Method and apparatus for detecting volatile fatty acids

Abstract

The present invention provides a method and device for sensing a volatile fatty acid with high sensitivity and measuring a volatile fatty acid with high precision. This method for sensing a volatile fatty acid comprises mounting a sample that potentially includes a volatile fatty acid in a closed space in which water vapor coexists with a resistance measurement device for measuring an electrical characteristic arising from electrical resistance between at least two electrodes disposed adjacent to each other across a minute interval on an insulating substrate, measuring the electrical characteristic when the relative humidity of the space in the vicinity between the electrodes satisfies a humidity condition immediately before condensation forms between the electrodes, and comparing the obtained measurement result and reference data obtained under a prescribed condition.

Description

[Technical Field]

[0001] The present invention relates to a method and an apparatus for detecting volatile fatty acids. [Background technology]

[0002] Volatile fatty acids (VFAs), such as propionic acid, butyric acid, and acetic acid, are substances that have a significant impact on the productivity and quality of dairy cattle. Furthermore, cows emit large amounts of methane gas, which contributes to global warming, when digesting food. It has been reported that this emission can be significantly reduced by controlling VFAs in the cow's rumen (first stomach). In light of this, there is a need for a method and device for detecting and quantitatively measuring VFAs, particularly VFAs volatilized from the liquid portion of the bovine rumen. In this specification, "detection" with respect to VFAs means confirming the presence or absence of VFAs, and "measurement" means measuring and determining (quantifying) the amount of VFAs.

[0003] Previously, attempts were made to detect and measure VFAs by adsorbing them onto an adsorbent and monitoring the change in the adsorbent's weight and infrared absorption spectrum due to VFA adsorption. However, this method has problems such as low detection sensitivity and measurement accuracy, and the equipment required is relatively large. Note that the practical detection sensitivity and measurement accuracy required for the liquid portion of the cow's rumen is 25 mM (millimolar). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 13544 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method and an apparatus for detecting VFA with high sensitivity and measuring it with high accuracy. [Means for solving the problem]

