EP0788597A1

EP0788597A1 - Chemical sensor with diffusion barrier

Info

Publication number: EP0788597A1
Application number: EP95931731A
Authority: EP
Inventors: Edward E. Parsonage; Mark K. Debe
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1994-10-24
Filing date: 1995-09-06
Publication date: 1997-08-13
Also published as: MX9702850A; WO1996012949A1; JPH10507831A; KR970707439A; BR9509481A

Abstract

A diffusion rate limited amperometric electrochemical sensor is provided. The sensor has at least two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, said diffusion barrier being a porous membrane containing within the pores of the membrane a low vapor pressure liquid phase in which a gas to be detected is soluble. Also provided are respirators, personal exposure indicators and environmental indicators, as well as a method of sensing the presence of gas in the air and a method of preparing the sensor.

Description

CHEMICAL SENSOR WITH DIFFUSION BARRIER

Field of the Invention

The present invention relates to a chemical sensor for gases which has a rate limiting diffusion barrier.

Background of the Invention

Electrochemical sensors have at least two electrodes which are connected by both an ionically conducting electrolyte and an electronically conducting external circuit. In a sensor, one electrode is exposed to the gas to be detected and is called the sensing electrode or working electrode. The gas may be oxidized or reduced at the sensing electrode, depending on the nature of the gas, the electrolyte, and the electrical potential of the electrode. The other electrode, called the counter electrode, supports a reaction to balance the oxidation or reduction of the gas of interest, and the rate of gas reacting at the sensing electrode is measured by the amount of current flowing in the external circuit. For a sensor, the electrolyte is most conveniently a piece of ionically conducting solid polymer electrolyte with the two electrodes attached on opposite sides.

If the rate of reaction of the gas at the sensing electrode is controlled by the rate of electron transfer, then the current measured in the external circuit will depend on the electrical potential of the sensing electrode. If, on the other hand, the rate controlling step is the rate of diffusion of the gas to the electrode, then the measured current will be independent of potential and will be a measure of the rate at which the gas molecules reach the electrode, which in turn is a measure of the concentration of the gas. This is called the limiting current mode and is the desirable condition for operating a sensor. While it is possible for a system to be operating in the limiting current mode and function as a sensor without a separate diffusion barrier, generally a diffusion barrier is required to ensure that the system will perform in the limiting current mode. Diffusion barriers can be solid nonporous films in which the gas to be detected can dissolve and diffuse by solid state diffusion. For example, in an electrochemical sensor for oxygen, suitable membrane materials include polypropylene, polyethylene, TEFLON™ and MYLAR™. Also disclosed for use in oxygen sensors are silicone rubber and other rubbers.

Generally, in solid membrane diffusion barriers, diffusion rates are low which requires that the membranes must be very thin in order to achieve sufficient sensitivity of the sensor. This can lead to handling and breakage problems. Also, the temperature dependence of the diffusion coefficients are high which makes the sensor response vary significantly with temperature changes.

Some of the deficiencies of solid membrane diffusion barriers may be overcome by using porous membranes with limited pore size or membranes with one or more capillary openings, in which the gas diffuses through the barrier in the gas phase rather than by solid state diffusion. Suitable materials which have been reported include, some grades of porous unsintered polytetrafluoroethylene tape and porous polycarbonate films. Where the porosity of the membrane is greater than desired, the porosity can be reduced by pre-pressing membranes of polytetrafluoroethylene or impregnating the pores with a liquid containing a solid component which partially fills the pores after removal of the liquid.

In porous membrane diffusion barriers, sharp changes from a cold to a warm environment may cause water vapor to condense on the membrane which can close the pores. Changes in temperature can also cause the sensor output to vary. Also with a porous membrane diffusion barrier an air pressure shock, such as the pressure wave inside a car when the door is slammed shut, or sudden movement of the sensor can cause a burst in the sensor output and may trigger an alarm. A liquid membrane may be formed by immobilizing it between two gas- permeable films. Such membranes have been used for separating gases from a mixture with one of the gas-permeable films having liquid-filled pores allowing the diffusive transfer of an active carrier species into the liquid membrane. A liquid can also be immobilized in the pores of a porous polymer membrane. Such membranes have been used for separating gases from a mixture. A two-step sensing system in which an immobilized liquid membrane is used to selectively separate a gas of interest from a mixture of gases, and the selected gas then passes into a chamber where it is detected with general purpose sensors such as semiconductor or catalytic gas sensors has been disclosed. In this two-step process, the immobilized liquid membrane is preferably a porous polymer membrane, the pores of which contain a hydrophobic, low vapor pressure, chemically compatible liquid which wets the matrix material and has a high solubility and diffusivity for the gas of interest.

