromatographic column in sequence and condense on the chip surface of the surface acoustic wave detector; the surface acoustic wave detector measures the amount corresponding to each component according to the frequency change, thereby achieving the quantification of different VOCs components, and at the same time, qualitative analysis of different components through retention time.

7. The rapid detection method of high-altitude chemical clouds according to claim 6, characterized in that: The surface acoustic wave detector comprises a 500MHz SAW resonator, an oscillation circuit, a reference oscillator and a semiconductor refrigerator; the resonator and the reference oscillator are mixed to convert the oscillation frequency from 500MHz to 1MHz.

8. The rapid detection method of high-altitude chemical clouds according to claim 6, characterized in that: The gas chromatographic column adopts a metal capillary column.

9. The rapid detection method of high-altitude chemical clouds according to claim 6, characterized in that: The gas chromatographic column adopts a polar column DB-WAX column.

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