JPH0422221B2 - - Google Patents

Description

BACKGROUND OF THE INVENTION [Technical Field] The present invention relates to a gas concentration measuring device for detecting and measuring the concentration of a gas such as carbon dioxide or oxygen contained in a mixed gas or blood. [Prior Art and its Problems] Electrolytic means is known as a means for detecting and measuring the concentration of each gas from a mixed gas of carbon dioxide and oxygen gas. Conventionally, oxygen electrodes for detecting oxygen used for such gas concentration measurements are broadly classified into bipolar separation type electrodes and Clark type electrodes based on the classification of electrode systems. From a practical point of view, Clark type electrodes are mainly used due to their economic efficiency, durability, response characteristics, measurement accuracy, etc. This Clark type electrode has a structure in which a platinum cathode and a silver anode are immersed in an internal electrolyte chamber separated by a polymer membrane. On the other hand, since the carbon dioxide electrode also uses a structure in which a PH glass membrane electrode is immersed in an internal electrode liquid chamber separated from the measurement liquid by a gas selective permeation membrane, it is difficult to miniaturize the electrode. In addition, the presence of impurities contained in the measurement liquid on the gas permeable membrane slows down the response speed, which is the main purpose of the detection element, or causes interference. Furthermore, since the carbon dioxide concentration is determined from changes in the PH value of the internal solution, there are problems such as a decrease in measurement accuracy and a slow response speed to reach the equilibrium potential. For this reason, it is desired to develop a detection element using the current method, similar to the oxygen concentration detection element. W. J. Albery and P. Barron (J. Electroanalytical Chem., 138, 79-87,
(1982) reported a carbon dioxide gas detection element with a dimethyl sulfoxide (DMSO) solution in its internal liquid. This carbon dioxide gas electrode is subject to interference from oxygen gas. Therefore, to solve this problem, oxygen electrodes are used in combination. For this reason, the electrode structure is complicated, and it has a double membrane structure such as an oxygen selective membrane or a carbon dioxide selective permeable membrane, which inevitably results in a slow response speed. . Also,
This electrode is used as a rotating electrode and must be used in a solution, and cannot perform measurements in a stationary state and in contact with a solid surface like a transdermal electrode. When used as a rotating electrode, it must be detected as a relatively large reduction current, and electrolysis takes a long time, making it susceptible to the effects of electrolysis products, and short-time measurements are difficult. It is inappropriate for. Purpose of the Invention The present invention has been made in view of the above points, and it is possible to make the electrode structure sufficiently simple, and it also provides excellent responsiveness to measured values and quick response time. The present invention also aims to provide a device for measuring the concentration of gases such as carbon dioxide and oxygen, which can be used for a long period of time. That is, the present invention includes an outer case having at least one opening, a gas permeable membrane that is set on the opening of the case so as to seal the opening and is in contact with a portion where a measurement gas exists, and an inner part of the case. a working electrode whose tip is set in contact with the gas permeable membrane, a reference electrode and a counter electrode set independently from the working electrode, a non-aqueous solvent filled and set in the outer case, It is an object of the present invention to provide a gas concentration measuring device characterized by comprising means for applying a pulsed electrolytic voltage with a specified potential between the working electrode and the reference electrode. Detailed Description of the Invention That is, in the gas concentration measuring device according to the present invention, a working electrode is set in a case in which a gas permeable membrane is set on one surface, and is in contact with the gas permeable membrane, and further A reference electrode and a counter electrode are set, and the case is filled with an electrode body filled with a non-aqueous solvent, and between the working electrode and the reference electrode,
For example, a pulsed signal having voltages set in two stages corresponding to the electrolytic potentials of carbon dioxide and oxygen is applied, and the concentration of the gas electrolyzed corresponding to the above potentials is measured. By setting the voltage in two stages in this way, the current responses corresponding to, for example, carbon dioxide and oxygen gas concentrations can be displayed and output separately, and these measurement results can be By performing calculation processing, it is possible to perform finer gas concentration measurement without requiring a complicated structure for the electrode base. Furthermore, it is possible to measure gas concentration on a solid surface like a transdermal detection element. The above electrode separates the gas to be measured from a gas-permeable membrane made of a polymer membrane, and allows carbon dioxide and oxygen gases that permeate through this membrane to be measured on the same working electrode surface. It has an internal liquid chamber, and has a structure in which three electrodes, the above-mentioned working electrode, a reference electrode, and a counter electrode are immersed in the liquid chamber, and the area around the working electrode is insulated. There is. Since the tip of this working electrode is in contact with the gas permeable membrane, the carbon dioxide and oxygen gases that permeate through the membrane are electrolyzed at this working electrode, and their reduction current response causes them to be electrolyzed. Then, the concentration of the gas passing through can be measured. In this way, only the end face portion of the working electrode that contacts the gas permeable membrane participates in the dissolved gas reduction reaction. Therefore, the following characteristics are exhibited. (1) It is possible to miniaturize the electrode to the limit of gas measurement accuracy. (2) The electrode reaction occurs instantaneously (usually within 1 second), and the amount of electrolyzed products is extremely small.
