Rapid detection method for volatile organic compounds

Abstract

The invention discloses a rapid detection method for volatile organic compounds, and relates to the field of environmental detection.The method comprises the steps that the carrier gas flow of a gas inlet pipeline is adjusted from the initial flow to the measured flow, and the flow speed of volatile organic compound sample gas is adjusted; the method comprises the following steps: adsorbing volatile organic compound sample gas in a pre-concentration pipe based on flow rate control of the volatile organic compound sample gas; when the temperature of the pre-concentration tube reaches a target temperature, the volatile organic compound sample gas is separated from the pre-concentration tube and enters the chromatographic column along with the carrier gas; controlling the temperature of the chromatographic column by utilizing a constant-temperature control mode and a proportional heating mode; through temperature control of the chromatographic column, different substances in the volatile organic compound sample gas are gradually separated from the chromatographic column and enter a photoionization detector; and detecting the components of different substances in the volatile organic compound sample gas by using a photoionization detector. The photoionization detector and the gas chromatography are combined, so that the detection efficiency of the volatile organic compounds is improved.

Description

Technical Field

[0001] The present invention relates to the field of environmental detection, and in particular to a method for rapid detection of volatile organic compounds. Background Art

[0002] Volatile organic compounds (VOCs) not only have adverse effects on the ecosystem, but also cause certain harm to the human body. Therefore, increasing the control of VOCs in the atmosphere is an important part of the current environmental protection cause. Gas chromatography (GC) has gradually become a common method for online monitoring of VOCs at home and abroad due to its high detection accuracy, good stability and wide application range.

[0003] In recent years, the gas chromatograph industry has developed rapidly, and the market demand for low-concentration volatile organic compound monitoring in the field of environmental testing has shown an upward trend. The photoionization detector (PID) is small in size and fast in detection speed, but the traditional PID is difficult to monitor multi-component VOCs qualitatively and quantitatively, and gas chromatography can separate compounds, which can better make up for this shortcoming of PID. However, the traditional gas chromatography column oven is large in size and uses an air bath to heat the chromatographic column, which consumes a lot of power and has poor cooling performance, thus limiting the portability of the instrument. At the same time, how to effectively combine the photoionization detector and gas chromatography to quickly detect volatile organic compounds is an urgent problem to be solved. Summary of the invention

[0004] In view of the problems in the related art, the present invention proposes a method for rapid detection of volatile organic compounds to overcome the above technical problems existing in the existing related art.

[0005] To this end, the specific technical solution adopted by the present invention is as follows:

[0006] A method for rapid detection of volatile organic compounds, the method comprising:

[0007] S1. According to the measurement command, the carrier gas flow rate of the air inlet pipeline is adjusted from the initial flow rate to the measurement flow rate, and the flow rate of the volatile organic compound sample gas is adjusted based on the state parameters in the pre-concentrator;

[0008] S2, based on the flow rate control of the volatile organic matter sample gas, the volatile organic matter sample gas is adsorbed in the pre-concentration tube;

[0009] S3. When the temperature of the pre-concentrator reaches the target temperature, the volatile organic sample gas separates from the pre-concentrator and follows the carrier gas with a measured flow rate into the chromatographic column; the temperature of the chromatographic column is controlled by a constant temperature control method and a proportional heating method;

[0010] S4. By controlling the temperature of the chromatographic column, different substances in the volatile organic sample gas are gradually separated from the chromatographic column and enter the photoionization detector; the photoionization detector is used to detect the components of different substances in the volatile organic sample gas.

[0011] Further, according to the measurement command, adjusting the carrier gas flow rate of the air inlet pipeline from the initial flow rate to the measurement flow rate, and adjusting the flow rate of the volatile organic compound sample gas based on the state parameters in the pre-concentrator tube includes:

[0012] After the measurement command is triggered, the carrier gas flow rate of the air inlet pipeline is adjusted from the initial flow rate to the measurement flow rate;

[0013] Obtaining state parameters in the pre-concentrator tube by means of an electronic flow meter, wherein the state parameters include pressure and temperature;

[0014] Adjust the flow rate of the volatile organic compound sample gas according to the state parameters in the pre-concentration tube.