[0006] The inventor discovered that by measuring the electrical characteristics resulting from the electrical resistance between electrodes spaced very close together in a humid environment just before condensation occurs, it is possible to detect VFAs contained in a sample with high sensitivity and quantify them with high accuracy, and thus came up with the present invention. The configuration of the present invention is shown below. (Configuration 1) A sample that may contain volatile fatty acids is placed in a closed space where water vapor is present, together with a resistance measuring device that measures electrical characteristics resulting from the electrical resistance between at least two or more electrodes that are adjacently arranged at a fine interval on an insulating substrate; measuring the electrical characteristics when the relative humidity in the space near the electrodes satisfies the humidity condition just before condensation occurs between the electrodes; A method for detecting volatile fatty acids, comprising comparing the obtained measurement results with reference data obtained under specified conditions. (Configuration 2) The reference data is data obtained by measuring the electrical characteristics using the sample when the relative humidity in the space near the electrodes is at a humidity condition where condensation does not occur between the electrodes, or data obtained by measuring the electrical characteristics using a reference sample that does not contain volatile fatty acids or a reference sample that contains volatile fatty acids at a certain ratio when the relative humidity in the space near the electrodes satisfies the humidity condition just before condensation occurs between the electrodes. (Configuration 3) The method for detecting volatile fatty acids according to Configuration 1 or 2, wherein the humidity condition is a relative humidity of 80% or more but less than 100%. (Configuration 4) 4. The method for detecting volatile fatty acids according to claim 3, wherein the humidity condition is a relative humidity of 80% or more and 95% or less. (Configuration 5) A method for detecting volatile fatty acids according to any one of configurations 1 to 4, wherein the electrode has a configuration in which first thin wire electrodes and second thin wire electrodes are arranged alternately in at least a portion of an insulating substrate. (Configuration 6) A method for detecting volatile fatty acids according to configuration 5, wherein the first thin wire electrode and the second thin wire electrode are arranged alternately in parallel with each other at a fixed interval. (Configuration 7) 7. The method for detecting volatile fatty acids according to claim 6, wherein the distance is 100 nm or more and 1000 nm or less. (Configuration 8) a first metal is formed on at least a portion of an exposed surface of the first thin wire electrode, and a second metal different from the first metal is formed on at least a portion of an exposed surface of the second thin wire electrode; The method for detecting volatile fatty acids according to any one of configurations 5 to 7, wherein the current flowing between the first thin wire electrode and the second thin wire electrode is measured. (Configuration 9) 9. The method for detecting volatile fatty acids according to claim 8, wherein the first metal is selected from the group consisting of gold, platinum, silver, titanium and alloys thereof, and carbon. (Configuration 10) 10. The method for detecting volatile fatty acids according to claim 8 or 9, wherein the second metal is selected from the group consisting of silver, copper, iron, zinc, nickel, cobalt, aluminum, tin, chromium, molybdenum, manganese, magnesium, and alloys thereof. (Configuration 11) 11. The method for detecting volatile fatty acids according to any one of claims 1 to 10, wherein the surface of the insulating substrate is hydrophilic. (Configuration 12) 12. The method for detecting volatile fatty acids according to any one of claims 1 to 11, wherein the relative humidity is controlled by a temperature adjusting device thermally connected to the resistance measuring device. (Configuration 13) 13. The method for detecting volatile fatty acids according to claim 12, wherein the temperature adjusting device is a Peltier element. (Configuration 14) 14. The method for detecting volatile fatty acids according to any one of claims 1 to 13, wherein the sample is in the form of an aqueous solution. (Configuration 15) The device includes a resistance measuring device, a humidity measuring means, a data extracting means, and a data analyzing means; The resistance measuring device has at least two electrodes arranged adjacent to each other at a fine interval on an insulating substrate, measures electrical characteristics resulting from the electrical resistance between the electrodes, and outputs the results; the humidity measuring means is disposed adjacent to the resistance measuring device, measures the relative humidity of a space in the vicinity of the electrodes of the resistance measuring device, and outputs the result of the measurement; the data extraction means is means for extracting output data from the resistance measurement device when the relative humidity measured by the humidity measurement means satisfies a predetermined humidity condition; The data analysis means is a means for comparing the data extracted by the data extraction means with calibration curve data obtained in advance, and outputting the results. (Configuration 16) 16. The device for measuring volatile fatty acids according to claim 15, wherein the electrodes have a configuration in which first thin wire electrodes and second thin wire electrodes are alternately arranged in parallel in at least a partial area on an insulating substrate. (Configuration 17) 17. The device for measuring volatile fatty acids according to claim 16, wherein the first thin wire electrode and the second thin wire electrode are arranged alternately and side by side at a fixed interval. (Configuration 18) 18. The device for measuring volatile fatty acids according to claim 17, wherein the interval is 100 nm or more and 1000 nm or less. (Configuration 19) a first metal is formed on at least a portion of an exposed surface of the first thin wire electrode, and a second metal different from the first metal is formed on at least a portion of an exposed surface of the second thin wire electrode; 19. The device for measuring volatile fatty acids according to any one of configurations 16 to 18, wherein the resistance measuring device measures the current flowing between the first thin wire electrode and the second thin wire electrode and outputs the result. (Configuration 20) 20. The device for measuring volatile fatty acids according to any one of claims 16 to 19, wherein the humidity measuring means comprises a humidity measuring electrode that determines humidity based on electrical resistance between electrodes, and the material constituting the humidity measuring electrode is the same as the material constituting the first thin wire electrode or the second thin wire electrode. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a method and an apparatus for detecting VFA with high sensitivity and measuring it with high accuracy. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram illustrating the configuration of a VFA detection and measurement device according to the present invention. [Figure 2] FIG. 1 is a structural diagram illustrating the configuration of a resistance measuring device (galvanic sensor), where (a) is a plan view and (b) is a cross-sectional view. [Figure 3] FIG. 2 is an explanatory diagram illustrating the operating principle when the resistance measuring device is a galvanic sensor. [Figure 4] FIG. 1 is an explanatory diagram for explaining a method for detecting and measuring VFA according to the present invention. [Figure 5] FIG. 1 is an explanatory diagram illustrating the method of the present invention based on the sensor output and time change of platinum resistance obtained using a prototype VFA detection and measurement system. [Figure 6] FIG. 6 is a characteristic diagram showing the propionic acid concentration dependency of the sensor output shown in FIG. 5. [Figure 7] FIG. 6 is a characteristic diagram showing the propionic acid concentration dependency of the sensor output shown in FIG. 5. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Concept> In the VFA detection and measurement method of the present invention, a sample that may contain VFA is placed in a closed space where water vapor is present together with a resistance measuring device, and the output from the resistance measuring device is monitored when the relative humidity in the space near the electrodes provided in the resistance measuring device satisfies the humidity condition just before condensation occurs between the electrodes.

[0010] Here, the resistance measuring device measures an electrical characteristic (either electrical resistance, conductivity, or current value) resulting from the electrical resistance between at least two or more electrodes arranged adjacent to each other at a fine interval on an insulating substrate, and the output from the resistance measuring device is the measurement result of the electrical characteristic. A specific example of such a resistance measuring device is a galvanic sensor that has two or more electrodes on an insulating substrate exposed to the outside air (in the present invention, the water vapor atmosphere in the closed space is intended), and the electrodes are made of at least two different metals, and measures the galvanic current flowing between the electrodes. Such a galvanic sensor is disclosed, for example, in Patent Document 1. Other examples include a resistance measurement sensor having two or more electrodes on an insulating substrate exposed to the outside air, the electrodes being made of the same material, and measuring the current flowing between the electrodes by applying a voltage between the electrodes, or a resistance value from a bridge circuit provided in the circuit. Among these, galvanic sensors are particularly preferred because they are small and do not necessarily require an external power source. Details of an embodiment in which a galvanic sensor is used as a resistance measuring device in the VFA detection and measurement method of the present invention will be described later.

