JP3815041B2 - Gas identification device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas identification device including a gas sensor. The gas sensor according to the present invention can be used, for example, in an odor measuring device for measuring odor components, quality inspection of food and fragrance, quantitative detection of bad odor pollution, fire alarm by burnt odor detection, quality inspection of food and fragrance Furthermore, it can be used in a wide range of fields, such as criminal investigations such as person tracking, identification, authentication, and drug inspection.
[0002]
[Prior art]
Conventionally, a gas sensor that detects gas by forming a sensitive film made of a metal oxide semiconductor between electrodes and measuring a change in resistance value between electrodes when a sample component is adsorbed to the sensitive film has been put into practical use. Yes. A gas identification device called “electronic nose” using a plurality of gas sensors is commercialized by Prime Tech, France. In this type of gas sensor, the sensitive film is heated to a high temperature (350 ° C. or higher) to cause an oxidation-reduction reaction with the sample components adhering to the surface of the sensitive film. In this process, movement of electrons occurs, and the electric density changes as the electron density in the sensitive film and the thickness of the depletion layer change.
[0003]
Therefore, in a gas sensor using a metal oxide semiconductor film, only target substances that cause a redox reaction can be detected, and substances that thermally decompose at the above temperature cannot be detected, so that target substances are extremely limited. . Further, it is necessary to wait for the gas sensor to rise to the operating temperature and stabilize at the time of analysis, and in particular, a long measurement time is required for repeated measurement. Furthermore, since the surface state of the sensitive film is relatively unstable, there is a problem that the change with time is large and the reliability is poor.
[0004]
On the other hand, for example, Japanese Patent Application Laid-Open No. 61-147145 proposes a gas sensor using a conductive polymer as a sensitive film. Further, an odor sensor using a conductive polymer mainly composed of polypyrrole as a sensitive film and utilizing a change in the resistance value thereof is commercialized by Aromascan and Neotronics, UK. In such a gas sensor, analysis can be performed while the sensitive film is maintained at room temperature.
[0005]
[Problems to be solved by the invention]
In general, the gas sensor does not respond only to a specific single component, but to multiple components. Therefore, even if a single gas sensor is exposed to an unknown sample gas and a change in the resistance value of the sensitive film is observed, it is impossible to determine the type of the sample component by itself. Therefore, in the conventional gas identification device, a plurality of gas sensors with different response characteristics are installed, and multivariate analysis processing is performed based on the detection signals acquired by each gas sensor to comprehensively determine the sample components. ing. For this reason, many gas sensors must be prepared in order to increase the discrimination capability, which increases the shape and the cost.
[0006]
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a gas identification device capable of obtaining a high identification capability with a single or a small number of gas sensors.
[0007]
[Means for Solving the Problems]
The gas identification device according to the present invention, which has been made to solve the above problems,
a) a gas sensor having a sensitive film made of a conductive polymer between a plurality of electrodes, and detecting an electrical change between the electrodes when a sample component adheres to the sensitive film;
b) a channel switching means for switching between a sample gas containing a sample component and a reference gas not containing the sample component and introducing the gas into the gas sensor;
c) When the gas sensor is switched from being exposed to the reference gas to being exposed to the sample gas, and / or when being switched from being exposed to the sample gas to being exposed to the reference gas Signal processing means for identifying a sample component based on a difference in transient characteristics of fluctuations in the detection signal of a gas sensor generated in
It is characterized by having.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
When the flow path switching means switches the gas introduced into the gas sensor from the reference gas to the sample gas, the sample component adheres to the sensitive film of the gas sensor, thereby changing the electrical characteristics of the gas sensor, such as resistance and impedance. When the gas introduced into the gas sensor is switched to the reference gas again, the sample component is detached from the sensitive film of the gas sensor, and the electrical characteristics of the gas sensor are restored. Such transient changes in characteristics depend on the ease of adsorption of sample components to the sensitive membrane, the rate of penetration / diffusion of sample components into the sensitive membrane, etc., and these vary depending on the sample components. . Therefore, the signal processing means acquires the fluctuation of the transient detection signal as data, and identifies the sample component by analyzing it. Accordingly, even in the steady state where the gas sensor is exposed to the sample gas, it is possible to identify even a sample component whose resistance value changes in the same manner and is difficult or impossible to identify.
[0009]
【Example】
An embodiment of a gas identification device according to the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of a gas identification device of the present embodiment, and FIG. 2 is a plan view showing a configuration of a gas sensor 1 in FIG.