[0015] Furthermore, the temperature control of the chromatographic column by using the constant temperature control method and the proportional temperature increase method includes:

[0016] The control parameters of the PID controller are adjusted based on fuzzy logic, and the chromatographic column is temperature-controlled according to the PID controller;

[0017] The incremental PID is used in combination with the temperature compensation control method to increase the temperature of the chromatographic column proportionally;

[0018] The control parameters of the PID controller adjusted based on fuzzy logic include a quantization process, a membership calculation process, a fuzzy output calculation process and a dequantization process.

[0019] Furthermore, the quantification process includes:

[0020] When the temperature input value of the quantization process is less than or equal to all the level arrays, the first preset value is returned; when the temperature input value of the quantization process is greater than all the level arrays, the second preset value is returned;

[0021] When the temperature input value of the quantization process is within the range of the quantization level, all level arrays are traversed. If the current temperature input value is less than or equal to the current quantization level, the current temperature input value is adjusted to the quantization value before the current quantization level.

[0022] Furthermore, the calculation formula of the quantization value is:

[0023] dat in =2.0*i-6.0+2.0*[d in -p quantiz (i)] / [p quantiz(i)-p quantiz (i-1)]

[0024] In the formula, dat in Represents a quantized value;

[0025] d in Indicates the temperature input value;

[0026] p quantiz (i) represents the i-th quantization level, p quantiz (i-1) represents the i-1th quantization level.

[0027] Furthermore, the membership calculation process includes:

[0028] Determine whether the quantized value is less than the first preset value. If so, set the membership of each fuzzy set to one of completely belonging and completely not belonging according to the preset rule; if not, calculate the membership value of the fuzzy set; wherein the calculation formula of the membership value of the fuzzy set is:

[0029] p ms [i] = -0.5*dat in -2.0+i;

[0030] In the formula, p ms [i] represents the membership degree of the i-th fuzzy set, dat in Represents a quantized value;

[0031] According to the membership of the fuzzy set, the error membership array and the error change rate membership array are determined.

[0032] Furthermore, the process of calculating the fuzzy output includes:

[0033] Each membership level of the error and the error change rate is traversed; when the error and the error change rate are not zero, the corresponding output membership level is determined according to the fuzzy rule table, and the membership array is obtained.

[0034] Furthermore, the dequantization process includes:

[0035] Iterate through each membership value in the membership array, and multiply each membership value by the corresponding quantization level;

[0036] The weighted sum of the results of multiplying all membership values ​​by the corresponding quantization levels is performed to obtain the control parameters of the PID controller.

[0037] Furthermore, the incremental PID is used in combination with the temperature compensation control method to proportionally increase the temperature of the chromatographic column, including:

[0038] The calculation formula for the compensated temperature is:

[0039] T comp =k'×v+T k ;

[0040] Where, T comp represents the compensated temperature, k' represents the temperature compensation coefficient, v represents the instantaneous heating rate of the current temperature acquisition point, T k Indicates the current kth collected temperature.

[0041] Furthermore, the photoionization detector is used to detect the components of different substances in the volatile organic compound sample gas, including:

[0042] The volatile organic sample gas enters the photoionization detector and is ionized after being excited by ultraviolet light, generating a sample gas detection current;

[0043] The sample gas detection current is amplified to obtain a measurable voltage; the measurable voltage is converted into an analog-to-digital value to obtain a detector response value;

[0044] The response value spectrum is drawn using the detector response value; qualitative and quantitative analysis is performed based on the characteristic peaks of different substances in the volatile organic compound sample gas in the response value spectrum and in combination with the retention time and peak area of ​​different substances.