[0011] The inventor discovered that when VFAs are present in a water vapor atmosphere and the electrical characteristics resulting from the electrical resistance between the electrodes of a resistance measuring device are measured when the relative humidity in the space near the electrodes satisfies the humidity conditions just before condensation occurs between the electrodes, the output from the resistance measuring device depends on the concentration of VFAs in the atmosphere, making it possible to detect VFAs with high sensitivity and quantify them with high accuracy. On the other hand, when measuring the electrical characteristics resulting from the electrical resistance between the electrodes of a resistance measuring device when condensation has formed between the electrodes, the output from the resistance measuring device does not stabilize until the space between the electrodes is completely filled with a liquid consisting of water and VFA, and the output does not correlate with the concentration of VFA in the atmosphere. Furthermore, even when the space between the electrodes of the resistance measuring device is completely filled with a liquid consisting of water and VFA, there is a problem in that the detection sensitivity and measurement accuracy for VFA are not high. Here, from the viewpoint of VFA detection sensitivity and measurement accuracy determined through numerous experiments, the humidity condition immediately before condensation occurs between the electrodes of the resistance measuring device is preferably a relative humidity of 80% or more but less than 100%, and more preferably a relative humidity of 80% or more but less than 95%. The space near the electrodes of the resistance measuring device refers to a space containing at least the outside air that contacts the electrodes of the resistance measuring device in a closed space in which the resistance measuring device is placed, and typically refers to a space containing the atmosphere surrounding the device. Specifically, for example, by placing any humidity measuring means in the closed space adjacent to the resistance measuring device, it is possible to measure the relative humidity of the space near the electrodes of the resistance measuring device.

[0012] The detection and measurement of VFAs according to the present invention is believed to be based on the following mechanism. When the relative humidity in the space near the electrodes of the resistance measuring device satisfies the humidity condition just before condensation occurs between the electrodes, water molecules from water vapor and VFAs volatilized from the sample and floating in the atmosphere in the closed space are co-adsorbed on the exposed surface of the insulating substrate between the electrodes of the resistance measuring device, where the target electrical characteristics are measured by the resistance measuring device. Then, the protons (H +) become carriers, which move in a hopping manner via water molecules and adsorbed VFAs, changing the electrical resistance (or conductivity) between the electrodes. In other words, under humidity conditions just before condensation occurs between the electrodes of a resistance measuring device, the humidity is not high enough to cause condensation of water vapor in the atmosphere on the exposed surface of the insulating substrate between the electrodes, but the exposed surface is in a state where adsorption of water molecules derived from water vapor can occur frequently. Typically, this state is considered to occur over a certain time range (time interval) rather than at a specific point in time (instantaneous). In the present invention, this phenomenon is utilized to detect VFAs in a sample by comparing the measurement results obtained under the above conditions with data obtained by measuring the electrical resistance between the electrodes using the same sample when the relative humidity is such that condensation does not occur between the electrodes. Alternatively, VFAs in the sample can be detected by comparing the measurement results obtained under the above conditions with the data obtained by measuring the electrical resistance between the electrodes using a reference sample containing no VFAs or a reference sample containing a certain proportion of VFAs under the above conditions (i.e., when the relative humidity satisfies the humidity condition just before condensation occurs between the electrodes). Furthermore, because the amount of protons described above is highly and monotonically dependent on the amount of VFA, the amount of VFA in the sample can be determined (quantified) using a calibration curve. Moreover, because this is an application of a type of interfacial phenomenon, high detection sensitivity and measurement accuracy can be easily achieved despite the compact configuration of the device. Furthermore, the insulating substrate preferably has a hydrophilic surface to maximize the adsorption of water molecules on the exposed surface of the insulating substrate between the electrodes. Using an insulating substrate with a hydrophilic surface promotes coadsorption of water molecules and VFAs, improving VFA detection sensitivity and measurement accuracy, as well as the stability of the output from the resistance measurement device and the reproducibility of output data. Here, a hydrophilic surface refers to a surface with a water contact angle of 0° to 50°.

[0013] On the one hand, when measuring the electrical resistance between the electrodes of the resistance measuring device under humidity conditions where condensation occurs between the electrodes, although droplets are formed between the electrodes, the electrical characteristics (electrical resistance) are affected by the droplet formation process and exhibit behavior following probability theory. As a result, the reproducibility of the output data from the resistance measuring device cannot be achieved, it is difficult to accurately detect the VFA contained in the sample, and the magnitude of the amount of VFA cannot be grasped either.

[0014] In addition, a method of forming a water layer in advance on the exposed surface of the insulating substrate between the electrodes, dissolving the VFA volatilized from the sample therein, and measuring the conductivity between the electrodes can be considered. However, in this method, it is difficult to obtain high detection sensitivity and measurement accuracy due to the low solubility of VFA in water, etc., and there is also a problem that the device tends to be large-sized.