Summary of the Invention The present invention provides a diffusion rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to the electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, said diffusion barrier being a porous membrane containing within the pores of the membrane a low vapor pressure liquid phase in which a gas to be detected is soluble.

The present invention further provides a method of sensing the presence of a gas comprising the steps of permitting passage of said gas through a diffusion barrier comprising a porous membrane containing within the pores of the membrane a low vapor pressure liquid phase in which the gas to be detected is soluble, said gas contacting a sensing electrode which is proximate said diffusion barrier and causing said gas to be oxidized or reduced, generating ions, permitting ions to pass through electrolyte between electrodes, permitting ions to contact a counter electrode, said counter electrode supporting a reaction to balance the oxidation or reduction of the gas of interest, and quantifying the rate of gas reacted at said sensing electrode by determining the current flow in an external circuit connecting said electrodes.

The present invention further provides a method of preparing a rate limited amperometric electrochemical sensor comprising the steps of providing a porous membrane, imbibing said membrane with a low vapor pressure liquid in which a gas to be detected is soluble, and bringing said imbibed membrane proximate an electrochemical sensor.

The present invention still further provides a respirator comprising a facepiece defining a space covering at least the mouth and nose of a wearer, at least one air inlet port, at least one air outlet port, means for filtering one or more components from external air drawn into said space, means for detecting a gas in said space comprising a rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, and a signal emitting means operatively connected to said external circuit, said diffusion barrier being a microporous membrane containing within the pores of the membrane a non-evaporating liquid phase in which a gas to be detected is soluble. The present invention also provides a supplied air respirator comprising a facepiece defining a space covering at least the mouth and nose of a wearer, at least one air inlet port, a supply of breathable air for transmission through said air inlet port, at least one air outlet port, means for detecting a gas in said space comprising a rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, and a signal emitting means operatively connected to said external circuit, said diffusion barrier being a microporous membrane containing within the pores of the membrane a non-evaporating liquid phase in which a gas to be detected is soluble.

The present invention also provides a personal exposure indicator or environmental indicator comprising a diffusion rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to the electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, said diffusion barrier being a porous membrane containing within the pores of the membrane a low vapor pressure liquid phase in which a gas to be detected is soluble and a signal emitting means operatively connected to said external circuit.

The rate limited amperometric electrochemical sensors of the present invention have fast response times, excellent sensitivity, are easy to handle, and exhibit few failures due to leakage or breakage of the diffusion barrier. The membranes of this invention are particularly easy to construct reliably and reproducibly. The pores of the membrane material are simply imbibed with the desired liquid and blotted to remove any excess. .Also, the liquid used in the diffusion barrier may be selected for optimum solubility for the substance being detected, providing a means for optimization not available with gas phase diffusion membranes. Additionally, immobilized liquid membrane diffusion barriers are less affected by condensation problems, air pressure shocks, or mechanical jolts than are gas phase diffusion membranes. The diffusion barrier in this invention is rate limiting, and the concentration dependence of the signal depends on the diffusion properties of the membrane. Further, selective separation of the gas to be detected is normally not required.

Brief Description of the Drawings FIG. 1 is a cross-sectional view of a sensor of the present invention.

FIG. 2 shows the response curve for the hydrogen sulfide sensor of Example 1.

FIG. 3 shows the response curve for the hydrogen sulfide sensor of Comparative Example 1. FIG. 4 shows the response curve for the hydrogen sulfide sensor of

Example 2.

FIG. 5 shows the response curve for the hydrogen sulfide sensor of Comparative Example 2.

FIG. 6 shows the response curve for the hydrogen sulfide sensor of Example 3. FIG. 7 shows the response curve for the hydrogen sulfide sensor of Comparative Example 3.

FIG. 8 shows the response curves as a function of hydrogen sulfide concentration for the hydrogen sulfide sensor of Example 4. FIG. 9 shows the sensitivity as a function of hydrogen sulfide concentration for the sensor of Example 4.

FIG. 10 shows the response curve as a function of humidity for the sensor of Example 4.

FIG. 1 1 shows the response curve for 40 minute exposure to 5 ppm hydrogen sulfide for the sensor of Example 4.

FIG. 12 shows the response curve of the sensor of Example 4 as a function of temperature at an exposure level of 10 ppm hydrogen sulfide.