[0010]
As shown in FIG. 1, the gas sensor 1 is arranged in a flow cell 2 having a temperature adjusting function, and a three-port valve 3 is connected to the inlet side flow path of the flow cell 2 and a pump 4 is connected to the outlet side flow path. ing. The valve 3 selectively switches between the reference gas sucked from the reference gas inlet 6 and the sample gas containing the sample component sucked from the sample gas inlet 7. A filter 5 made of activated carbon or the like for removing undesired components is provided on the reference gas flow path between the reference gas intake 6 and the valve 3. The control unit 8 controls the temperature adjustment of the flow cell 2, the switching of the valve 3, and the flow rate by pump suction. On the other hand, the signal processing unit 9 receives the detection signal of the gas sensor 1 and executes signal processing for gas identification as described later.
[0011]
As shown in FIG. 2, the gas sensor 1 includes two comb-shaped electrodes 11 that are opposed to each other on an insulating substrate 10, and a sensitive film 12 is formed by covering the electrodes 11. As the sensitive film 12, for example, a film made of a conductive polymer such as polypyrrole or polythiophene can be used. In the conductive polymer film, the conductivity is adjusted by controlling the amount of dopant introduced into the film. When such a gas sensor 1 is exposed to the sample gas, the sample components are adsorbed on the sensitive film 12, and the conductivity of the conductive polymer changes due to the direct or indirect involvement of the component molecules. Thereby, the resistance value between the electrodes 11 changes.
[0012]
When measuring the sample gas with the gas identification device of the present embodiment, the control unit 8 adjusts the temperature of the flow cell 2 to a predetermined temperature (for example, 40 ° C.), sets a predetermined flow rate, and drives the pump 4. The valve 3 is switched every predetermined time (t seconds). Thus, the sample gas and the reference gas alternately flow into the flow cell 2 every t seconds. The signal processing unit 9 reads a resistance value between the electrodes 11 of the gas sensor 1 at a predetermined sampling time interval, converts the resistance value into a digital value by a built-in A / D converter, and stores the digital value in a memory or the like. Then, when predetermined data is collected, signal processing as described later is started.
[0013]
When the gas sensor 1 is switched from the state exposed to the reference gas to the state exposed to the sample gas, and conversely, the state exposed to the sample gas is switched to the state exposed to the reference gas. A so-called transient characteristic occurs in the resistance value between the electrodes 11. That is, for example, when switching from the state exposed to the reference gas to the state exposed to the sample gas, the sample component is first adsorbed on the surface of the sensitive film 12 and then diffuses and penetrates into the sensitive film 12. When the conductive polymer has a property of easily swelling, swelling also occurs in this process. Therefore, even if it is a sample component that ultimately causes a change in conductivity, it is easy to adsorb on the surface of the sensitive film 12, easy to penetrate and diffuse into the sensitive film 12, and the speed of swelling. The transient characteristics differ depending on the factors.
[0014]
This will be specifically described below on the basis of the measurement results of sample gases each containing a single component of toluene, butanol, benzene, and butyl acetate as sample components. Here, the flow rate of the pump 4 is 300 mL / min, and the switching time t of the valve 3 is 3 seconds. In addition, these numerical values are not limited to this, It is good to choose an appropriate value according to the kind of gas sensor 1, the kind of sample component made into object, etc. However, the switching time t is preferably set to be longer than the time for which the transient response of the resistance value of the gas sensor 1 generated when switching between the sample gas and the reference gas converges.
[0015]
The results of measuring the above four types of sample gases under the above conditions are shown in FIG. 3A, 3B, 3C and 3D are measurement waveforms corresponding to toluene, butyl acetate, benzene and butanol, respectively.
[0016]
Analysis processing is performed on the waveform thus obtained as follows. First, a waveform for 6 seconds (one period) from the time when the gas sensor 1 is exposed to the sample gas and the resistance value starts to rise to the next simultaneous point is cut out from the obtained waveform. For example, in the case of benzene shown in FIG. 3C, the waveform for one cycle is as shown in FIG. In this waveform, the region I where the resistance increase is steep, the region II where the resistance increase is slow, the region III where the resistance decrease is steep, and the resistance value is kept low. Each of the areas IV is set as shown in FIG. Here, the region I and the region II are periods in which the gas sensor 1 is exposed to the sample gas, and the region III and the region IV are periods in which the gas sensor 1 is exposed to the reference gas.
[0017]
Next, in the period in which the gas sensor 1 is exposed to the sample gas, the resistance value of the asymptote of the rising curve of the resistance value in FIG. 4 is Rmax, and the difference r () between the resistance value R (t) with respect to each time t ( t) is defined by equation (1).
r (t) = Rmax−R (t) (1)
This r (t) is expressed as r (t) = a1 · exp (−A1 · t) + a2 · exp (−A2 · t) (2)
, The first term of the above equation (2) indicates the property of region I, and the second term indicates the property of region II.