[0015] <VFA Detection and Measurement Device> The configuration of the VFA detection and measurement device of the present invention is shown in FIG. 1. The VFA detection and measurement device 101 of the present invention includes a resistance measuring device 11, a humidity measuring means 12, a data extraction means 13, and a data analysis means 14. The output data from the resistance measuring device 11 and the humidity measuring means 12 are sent to the data extraction means 13 via the signal line 15. Further, the output data from the resistance measuring device 11 when satisfying a predetermined humidity condition, extracted by the data extraction means 13, is sent to the data analysis means 14 via another signal line, and this data is compared with the calibration curve data etc. obtained in advance by the data analysis means 14, and the presence or absence of VFA in the sample 16 is confirmed, and / or the quantification of VFA is performed. Here, the resistance measuring device 11 and the humidity measuring means 12 are arranged in a closed space 17 where water vapor exists together with the sample 16.

[0016] 2(a), the resistance measuring device 11 has a substrate (insulating substrate) 21, a first thin wire electrode 22, a second thin wire electrode 23, a first electrode (first collecting electrode) 24, and a second electrode (second collecting electrode) 25. The first thin wire electrode 22 and the second thin wire electrode 23 are arranged adjacent to each other with a small gap between them on the insulating substrate 21. The resistance measuring device 11 measures electrical characteristics resulting from the electrical resistance between the first thin wire electrode 22 and the second thin wire electrode 23. Here, it is preferable that the first thin-wire electrodes 22 and the second thin-wire electrodes 23 are arranged alternately in at least a partial region on the insulating substrate 21. With such a configuration, the facing surface between the first thin-wire electrodes 22 and the second thin-wire electrodes 23 becomes wider, which increases the sensitivity to the VFA and also increases the stability (reproducibility) of the results obtained. Furthermore, it is preferable that the first thin-wire electrodes 22 and the second thin-wire electrodes 23 are alternately arranged side by side with a constant gap between them, i.e., the values ​​of d1 and d2 in the cross-sectional view of Fig. 2(b) are equal and there is no in-plane distribution (they are constant within the plane). When this gap is constant, the VFA detection and measurement mechanism according to the present invention described above functions effectively, and the change in electrical resistance between the first thin-wire electrodes 22 and the second thin-wire electrodes 23 becomes steeper, improving detection sensitivity and measurement accuracy.

[0017] Alternatively, the first thin-wire electrode 22 and the second thin-wire electrode 23 may be made of the same conductive material, and a voltage may be applied between the two electrodes via the first electrode 24 and the second electrode 25 to measure the current flowing between the first electrode 24 and the second electrode 25, thereby forming a resistance measurement sensor that measures the electrical resistance. Alternatively, the first thin-wire electrode 22 and the second thin-wire electrode 23 may be made of different metals, and a galvanic sensor may be used to measure the electrical resistance by measuring the galvanic current flowing between the first electrode 24 and the second electrode 25. Note that the first thin-wire electrode 22 and the first electrode 24 may be made of the same conductive material or different conductive materials. The same applies to the second thin-wire electrode 23 and the second electrode 25.

[0018] Hereinafter, the resistance measuring device 11 will be described in more detail, taking as an example a case where the resistance measuring device 11 is a galvanic sensor. As shown in Figure 3, the galvanic sensor 11 is a current detection sensor that has a first thin wire electrode 22 and a second thin wire electrode 23 made of different metals (metals A and B) arranged side by side on an insulating substrate, and utilizes the phenomenon that when a conductive liquid droplet such as a water droplet touches the electrodes of this thin wire electrode pair, a galvanic current flows between the electrodes.

[0019] 2(a) and 2(b), the galvanic sensor 11 includes a first thin-wire electrode 22 made of a first metal and a second thin-wire electrode 23 made of a second metal or semiconductor different from the first metal, which are arranged side by side on an insulating substrate 21. In other words, the first thin-wire electrode 22 and the second thin-wire electrode 23 are arranged adjacent to each other with a small gap between them on the insulating substrate 21. Here, the gap between the first thin-wire electrode 22 and the second thin-wire electrode 23 (i.e., the values ​​of d1 and d2 in FIG. 2(b)) is preferably 100 nm or more and 1000 nm or less from the viewpoint of detecting VFA with high sensitivity and measuring it with high accuracy.

[0020] A silicon substrate having an oxide film (SiO2 film) formed on its surface is preferably used as insulating substrate 21, but is not limited to silicon substrates and various insulating materials such as plastics such as polycarbonate, rubber, etc. Also, even if the substrate itself is a conductor such as metal, a substrate having insulating properties in relation to first fine-wire electrode 22 and second fine-wire electrode 23 formed thereon by forming an insulating coating or covering thereon is also included in the category of "insulating substrate" in this application.

[0021] The first thin wire electrode 22 is connected to a first electrode 24, and the second thin wire electrode 23 is connected to a second electrode 25, and an electrical signal is transmitted to the data extraction means 13 via electrical wiring (not shown) connected to the first electrode 24 and the second electrode 25. Here, an amplifier may be connected to the first electrode 24 and the second electrode 25 to amplify the galvanic current and transmit the electrical signal to the data extraction means 13. There may be a space between first thin wire electrode 22 and second thin wire electrode 23, or an insulator may be embedded therein. If an insulator is embedded therein, it is preferable that the surface thereof is hydrophilic so that water molecules are easily adsorbed thereon.