[0045] The beneficial effects of the present invention are:

[0046] (1) The present invention utilizes the different retention capacities of different substances in the chromatographic column and separates them one by one under certain temperature and carrier gas flow rate conditions. The gas molecules to be measured are ionized after being excited by ultraviolet light to generate weak current, which is converted into a measurable voltage through a preamplifier and a high-impedance resistor. The detector response value is collected using an analog-to-digital conversion chip, and a spectrum is drawn based on the response value. Different components show different characteristic peaks in the spectrum. Finally, qualitative and quantitative analysis is performed based on retention time and peak area to identify the components and calculate the concentration value.

[0047] (2) Portability and rapidity: The LTM chromatographic column of the present invention greatly reduces the volume of the chromatographic column module by winding and assembling the chromatographic column, thermistor wire, heating wire and insulating fiber. It also has the advantages of low power consumption and fast heating and cooling. Compared with common gas chromatography detectors such as hydrogen flame ionization detector, thermal conductivity detector, flame photometry detector, etc., the PID detector is more suitable for portable gas chromatography due to its small size and convenient application. The present invention combines the LTM chromatographic column with the PID detector in design, which greatly enhances the portability of the instrument. The reasonable design of the ten-way valve gas connection and detection process can also accelerate the qualitative and quantitative analysis of multi-component VOCs scenarios.

[0048] (3) The present invention combines a photoionization detector with a gas chromatograph, which can separate compounds, making up for the deficiency of traditional PID in the qualitative and quantitative monitoring of multi-component VOCs. PID has a fast detection speed and simplifies the traditional detection process, greatly accelerating the detection efficiency of VOCs.

[0049] (4) Reduce costs and save resources: The present invention controls the use of carrier gas (such as helium) by optimizing the detection process, reducing unnecessary resource consumption and reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 is a flow chart of a method for rapid detection of volatile organic compounds according to an embodiment of the present invention;

[0052] Figure 2 It is a schematic diagram of gas circuit connection in standby or measurement state;

[0053] Figure 3 It is a schematic diagram of the gas path connection in the injection state. DETAILED DESCRIPTION

[0054] To further illustrate each embodiment, the present invention provides drawings, which are part of the disclosure of the present invention and are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these contents, ordinary technicians in the field should be able to understand other possible implementations and advantages of the present invention. The components in the figures are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0055] According to an embodiment of the present invention, a method for rapid detection of volatile organic compounds is provided.

[0056] The present invention will now be further described with reference to the accompanying drawings and specific embodiments. Figure 1-Figure 3 As shown, according to the method for rapid detection of volatile organic compounds in an embodiment of the present invention, the method comprises:

[0057] S1. According to the measurement command, the carrier gas flow rate of the air inlet pipeline is adjusted from the initial flow rate to the measurement flow rate, and the flow rate of the volatile organic compound sample gas is adjusted based on the state parameters in the pre-concentrator;

[0058] S2, based on the flow rate control of the volatile organic matter sample gas, the volatile organic matter sample gas is adsorbed in the pre-concentration tube;

[0059] S3. When the temperature of the pre-concentrator reaches the target temperature, the volatile organic sample gas separates from the pre-concentrator and follows the carrier gas with a measured flow rate into the chromatographic column; the temperature of the chromatographic column is controlled by a constant temperature control method and a proportional heating method;

[0060] S4. By controlling the temperature of the chromatographic column, different substances in the volatile organic sample gas are gradually separated from the chromatographic column and enter the photoionization detector; the photoionization detector is used to detect the components of different substances in the volatile organic sample gas.

[0061] In order to facilitate understanding of the above technical solutions of the present invention, the above technical solutions of the present invention are further explained below from the perspectives of architecture and principle.