FIG. 13 shows the response curve of the sensor of Example 4 as a function of carrier flow rate. FIG. 14 shows a respirator of the invention utilizing the diffusion rate limited amperometric electrochemical sensor.

FIG. 15 shows a supplied air respirator of the invention utilizing the diffusion rate limited amperometric electrochemical sensor.

FIG. 16 shows a personal exposure or environmental indicator of the invention utilizing the diffusion rate limited amperometric electrochemical sensor.

FIG. 17 is a sectional view of a sensing device suitable for use with the respirators and exposure indicators of the invention

Detailed Description of the Invention Amperometric electrochemical sensors useful in the present invention include any sensors that are capable of electrochemically oxidizing or reducing an analyte and generating a proportional current. Such sensors are described, for example, in U.S. Pat. No. 4,865,717 (Stetter et al.), U.S. Pat. No. 4,913,792 (Nagata et al ), U.S. Pat. No. 4,227,984 (Dempsey et al.), U.S. Pat. No. 4,025,412 (LaConti), U.S. Pat. No. 4,894,138 (Gambert et al.), U.S. Pat. No. 4,633,704 (Tantram et al ), U.S. Pat. No. 4,948,496 (Chand), and U.S. Pat. No. 4,591,414 (Zaromb et al ). Particularly preferred are sensors fabricated with the nanostructured electrode membranes described in U.S. Pat. No. 5,338,430 (Parsonage et al ).

The porous membranes useful in the present invention are any porous membrane capable of imbibing a liquid. The membranes have a porosity such that simply immersing the membrane in the liquid causes the liquid to spontaneously enter the pores by capillary action. The membranes, before imbibing preferably have a porosity of at least about 50%, more preferably at least about 75%. The porous membranes preferably have a pore size of about 10 n to 100 μm, more preferably 0.1 μm to 10 μm and a thickness of about 2.5 μm to 2500 μm, more preferably about 25 μm to 250 μm. The membranes are generally prepared of polytetrafluoroethylene or thermoplastic polymers such as polyolefins, polyamides, polyimides, polyesters, polyether sulfones, polycarbonates, cellulosic polymers, polyvinyl chloride, polyvinylidene fluoride and the like. Examples of suitable membranes include, for example, those disclosed in U.S. Pat. No. 4,539,256 (Ship an), U.S. Pat. No. 4,726,989 (Mrozinski), U.S. Pat. No. 4,247,498 (Castro) and U.S. Pat. No. 3,953,566 (Gore).

Suitable liquids for imbibing the membrane include, for example, mineral oil, polypropylene glycol, silicones, and other liquid-like oligomers and polymers. The diffusion barriers may be prepared by soaking the porous membrane in the liquid until the liquid has been imbibed and then removing excess liquid, for example, by blotting. Alternatively, the diffusion barriers can be prepared by soaking the porous membrane in a solution of the liquid and then evaporating the solvent. Further, the diffusion membrane can be prepared utilizing liquid-solid or liquid-liquid phase separation techniques according to U.S. Pat. No. 4,539,256 (Shipman), U.S. Pat. No. 4,726,989 (Mrozinski) or U.S. Pat. No. 4,247,498 (Castro) and the blending compound may be left in the membrane if the gas to be detected is soluble in the blending compound.

FIG. 1 shows a preferred embodiment of the diffusion rate limiting amperometric electrochemical sensor 10 having a sensor 12 comprising a sensing electrode 13, a counter electrode 14 and electrolyte 15 and a diffusion barrier 16 comprising porous membrane 17 containing a low vapor pressure liquid 18 in the pores of the membrane. The sensor may further comprise a porous buffer layer of, for example tissue or scrim (not shown) between the sensor and the membrane to prevent direct contact of the low vapor pressure liquid and the sensing electrode. FIG. 14 shows a respirator 20 of the invention. Respirator 20 contains a pair of air purifying respirator cartridges 22, 23 disposed laterally from a face mask 24. The diffusion rate limiting amperometric electrochemical sensor is contained in flow through housing 26 on which is mounted a signal emitter 27. FIG. 15 shows a supplied air or powered air purifier respirator 30 which has a face piece 32 connected to a shroud 33 and an air supply hose 35. Air is provided by air supply 37. a sensing device 36 can be located in the air supply line to monitor the air supply. When the respirator is the supplied air respirator, the air supply would be a self-contained unit. When the respirator is a powered air purifier respirator, ambient air would be blown through a filtering system into air supply hose 35.