[0022] The first thin wire electrode 22 and the second thin wire electrode 23 are made of a first metal and a second metal having a different electrochemical potential, respectively. When the metals are co-adsorbed with water molecules and VFA at a density of a certain level or higher, protons (H + ) become carriers, which move in a hopping manner via water molecules and adsorbed VFA, causing a galvanic interaction between the electrodes, resulting in the flow of a galvanic current.

[0023] Increasing the length of the portion where the first thin-wire electrode 22 and the second thin-wire electrode 23 face each other in close proximity increases the battery capacity, which is effective in increasing the galvanic current. Therefore, it is preferable to arrange these thin-wire electrodes approximately parallel to each other over a long distance. Examples of configurations that increase the length of the portion where the thin-wire electrodes are close to each other (hereinafter referred to as the parallel running distance) by arranging the thin-wire electrodes in parallel include a comb structure and a double spiral structure. Other structures that maximize the parallel running distance of two wires within a certain planar area are well known in the field of semiconductor devices, and such structures may also be employed as needed. In the present invention, "arranging thin-wire electrodes side by side on a substrate" does not specify the relative orientation of the multiple thin-wire electrodes placed on the substrate, but refers to arranging the thin-wire electrodes spaced apart on the same plane of the substrate.

[0024] When the first thin wire electrode 22 is used as a cathode, examples of the material for the first thin wire electrode 22 include gold (Au), platinum (Pt), silver (Ag), titanium (Ti), and alloys thereof, as well as carbon (C) and its allotropes. When the second thin wire electrode 23 is used as an anode, examples of the material for the second thin wire electrode 23 include silver (Ag), copper (Cu), iron (Fe), zinc (Zn), nickel (Ni), cobalt (Co), aluminum (Al), tin (Sn), chromium (Cr), molybdenum (Mo), manganese (Mn), magnesium (Mg), and alloys thereof. However, when silver and its alloys are used as the first thin wire electrode 22, a material other than silver and its alloys is used as the material for the second thin wire electrode 23.

[0025] Naturally, the output (galvanic current value) of the galvanic sensor 11 depends on the combination of metal materials used for the first thin-wire electrode 22 and the second thin-wire electrode 23. For example, between silver / iron and gold / silver, the silver / iron combination has a higher corrosion rate per unit area, resulting in a larger galvanic current value. On the other hand, gold / silver has a longer lifespan due to less electrode wear. Here, silver is preferably used for the first thin-wire electrode 22 or the second thin-wire electrode 23 because it has the effect of preventing mold growth on the galvanic sensor 11. Note that using the same material for the first electrode 24 and the same material for the first thin-wire electrode 22 and the second electrode 25 and the second thin-wire electrode 23 is preferable because it simplifies the manufacturing process of the galvanic sensor 11.

[0026] When a galvanic current repeatedly flows in the galvanic sensor 11, the metal of the anode electrode, which is the second thin wire electrode 23, is ionized, and the anode electrode (second thin wire electrode) gradually wears out. To address this issue while maintaining the density of the thin-wire electrodes, it is possible to, for example, thicken the anode electrodes, or widen the anode electrodes and narrow the cathode electrodes (first thin-wire electrodes). Furthermore, if the spacing between the thin-wire electrodes is made very short, even a slight increase in the spacing between the thin-wire electrodes due to wear of the anode electrodes will have a significant effect on the measurement results of the electrical resistance (galvanic current). If this effect becomes a problem, it is possible to take measures for the entire measurement system, such as performing a compensation calculation on the measurement results, taking advantage of the fact that wear of the metal in the anode electrodes is, in principle, proportional to the time integral of the galvanic current.

[0027] The humidity measurement means 12 is disposed adjacent to the resistance measurement device 11 and is a means for measuring the relative humidity in the vicinity of the part of the resistance measurement device 11 that measures electrical characteristics (galvanic current if the resistance measurement device 11 is a galvanic sensor) (hereinafter, in the same context, this will also be referred to simply as the "measurement part"), that is, in the vicinity of two or more thin-wire electrodes of the resistance measurement device 11. Examples of such humidity measurement means include a resistance change type humidity sensor that measures the amount of moisture absorbed by a wet-dry material such as a polymer or ceramic as electrical resistance, a capacitance type humidity sensor that uses a polymer film or the like as a wet-dry response material, and a method that uses a temperature sensor and humidity conversion system that monitors the temperature in a saturated water vapor environment to determine the converted humidity. Here, when VFA detection and measurement are performed by placing sample 16, resistance measurement device 11, and humidity measurement means 12 in the same closed space environment, a preferred method is to place platinum electrodes near first thin-wire electrode 22 and second thin-wire electrode 23 of resistance measurement device 11, monitor the temperature from the resistance value of the platinum electrodes, and determine the converted humidity. In particular, when platinum is used as first thin-wire electrode 22 or second thin-wire electrode 23 of resistance measurement device 11, it becomes possible to easily provide a temperature measurement unit of humidity measurement means 12 employing the above method in resistance measurement device 11, which has the advantage of enabling the overall VFA detection and measurement device of the present invention to be miniaturized and cost-reduced.