[0062] In standby mode, the ten-way valve is in Figure 2 In the state shown, the electronic pressure controller (EPC) 1 controls the carrier gas flow rate to the standby flow rate, which lasts for several minutes until the detector baseline is stable, to ensure the cleanliness of the PID detector and avoid the influence of residual air in the pipeline on the result analysis during subsequent detection. EPC2 controls the carrier gas flow rate to a fixed flow value to ensure that the gas line is clean and free of impurities, and also avoid residual air affecting the result data. In actual application, the instrument will not use large carrier gas tanks in consideration of its overall portability. In order to save carrier gas, the standby flow rate is less than the measured flow rate. At the same time, the column box is heated to the initial temperature. When the carrier gas flow rate and column box temperature are stable at the preset values, the instrument enters the ready state.

[0063] After the measurement command is triggered, the motor rotates forward and the ten-way valve switches to the injection position. Figure 3 As shown. The carrier gas flow rate controlled by EPC1 changes from the standby flow rate of 2ml / min to the measurement flow rate of 3.5ml / min. The injection pump is turned on, and the sample gas enters from the injection port. When passing through the pre-concentrator, the VOCs in the sample gas will be adsorbed on the pre-concentrator filled with activated carbon at room temperature. In order to optimize the adsorption efficiency of VOCs, an electronic flowmeter EPC3 is added between the injection pump and the connecting gas line, so that the sample gas flow rate can be adjusted in real time according to the pressure or temperature in the pre-concentrator, thereby ensuring the best adsorption effect at the corresponding pressure or temperature. After the injection is completed, the injection pump stops, the motor reverses, and the ten-way valve switches to the measurement position. The pre-concentrator is then quickly heated to the target temperature, so that the VOCs adsorbed on the pre-concentrator are quickly desorbed at high temperature and enter the chromatographic column with the carrier gas.

[0064] After the pre-concentrator tube is heated up, the chromatographic column starts to heat up programmatically, which includes constant temperature control and proportional heating. Although fuzzy PID has a slow response speed, it has good robustness and high control accuracy. Therefore, fuzzy PID is used for constant temperature control. First, fuzzy control rules and quantization parameters are set, and the fuzzy PID controller is called. According to input parameters such as error (target temperature-actual temperature) and error change, the parameters of the output PID controller (proportional Kp, integral Ki, differential Kd) are adjusted through fuzzy logic, which specifically includes quantization, calculation of membership, fuzzy output, and inverse quantization process.

[0065] During the quantization process, a bounds check is first performed. If the input value d in (error or error change) is less than or equal to the first data in the quantization level array, then -6.0 is returned directly. in If the input value is greater than all quantization levels, 6.0 is returned, which means that the input value is outside the range of quantization levels. Then a for loop is used to traverse the quantization level array p quantiz , inside the loop, if the input value d in Less than or equal to the current quantization level p quantiz (i), that is, the i-th quantization level, the following calculation is performed:

[0066] 2.0*i-6.0+2.0*[d in -p quantiz (i)] / [p quantiz (i)-p quantiz (i-1)]

[0067] The above formula calculates the linear interpolation of the current input value between two quantization levels, which maps the input value to a quantization value between the two quantization levels, thereby obtaining the quantization values ​​of the error and the error change respectively.

[0068] In the process of calculating the membership, the array storing the membership value is first cleared. If the input quantization value dat in is less than or equal to -6.0, then the membership of the first fuzzy set is set to 1.0, and the membership of the other fuzzy sets remains at 0.0. For each fuzzy set, check dat in Is it less than 2.0*i-4.0. If so, p ms [i] is set to -0.5*dat in -2.0+i, this formula calculates the membership of the i-th fuzzy set. It is a linear function, as dat in decreases with the increase of p ms [i+1] is set to 1.0-p ms[i], this formula calculates the membership of the i+1th fuzzy set, which is the complement of the membership of the i-th fuzzy set, representing the membership of the next fuzzy set. The error membership arrays e are obtained respectively. ms , error change rate membership array ec ms .

[0069] In the process of calculating fuzzy output, its input includes the error membership array e ms , error change rate membership array ec ms And the fuzzy rule table rule, the output is the array res storing the result membership ms , first res ms The array is cleared, and two nested for loops are used to iterate over each membership level of the error and error change rate. If e ms [i] and ec ms [j] is not zero, then the corresponding output membership level index is determined according to the fuzzy rule table rule. ms [i] and ec ms The products of [j] are accumulated to res ms [index] on.