Even if used for a long period of time, it will not be affected by current changes due to obstructions, etc. (3) The internal solution of the electrode uses a non-aqueous solvent (for example, dimethyl sulfoxide), and the electrolytic products produced by the reduction reaction of carbon dioxide are reduced in the method of this invention compared to ordinary electrolytic methods. It contains almost no organic acids and decomposition products such as carbon monoxide. (4) Furthermore, since a non-aqueous solvent is used, hydrogen gas is not generated in the system unlike in conventional aqueous methods, and there is no interference with the carbon dioxide reduction reaction indication. Therefore, the electrolytic surface is less disturbed by organic acids such as oxalic acid, glyoxylic acid, glycolic acid, and formic acid, and the current response speed is fast. In this case, the gas selective permeable membrane is made of Teflon, a fluorine-containing compound, and its copolymer silicone,
Polymers of silicon-containing compounds and copolymers thereof, polypropylene, polyethylene, polyvinidene fluoride, and polymers of chlorine-containing compounds and copolymers thereof, polyurethane, polycarbonate, condensation polymers containing bisphenol A as one component, A membrane made of a polystyrene-containing polymer, a styrene copolymer, or the like is used. Hereinafter, one embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the structure of a detection electrode body 11 for measuring gas concentration.
The opening surface in the direction of the detection end is hermetically sealed by a gas permeable membrane 13 made of Teflon or the like using an O-ring. Inside the case 12, a working electrode 14 made of a silver wire with a diameter of 0.2 mm is set corresponding to the central axis portion thereof, and a reference electrode 15 and a counter electrode 16 are placed in a state sandwiching the working electrode 14. Set each independently. In this case, these electrodes 14 to 16 are arranged in parallel and embedded inside an inner cylinder 17 made of cylindrical polypropylene or the like, and this inner cylinder 17 is placed inside the outer case 12. A space is set inside the case corresponding to the electrode portion that is contacted via a ring, and the space is filled with an internal liquid 18 made of a non-aqueous solution. Here, the reference electrode 15 and the counter electrode 16 are each a silver wire with a diameter of 1.0 mm,
and platinum wire. Further, in the electrode body 11 configured in this way, the working electrode 14 that detects the gas that has passed through the gas permeable membrane 13 is set to be in contact with the gas permeable membrane 13. FIG. 2 shows an example of an effective structure when the electrode body 11 as described above is made smaller.
is formed into a cylindrical shape that does not contact the inner circumferential surface of the outer case 12, and an O-ring 19 is interposed between the inner circumferential surface of the case 12 and the outer circumferential surface of the inner cylinder 17 to form a storage portion for the internal liquid 18. I'll do what I do. And inner cylinder 17
The working electrode 1 is placed on the surface that contacts the gas permeable membrane 13.
4 is configured such that only the end surface of the working electrode 14 is in contact with the gas permeable membrane 13. FIG. 3 explains the principle means for measuring carbon dioxide and oxygen gas concentrations using the electrode body 11 configured as described above.
Carbon dioxide and oxygen gas from the gas mixer 23
This gas mixer 23 is filled with the mixed gas to be measured. The mixed gas from this gas mixer 23 is supplied to a measuring liquid 25 set in a container 24, and is stirred so that the gas partial pressure in this measuring liquid 25 becomes uniform. The gas permeable membrane 13 portion of the electrode body 11 is immersed in the liquid. Then, a pulsed voltage signal with a voltage setting is supplied to the electrodes of the electrode body 11 from the pulse electrolyzer 26, and the current -
The time response and current-voltage response are observed using a display/recording device 27 consisting of a digital oscilloscope or an X-Y recorder. FIG. 4 shows a specific example of the configuration of the pulse electrolysis device 26 section, in which a programmable voltage pulse generator 28 is controlled by a timing signal from a timing control circuit 27 to generate a pulse-like signal with a set voltage value. occurs. Then, this voltage-set pulse signal is sent to the potentiostat 29.