[0028] The data extraction means 13 has a function of extracting output data from the resistance measuring device 11 and sending it to the data analysis means 14 when the relative humidity measured by the humidity measuring means 12 satisfies a predetermined humidity condition. Here, as described above, the predetermined humidity condition for relative humidity is preferably 80% or more and less than 100%, more preferably 80% or more and 95% or less. In other words, when data extraction means 13 extracts output data from resistance measurement device 11, the relative humidity of the space near the electrodes of resistance measurement device 11 measured by humidity measurement means 12 is preferably 80% or more and less than 100%, more preferably 80% or more and 95% or less. When the relative humidity satisfies this humidity condition, VFA can be detected with high sensitivity and measured with high accuracy.

[0029] The data analysis means 14 compares the output data of the resistance measuring device 11 sent via the data extraction means 13 (measurement data of the galvanic current if the resistance measuring device 11 is a galvanic sensor) with pre-obtained calibration curve data, etc., and outputs the amount of VFA corresponding to the output data, or compares it with a predetermined threshold value and outputs the presence or absence of VFA in the sample 16 from which the output data was obtained.

[0030] A specific embodiment of the detection and measurement of VFA according to the present invention will be further explained with reference to FIG. In the VFA detection and measurement system shown in FIG. 4, a sample 16 consisting of an aqueous VFA solution is placed on a base 41 together with a resistance measurement device 11 in a closed space 17 made of an acrylic box or the like. The system is configured so that water (water vapor) 31 and VFA 32 evaporated from the sample 16 can reach the measurement unit (at least the portion including the electrodes) of the resistance measurement device 11. Also, a humidity measurement means 12 is disposed adjacent to the resistance measurement device 11 in the closed space 17, so that the relative humidity near the electrodes of the resistance measurement device 11 can be measured. Here, in the system shown in FIG. 4, a temperature adjustment device 42 is preferably disposed below the resistance measurement device 11 to thermally connect the resistance measurement device 11 and the temperature adjustment device 42. The temperature adjustment device 42 adjusts the temperature of the resistance measurement device 11, or more precisely, the temperature of the measurement unit of the resistance measurement device 11, thereby enabling control of the relative humidity in the space near the electrodes present in the measurement unit. Examples of the temperature adjustment device 42 include a Peltier element. In order to increase the efficiency of heat conduction, it is preferable that the temperature adjusting device 42 and the resistance measuring device 11 are thermally connected by a heat pump.

[0031] By configuring the VFA detection and measurement system in this manner, the relative humidity in the space near the electrodes of the resistance measuring device 11 is monitored by the humidity measuring means 12, while the temperature is adjusted by the temperature adjustment device 42 to ensure that the relative humidity near the electrodes in the measurement section of the resistance measuring device 11 satisfies the humidity conditions just before condensation occurs between the electrodes. Specifically, the relative humidity can be efficiently controlled to be between 80% and less than 100%, or between 80% and 95%.

[0032] Since the sample 16 and the measurement part of the resistance measuring device 11 are placed in a closed space 17, when this system is used in an equilibrium state, the water 31 evaporated from the sample 16 and the VFA 32 are co-adsorbed onto the measurement part of the resistance measuring device 11, and since the relative humidity of the space near the electrodes in the measurement part is controlled within the desired range (the humidity conditions just before condensation occurs between the electrodes), it becomes possible to detect and measure the VFA with high sensitivity and high accuracy. Although the data extraction means 13 and the data analysis means 14 are not shown in FIG. 4, the output data from the resistance measuring device 11 and the humidity measuring means 12 can be sent to the data analysis means 14 via the data extraction means 13. [Example]

[0033] Example 1 In Example 1, a VFA detection and measurement system shown in FIG. 4 was fabricated and its characteristics were investigated. The results are described. However, it should be noted that the present invention is not limited to such a specific form, and the technical scope of the present invention is defined by the claims.

[0034] The resistance measuring device 11 was a galvanic sensor in which a first thin-wire electrode 22 and a second thin-wire electrode 23 were arranged in parallel in a comb-like pattern on an insulating substrate 21 as shown in Figure 2. Here, the first thin-wire electrode 22 was made of aluminum, and the second thin-wire electrode 23 was made of gold. Both the first thin-wire electrode 22 and the second thin-wire electrode 23 had a line width of 2 μm, a height of 200 nm, and a length of 1300 μm, and the distance between the first thin-wire electrode 22 and the second thin-wire electrode 23 was 500 nm. There were 92 pairs of first thin-wire electrodes 22 and second thin-wire electrodes 23.