[0070] In the dequantization process, the input is the output membership array res obtained above. ms The output is the parameters of the PID controller after inverse quantization, and the membership array res is output through a for loop. ms For each element in, for each membership value res ms [i], and compare it with the corresponding quantization level p quantiz [i] are multiplied, and the result is then added to the res variable. After the loop ends, the res variable stores the weighted sum of all membership values, which is the inverse quantized control output value.

[0071] Different heating rates will significantly affect the separation effect of chromatography. If the heating rate is too fast, some compounds may not be completely separated, resulting in overlapping chromatographic peaks, thus affecting the detection accuracy. Here, the incremental PID plus temperature compensation control method is used to achieve proportional heating. The temperature calculation formula after compensation is T comp =k'×v+T k , where T k represents the current kth temperature collection, k' represents the temperature compensation coefficient, which is obtained based on experience. If it is too small, the compensation effect is not obvious, and if it is too large, it will lead to over-compensation. v represents the instantaneous heating rate of the current temperature collection point, and its expression is v = (T k -T k-1 ) / T pid , T pidRepresents the PID control cycle, that is, the inverse of the AD chip data output rate, where T pid The time interval is 0.02s. The temperature measurement module selects thermocouples. The incremental pid and thermocouples have faster response speeds, which can eliminate the temperature measurement hysteresis of the temperature measurement module and the influence of the proportional heating effect to a certain extent. During the programmed heating process, the VOCs in the sample gas are gradually separated and enter the PID detector in turn. After the programmed heating is completed, the instrument enters the cleaning process. The pre-concentrator tube is quickly heated to 10°C above the target temperature, and then naturally cooled to prevent pollutants from being adsorbed on the pre-concentrator tube. EPC1 controls the carrier gas flow rate to the standby flow rate and turns on the fan. The chromatographic column is quickly cooled and finally stabilized at the initial temperature. The instrument is ready again to facilitate the rapid start of the next round of detection.

[0072] Considering the portability of the instrument design and shortening the sample analysis time as much as possible, a low thermal mass (LTM) chromatographic column is selected to replace the column oven of the traditional gas chromatography for the separation of compounds. The LTM chromatographic column of the present invention greatly reduces the volume of the chromatographic column module by winding and assembling the chromatographic column, thermistor wire, heating wire and insulating fiber, and also has the advantages of low power consumption and fast heating and cooling.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for rapid detection of volatile organic compounds, characterized in that: The method includes: S1. According to the measurement command, the carrier gas flow rate of the air inlet pipeline is adjusted from the initial flow rate to the measurement flow rate, and the flow rate of the volatile organic compound sample gas is adjusted based on the state parameters in the pre-concentrator; S2, based on the flow rate control of the volatile organic matter sample gas, the volatile organic matter sample gas is adsorbed in the pre-concentration tube; S3. When the temperature of the pre-concentrator reaches the target temperature, the volatile organic sample gas separates from the pre-concentrator and follows the carrier gas with a measured flow rate into the chromatographic column; the temperature of the chromatographic column is controlled by a constant temperature control method and a proportional heating method; S4. By controlling the temperature of the chromatographic column, different substances in the volatile organic sample gas are gradually separated from the chromatographic column and enter the photoionization detector; the photoionization detector is used to detect the components of different substances in the volatile organic sample gas.

2. A method for rapid detection of volatile organic compounds according to claim 1, characterized in that: The method of adjusting the carrier gas flow rate of the air inlet pipeline from the initial flow rate to the measurement flow rate according to the measurement command, and adjusting the flow rate of the volatile organic compound sample gas based on the state parameters in the pre-concentrator tube includes: After the measurement command is triggered, the carrier gas flow rate of the air inlet pipeline is adjusted from the initial flow rate to the measurement flow rate; Obtaining state parameters in the pre-concentrator tube by means of an electronic flow meter, wherein the state parameters include pressure and temperature; Adjust the flow rate of the volatile organic compound sample gas according to the state parameters in the pre-concentration tube.