[0018]
Similarly, during the period in which the gas sensor 1 is exposed to the reference gas, the resistance value of the asymptote of the descending curve of the resistance value is Rmin, and the difference r (t) from the resistance value R (t) for each time t is ( 3) Define with equation.
r (t) = R (t) -Rmin (3)
The approximate expression is
r (t) = b1 · exp (−B1 · t) + b2 · exp (−B2 · t) (4)
Thus, the first term of the above equation (4) indicates the property of region III, and the second term indicates the property of region IV.
[0019]
FIG. 5 is a graph showing r (t) in the above equations (1) and (3) as a logarithm. In FIG. 5, the constants A1, A2, B1, B2, a1, a2, b1, b2 of the above equations (2) and (4) can be obtained from the tangents of the curves in the regions I to IV. Regarding the example of FIG. 5, these constants are obtained as follows.
A1 = 0.694 log (a1) = 18
A2 = 0.096 log (a2) = 15
B1 = 4.233 log (b1) = 48
B2 = 0.166 log (b2) = 14
[0020]
FIG. 6 shows the result of normalization by performing the same analysis process for three types of sample gases other than benzene and obtaining constants A1, A2, B1, and B2. FIG. 6 shows the standard values of the constants A1, A2, B1, and B2 when B1 is 1 in each sample gas. As is clear from FIG. 6, the aromatic compounds (toluene, benzene), alcohols (butanol), and esters (butyl acetate) clearly have different characteristics. On the other hand, toluene and benzene belonging to the same aromatic compound show very similar characteristics. Thus, it can be seen that the components of the sample gas can be identified by the constants A1, A2, B1, and B2 obtained by the analysis process. The constants a1, a2, b1, and b2 depend on the component concentration of the sample gas, but this is not described in detail here.
[0021]
The analysis process as described above is performed in advance, and standard values of constants A1, A2, B1, and B2 for known components are stored in, for example, a memory. When measuring an unknown sample gas, the standard values of constants A1, A2, B1, and B2 calculated by the same analysis process as described above are compared with the values in the memory to find the component of the closest pattern. . Thereby, the kind of component contained in sample gas can be identified.
[0022]
In the above example, it is difficult to distinguish between benzene and toluene, but another type of gas sensor, for example, a gas sensor having a characteristic capable of identifying a methyl group (CH ₃ ) is additionally provided in the flow cell 2, If the detection signal of the gas sensor is used for identification of sample components, benzene and toluene can be identified.
[0023]
Furthermore, by using a combination of a plurality of gas sensors having different characteristics, the types of components that can be identified are further increased, and the accuracy of identification is improved.
[0024]
【The invention's effect】
As described above, in the gas identification device according to the present invention, by alternately exposing the gas sensor to the sample gas and the reference gas, the detection signal of the fluctuating gas sensor is obtained, and the sample component is based on the transient characteristics. Is estimated. For this reason, even in the case of a plurality of sample components that similarly cause a resistance change when being continuously exposed to the sample gas, different transient characteristics can be detected and each sample component can be identified. . This makes it possible to provide high discrimination with a single gas sensor or a small number of gas sensors, which conventionally required many gas sensors having different characteristics to be combined. As a result, downsizing and cost reduction of the apparatus can be achieved.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a gas identification device according to an embodiment of the present invention.
FIG. 2 is a plan view showing the configuration of the gas sensor in FIG.
FIG. 3 is a waveform diagram of response characteristics of a gas sensor to a sample gas containing a single component of toluene, butyl acetate, benzene, and butanol in the present example.
4 is a waveform diagram showing one cycle of response characteristics with respect to benzene in FIG.
5 is a graph obtained by analyzing the waveform diagram of FIG.
FIG. 6 is a graph showing standard values of A1, A2, B1, and B2 calculated for four types of components.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Gas sensor 11 ... Electrode 12 ... Sensing membrane 2 ... Flow cell 3 ... Three port valve 4 ... Pump 6 ... Reference gas inlet 7 ... Sample gas inlet 8 ... Control part 9 ... Signal processing part

Claims (1)

a) a gas sensor having a sensitive film made of a conductive polymer between a plurality of electrodes, and detecting an electrical change between the electrodes when a sample component adheres to the sensitive film;
b) a channel switching means for switching between a sample gas containing a sample component and a reference gas not containing the sample component and introducing the gas into the gas sensor;
c) When the gas sensor is switched from being exposed to the reference gas to being exposed to the sample gas, and / or when being switched from being exposed to the sample gas to being exposed to the reference gas Signal processing means for identifying a sample component based on a difference in transient characteristics of fluctuations in the detection signal of a gas sensor generated in
A gas identification device comprising:

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