[0035] A Si wafer having a thermally oxidized film with a thickness of 100 nm formed thereon was used as the substrate 21. The water contact angle on the flat portion of the substrate 21 was 33°, and the surface was hydrophilic. The humidity measuring means 12 monitors the temperature from the resistance value of a platinum electrode, and calculates the converted humidity therefrom. The temperature adjusting device 42 was a Peltier element. Aqueous solutions of propionic acid with concentrations set to five levels (0 mM, 10 mM, 20.7 mM, 100 mM, and 207 mM) were used as Sample 16. The amounts of the aqueous solutions were all the same, 50 mL each. A polypropylene resin box was used to provide a closed space 17. The volume of the closed space 17 was 430 mL, and the shortest distance between the container containing the sample 16 and the resistance measuring device 11 was approximately 3 cm.

[0036] The experimental results are shown in Figure 5. The time when recording of the output (sensor output) from the resistance measurement device (galvanic sensor) 11 began was designated as 0 (zero). At 900 and 1800 s, the temperature of the sensor's measurement section was changed (lowered) using the temperature control device 42 from the backside of the insulating substrate to control the humidity in the space between the sensor's electrodes. After 1900 s, condensation was confirmed visually (using a close-up camera). The time ranges of the sensor output before 1900 s with particularly high output stability were designated zone 1a (600-800 s) and zone 1b (1500-1700 s). Based on the resistance values ​​(platinum resistance values) of the platinum electrodes of the humidity measurement means, the relative humidity in zone 1a was estimated to be 85%, and the relative humidity in zone 1b was estimated to be 92%, confirming that these were within the desired humidity range just before condensation occurred between the electrodes of the resistance measurement device. After 1900 s, when condensation occurred, the sensor output no longer corresponded to the propionic acid concentration, and its behavior became stochastic and irregular. Specifically, the sensor outputs of the 0 mM and 10 mM propionic acid samples were nearly identical after approximately 1950 s, while the sensor output of the 20.7 mM sample exceeded those of the 100 mM and 207 mM samples after 1900 s. After approximately 2300 s, the sensor output of the 100 mM sample exceeded that of the 207 mM sample.

[0037] On the other hand, in Zones 1a and 1b, just before condensation, the sensor output exhibits a monotonic correlation with the propionic acid concentration. Figure 6 shows the propionic acid concentration dependence of sensor output in Zone 1a (relative humidity 85%), and Figure 7 shows that in Zone 1b (relative humidity 92%). In both figures, an inflection point is evident at a propionic acid concentration of approximately 20 mM. In Zone 1a, the characteristic curve is steep at propionic acid concentrations below 20 mM and relatively gentle above 20 mM. In Zone 1b, the characteristic curve is steep below 20 mM and saturated above 20 mM. These characteristic curves demonstrate that propionic acid (VFA) at concentrations as low as 10 mM can be detected and measured with sufficiently high linearity.

[0038] Here, the concentration of the propionic acid solution was evaluated, but the relative concentration of propionic acid in the gas and its partial pressure are as follows: Raul The conversion results are shown in Table 1. Here, "relative" refers to the ratio to water (water vapor). As shown in Table 1, the propionic acid contained in the sample in the form of an aqueous solution can be considered to exist in the water vapor atmosphere in a closed space at a relative concentration and partial pressure that correlates with the concentration in the aqueous solution. Therefore, it can be said that the system prototyped in this example has achieved highly sensitive detection and highly accurate measurement of VFAs.

[0039] [Table 1] [Industrial Applicability]

[0040] According to the present invention, a method and an apparatus for detecting VFA with high sensitivity and measuring it with high accuracy are provided. As mentioned in the Background Art section, VFAs are substances that have a significant impact on improving the productivity and quality of dairy farming, including cows, and are also closely related to the emission of methane gas through the digestion of food by cows. Therefore, if VFAs could be detected with high sensitivity, measured with high accuracy, and the results fed back, it is believed that the productivity and quality of dairy farming would be greatly improved and that it would also make a significant contribution to the prevention of global warming. Furthermore, the method of the present invention, which detects VFAs with high sensitivity and measures them with high accuracy, will lead to the quantitative evaluation of the activity of anaerobic bacteria, and is expected to be used in a wide range of cases where VFAs are generated by microbial decomposition, such as quality control in feed production, monitoring of methane fermentation tanks, operation management of final disposal sites (landfills), and environmental management of rice paddies and lakes. [Explanation of symbols]

[0041] 11: Resistance measuring device (galvanic type sensor) 12: Humidity measuring means 13: Data extraction method 14: Data analysis methods 15: Signal line 16: Sample 17: Closed space 21: Substrate (insulating substrate) 22: First thin wire electrode 23: Second thin wire electrode 24: First electrode (first collecting electrode) 25: Second electrode (second collecting electrode) 31: Water (water vapor) 32:VFA 41: Foundation 42: Temperature control device (Peltier element) 101: VFA detection and measurement equipment

Claims

1. A sample that may contain volatile fatty acids is placed in a closed space where water vapor is present, together with a resistance measuring device that measures electrical characteristics resulting from electrical resistance between at least two or more electrodes that are adjacently arranged at a small interval on an insulating substrate; measuring the electrical characteristics when the relative humidity in the space near the electrodes satisfies the humidity condition just before condensation occurs between the electrodes; comparing the obtained measurement results with reference data obtained under predetermined conditions; The method for detecting volatile fatty acids, wherein the humidity condition is a relative humidity of 80% or more and less than 100%.