3. A method for rapid detection of volatile organic compounds according to claim 1, characterized in that: The method of controlling the temperature of the chromatographic column by using a constant temperature control method and a proportional temperature increase method comprises: The control parameters of the PID controller are adjusted based on fuzzy logic, and the chromatographic column is temperature-controlled according to the PID controller; The incremental PID is used in combination with the temperature compensation control method to increase the temperature of the chromatographic column proportionally; The control parameters of the PID controller adjusted based on fuzzy logic include a quantization process, a membership calculation process, a fuzzy output calculation process and a dequantization process.

4. A method for rapid detection of volatile organic compounds according to claim 3, characterized in that: The quantification process includes: When the temperature input value of the quantization process is less than or equal to all the level arrays, the first preset value is returned; when the temperature input value of the quantization process is greater than all the level arrays, the second preset value is returned; When the temperature input value of the quantization process is within the range of the quantization level, all level arrays are traversed. If the current temperature input value is less than or equal to the current quantization level, the current temperature input value is adjusted to the quantization value before the current quantization level.

5. A method for rapid detection of volatile organic compounds according to claim 4, characterized in that: The calculation formula of the quantized value is: dat in =2.0*i-6.0+2.0*[d in -p quantiz (i)] / [p quantiz (i)-p quantiz (i-1)] In the formula, dat in Represents a quantized value; d in Indicates the temperature input value; p quantiz (i) represents the i-th quantization level, p quantiz (i-1) represents the i-1th quantization level.

6. A method for rapid detection of volatile organic compounds according to claim 4, characterized in that: The membership calculation process includes: Determine whether the quantized value is less than the first preset value. If so, set the membership of each fuzzy set to one of completely belonging and completely not belonging according to the preset rule; if not, calculate the membership value of the fuzzy set; wherein the calculation formula of the membership value of the fuzzy set is: p ms [i]=-0.5*dat in -2.0+i; In the formula, p ms [i] represents the membership degree of the i-th fuzzy set, dat in Represents a quantized value; According to the membership of the fuzzy set, the error membership array and the error change rate membership array are determined.

7. A method for rapid detection of volatile organic compounds according to claim 6, characterized in that: The calculation fuzzy output process includes: Each membership level of the error and the error change rate is traversed; when the error and the error change rate are not zero, the corresponding output membership level is determined according to the fuzzy rule table, and the membership array is obtained.

8. A method for rapid detection of volatile organic compounds according to claim 7, characterized in that: The dequantization process includes: Iterate through each membership value in the membership array, and multiply each membership value by the corresponding quantization level; The weighted sum of the results of multiplying all membership values ​​by the corresponding quantization levels is performed to obtain the control parameters of the PID controller.

9. A method for rapid detection of volatile organic compounds according to claim 8, characterized in that: The use of incremental PID combined with a temperature compensation control method to proportionally heat the chromatographic column includes: The calculation formula for the compensated temperature is: T comp =k’×v+T k ; Where, T comp represents the compensated temperature, k' represents the temperature compensation coefficient, v represents the instantaneous heating rate of the current temperature acquisition point, T k Indicates the current kth collected temperature.

10. A method for rapid detection of volatile organic compounds according to claim 1, characterized in that: The method of using a photoionization detector to detect the components of different substances in the volatile organic compound sample gas includes: The volatile organic sample gas enters the photoionization detector and is ionized after being excited by ultraviolet light, generating a sample gas detection current; The sample gas detection current is amplified to obtain a measurable voltage; the measurable voltage is converted into an analog-to-digital value to obtain a detector response value; The response value spectrum is drawn using the detector response value; qualitative and quantitative analysis is performed based on the characteristic peaks of different substances in the volatile organic compound sample gas in the response value spectrum and in combination with the retention time and peak area of ​​different substances.

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