It is supplied to the electrode portion of the electrode body 11 via. This potentiostat 29 has a solution resistance value correction function, and a pulse generator 28 is connected between the working electrode 14 and the reference electrode of the electrode body 11.
A voltage set in is applied, and the current flowing through the counter electrode 16 is controlled so that the voltage between the electrodes does not change due to a change in the resistance value of the internal liquid 18. Then, the electrolytic current at the working electrode 14 detected via the potentiostat 29 is transferred to the first or second sample and hold circuit 30, 3.
This is to hold the value at 1. Here, when looking at the electrolytic potentials of carbon dioxide and oxygen gas, as shown in Figure 5, the electrolytic potentials of carbon dioxide and oxygen gas are greatly different, and the measured gas is a mixed gas of carbon dioxide and oxygen gas. When used as a device, when the potential applied to the working electrode is changed from a high state to a negative potential direction, oxygen gas is first electrolyzed and a reduction current corresponding to the oxygen gas concentration is detected. Then, when the electrode voltage is further made negative and reaches the electrolytic potential of carbon dioxide, the entire mixed gas of oxygen gas and carbon dioxide is electrolyzed, and a reduction current corresponding to the concentration of both gases is detected. Become so. Therefore, first measure the reduction current at the electrolysis potential of oxygen gas, then measure the reduction current at the electrolysis potential of carbon dioxide, and subtract the reduction current due to the oxygen gas electrolysis potential from this reduction current due to the carbon dioxide potential. For example, the reduction current of only carbon dioxide can be calculated, and the carbon dioxide concentration can be measured from the result of this subtraction. For this reason, in the pulse electrolysis device 26, when the timing control circuit 27 is in a state where a command is given to the voltage pulse generator 28 to set the oxygen gas electrolysis potential, the first sample hold circuit 30 is A hold command is given, and a hold command is given to the second sample and hold circuit 31 while also commanding the carbon dioxide electrolytic potential. That is, the first hold circuit 30 is set to hold the reduction current of oxygen gas, and the second hold circuit 31 is set to hold the reduction current value of the mixed gas containing carbon dioxide. An output signal corresponding to the oxygen gas concentration is obtained from the first sample hold circuit 30, and this signal is supplied to the display/recording device as an oxygen gas concentration signal. Further, the read output signal from the second sample and hold circuit 31 is supplied to a subtraction circuit 32 to perform an operation of subtracting the output signal of the first sample and hold circuit 30 from the output signal of the second sample and hold circuit 31. The subtraction circuit 32 is configured to perform this operation so that an output signal corresponding to the carbon dioxide concentration can be obtained from the subtraction circuit 32. In this case, the output signals from the first sample hold circuit 30 and the subtraction circuit 32 are supplied to the display/recording device after subtracting the residual current value by the calibration circuit 33 as appropriate, and the oxygen reduction limit current is , and the carbon dioxide reduction limit current can be directly read. FIG. 6 shows an example of the state of a pulse signal generated by the voltage pulse generator 28 according to a command from the timing control circuit 27. pulses a and b are generated. Here, the first
The voltage of the pulse corresponds to the electrolytic potential of the oxygen gas, the voltage of the second pulse corresponds to the electrolytic potential of carbon dioxide, and the voltage of the second pulse corresponds to the electrolytic potential of carbon dioxide. A hold command is given to the first sample and hold circuit 30, and a hold command is given to the second sample and hold circuit in response to the generation of the second pulse. In this way, in the case of a pulse electrolyzer,
Since pulse electrolysis is carried out instantaneously at a constant potential, electrolytic products are difficult to adhere to the electrode surface, thus effectively protecting against obstructions. It becomes like this.