2. The method for detecting volatile fatty acids according to claim 1, wherein the reference data is data obtained by measuring the electrical characteristics using the sample when the relative humidity in the space near the electrodes is at a humidity condition that does not cause condensation between the electrodes, or data obtained by measuring the electrical characteristics using a reference sample that does not contain volatile fatty acids or a reference sample that contains a certain proportion of volatile fatty acids when the relative humidity in the space near the electrodes satisfies the humidity condition just before condensation occurs between the electrodes.

3. The method for detecting volatile fatty acids according to claim 1 , wherein the humidity condition is a relative humidity of 80% or more and 95% or less.

4. The method for detecting volatile fatty acids according to claim 1 , wherein the electrode has a configuration in which first thin wire electrodes and second thin wire electrodes are alternately arranged in parallel in at least a partial area on an insulating substrate.

5. 5. The method for detecting volatile fatty acids according to claim 4, wherein the first thin wire electrodes and the second thin wire electrodes are arranged alternately in parallel with each other at a fixed interval.

6. The method for detecting volatile fatty acids according to claim 5 , wherein the distance is between 100 nm and 1000 nm.

7. a first metal is formed on at least a portion of an exposed surface of the first thin wire electrode, and a second metal different from the first metal is formed on at least a portion of an exposed surface of the second thin wire electrode; 5. The method for detecting volatile fatty acids according to claim 4, wherein the current flowing between the first thin wire electrode and the second thin wire electrode is measured.

8. 8. The method for detecting volatile fatty acids according to claim 7, wherein said first metal is selected from the group consisting of gold, platinum, silver, titanium and alloys thereof, and carbon.

9. 8. The method for detecting volatile fatty acids according to claim 7, wherein the second metal is selected from the group consisting of silver, copper, iron, zinc, nickel, cobalt, aluminum, tin, chromium, molybdenum, manganese, magnesium, and alloys thereof.

10. The method for detecting volatile fatty acids according to claim 1 , wherein the surface of the insulating substrate is hydrophilic.

11. The method for detecting volatile fatty acids according to any one of claims 1 to 9, wherein the relative humidity is controlled by a temperature adjusting device thermally connected to the resistance measuring device.

12. The method for detecting volatile fatty acids according to claim 11, wherein the temperature adjusting device is a Peltier element.

13. The method for detecting volatile fatty acids according to any one of claims 1 to 9, wherein the sample is in the form of an aqueous solution.

14. The device includes a resistance measuring device, a humidity measuring means, a data extracting means, and a data analyzing means; The resistance measuring device has at least two electrodes arranged adjacent to each other at a fine interval on an insulating substrate, measures electrical characteristics resulting from the electrical resistance between the electrodes, and outputs the results; the humidity measuring means is disposed adjacent to the resistance measuring device, measures the relative humidity of a space in the vicinity of the electrodes of the resistance measuring device, and outputs the result of the measurement; the data extraction means is means for extracting output data from the resistance measurement device when the relative humidity measured by the humidity measurement means satisfies a predetermined humidity condition; the data analysis means is means for comparing the data extracted by the data extraction means with calibration curve data obtained in advance and outputting the results; The device for measuring volatile fatty acids, wherein the predetermined humidity conditions are humidity conditions immediately before condensation occurs between the electrodes, and the humidity conditions are humidity conditions in which the relative humidity is 80% or more and less than 100%.

15. A volatile fatty acid measuring device as described in claim 14, wherein the humidity conditions are such that the relative humidity is 80% or more and 95% or less.

16. 15. The device for measuring volatile fatty acids according to claim 14, wherein the electrodes have a configuration in which first thin wire electrodes and second thin wire electrodes are alternately arranged in parallel in at least a partial area on an insulating substrate.

17. 17. The device for measuring volatile fatty acids according to claim 16, wherein the first thin wire electrodes and the second thin wire electrodes are arranged alternately at regular intervals.

18. 18. The device for measuring volatile fatty acids according to claim 17, wherein the interval is 100 nm or more and 1000 nm or less.

19. a first metal is formed on at least a portion of an exposed surface of the first thin wire electrode, and a second metal different from the first metal is formed on at least a portion of an exposed surface of the second thin wire electrode; 19. The device for measuring volatile fatty acids according to claim 16, wherein the resistance measuring device measures the current flowing between the first thin wire electrode and the second thin wire electrode and outputs the result.

20. 19. The device for measuring volatile fatty acids according to claim 16, wherein the humidity measuring means comprises a humidity measuring electrode that determines humidity based on electrical resistance between electrodes, and the material constituting the humidity measuring electrode is the same as the material constituting the first thin wire electrode or the second thin wire electrode.

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