Therefore, the electrolytic response speed becomes fast,
Moreover, it can be made to have excellent durability even when used for a long time. A specific example of an experiment using the gas concentration measuring device configured as described above will be described next. In this experimental example, the concentration of carbon dioxide gas and oxygen gas dissolved in the blood of a cow is measured.The measurement surface of the electrode body 11 as shown in FIG. insert. In this case, a mixed gas of carbon dioxide and oxygen gas is dissolved in the blood through a holofiber oxygenator, and this gas-exchanged blood is circulated by a roller pump, and the blood is heated to the electrodes. It was made to make contact with the body 11. here,
The mixing ratios of carbon dioxide and oxygen in the blood in the experimental examples were 713/0, 570.4/142.6,
499.1/213.9, 356.5/356.5, 213.9/499.1,
142.6/570.4, 0/713 [mmHg/mmHg], and the temperature of the measurement liquid was set to 37°C ± 0.1 by flowing constant temperature water through the jacket of the measurement electrode.
In addition, the pulse voltage signal used for electrolysis is as shown in Fig. 7, with the wave height of the first pulse being -1.3V (reduction of oxygen) and the wave height of the second pulse being -2.3V (reduction of carbon dioxide). The electrolysis time was 1.0001 seconds, the sampling time was 1 second, and the potential at which no electrolysis was performed was -0.3V. Table 1 shows the measurement results.

[Table] Based on these experimental results, when the oxygen reduction current value was plotted against the oxygen partial pressure (mmHg) in the supplied mixed gas, a good linear relationship was obtained as shown in FIG. On the other hand, when the limiting current value of carbon dioxide was plotted against the partial pressure of carbon dioxide in the supplied mixed gas, it became as shown in FIG. 9, and good linearity was also obtained. In other words, it is confirmed that by using such a measuring device, the concentrations of carbon dioxide and oxygen can be measured independently in blood in which carbon dioxide and oxygen gases are dissolved, using the same electrode. It is. Specific Effects of the Invention According to the measuring device according to the present invention as explained above, the concentration of each dissolved gas in a solution is measured using the same electrode as a reduction current under a voltage corresponding to each dissolved gas in a solution using a pulse electrolysis method. The measurement can be carried out instantaneously and with a very small amount of electrolysis products. In addition, in the above pulse electrolysis method, the reduction reaction of a gas corresponding to the concentration of carbon dioxide or oxygen gas occurs intermittently and instantaneously at a constant potential, so it takes time to reach gas-liquid equilibrium on the electrode surface. The current response time is fast, within 1 second. Furthermore, since the working electrode of the electrode body can be set to a state where gas concentration is detected at its cross section, it is highly effective in miniaturizing the electrode. This working electrode can operate even when in contact with a solid surface, and can be effectively used as a transdermal electrode, and can be effectively used for measuring carbon dioxide and oxygen concentrations in blood, etc. It is possible to do it.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an electrode body structure for explaining a gas concentration measuring device according to an embodiment of the present invention, FIG. 2 is a sectional view showing another example of the electrode body structure, and FIG. 3 is a sectional view showing another example of the electrode body structure. Fig. 4 is a block diagram explaining the principle of the measuring means using the above electrode body, Fig. 4 is a block diagram explaining the pulse electrolyzer used in the above measuring means, and Fig. 5 is the state of the electrolytic potential of carbon dioxide and oxygen gas. FIG. 6 is a diagram showing the state of the pulse voltage signal supplied to the electrode section in the pulse electrolyzer, FIG. 7 is a diagram showing the state of the electrode supply pulse signal in an experimental example of the above device, and FIG. FIGS. 8 and 9 are diagrams plotting the states of oxygen and carbon dioxide reduction currents in the above experimental example, respectively. 11... Electrode body, 12... Outer case, 13...
Gas permeable membrane, 14... Working electrode, 15... Reference electrode, 16... Counter electrode, 18... Internal liquid, 26...
...Pulse electrolyzer, 27...Timing control circuit, 28...Programmable voltage pulse generator,
29... Potentiostat, 30, 31... First and second sample and hold circuits, 32...
Subtraction circuit.

Claims (1)

[Scope of Claims] 1: an outer case having at least one opening; a gas permeable membrane configured to seal the opening of the case and facing a portion where a measurement gas exists; A working electrode set inside the case with its tip in contact with the gas permeable membrane, a reference electrode and a counter electrode set independently from the working electrode, and a non-aqueous system filled and set inside the outer case. a solvent; and means for applying a pulsed electrolytic voltage in which at least two specified potentials are set between the working electrode and the reference electrode, and the voltage is set to the pulsed electrolytic voltage as described above. A gas concentration measuring device characterized in that a reduction current is measured corresponding to at least two types of potentials, and the concentration of a predetermined gas is determined based on the measured values.