FIG. 16 shows a personal exposure indicator or an environmental indicator 40 which can be worn on a user's clothing or located in a specific area. A clip 42 may optionally be provided to attach the indicator onto a user's pocket or belt. The device has a fluid coupling membrane 44 beneath which the sensor is located and a signal 46 may be provided as a light emitting diode.

FIG. 17 shows a sectional view of the type of sensing device 50 that could be used in each of the respirators and indicators. A fluid coupling membrane 51 covers the sensing port beneath which lies the diffusion rate limited amperometric electrochemical sensor which comprises rate limiting diffusion barrier 52, sensing electrode 54, electrolyte 53 and counter electrode 55. The diffusion rate limited sensor is connected to microprocessor 56 which is connected to signal means 57.

In the following examples, all parts and percentages are by weight unless otherwise specified. Example 1 and Comparative Example C 1

An amperometric electrochemical sensor for hydrogen sulfide was constructed as described in U. S. Patent 5,338,430 (Parsonage et. al.). A polynuclear aromatic hydrocarbon, N',N'-di(3,5-xylyl)perylene-3,4,9,10- bis(dicarboximide), available from American Hoechst Corp. as C.I. Pigment Red 149, hereinafter called "perylene red", was vacuum vapor deposited onto a flexible, copper-coated polyimide temporary substrate, near room temperature, to a thickness of about 0.1 to 0.15 micrometers. This was annealed in a vacuum, causing the perylene red film to convert to a layer comprising discreet, oriented whiskers 1-2 micrometers in length. The whiskers were then coated with a mass equivalent thickness of 175 nm of palladium by vacuum evaporation, producing the nanostructured elements.

Next, a curable solid polymer electrolyte formulation was prepared consisting of 0.06 g 90% benzene sulfonic acid (Aldrich Chemical Co.) in 1 ml tetrahydrofuran, 1 ml catalyst solution consisting of 25 microliters dibutyl tin dilaurate in 10 ml tetrahydrofuran, 2 ml 600 molecular weight poly(ethylene glycol) and 1 ml DESMODUR™ N100 (available from Farbenfabriken Bayer AG) multifunctional isocyanate. The sensor was made as follows: Approximately 0.1 ml of the curable solid polymer electrolyte solution was placed between two 10 mm diameter discs cut from the temporary substrate with the nanostructured elements encapsulated in the solid polymer electrolyte supporting the nanostructured elements. The sample was cured at approximately 40°C for a period of about 1 hour. The temporary substrate of the nanostructured elements was then peeled away from the cured solid polymer electrolyte leaving the fresh, Pd-coated nanostructured electrodes embedded in the surface of each side of the solid electrolyte disc. Electrical contact to both sides of the nanostructured electrode membrane was made using 0.3 mm diameter copper wire adhered to the electrode membrane with a trace amount of conducting silver paint ( available from GC Electronics, Rockford, IL). One side of the membrane (counter electrode) was then isolated by covering the entire surface with a 10 mm diameter piece of vinyl electrical tape. A microporous polypropylene was prepared as described in U.S. Pat. No. 4,726,989 (Mroanski). About 0.30 weight percent of a dibenzylidine sorbitol nucleating agent (MILLAD™ 3905, available from Milliken Chemical Co.) was dry blended with polypropylene resin (PRO-FAX™ 6823, available from Himont Incorporated, Wilmington, Del.). This was melt-blended with 52.6 weight percent mineral oil (Amoco White Mineral Oil #31 USP Grade, available from Amoco Oil Co.) and extruded at a melt temperature of about 205°C on a BERSTORFF™ 40 mm twin screw extruder fitted with a 30.5 cm by 0.04 mm slit gap sheeting die positioned above a water quench tank maintained at about 37.8°C . The extruder was operated at about a 227 cc/min throughput rate to produce a film collected at the rate of about 7.6 meters per minute. The resultant film was solvent washed in 1,1,1-trichloroethane for five minutes in a restraining device to remove the mineral oil, and was then dried at room temperature. The film was then stretched about 1.5 to 2 times its original length and width. The properties of the stretched film were: Gurley Value - 30 seconds (the time to pass 50 cc of air through the film according to ASTM-D-726-58 Method A); Bubble Point - 0.39 micrometers (largest effective pore size measured according to ASTM-F-316-80); Thickness - 0.017 cm; Void Volume - 72%; and Residual Oil - 1 1.6%.

A diffusion barrier was formed by immersing the porous membrane material in heavy white mineral oil (Mineral Oil, Heavy, White, catalog no.

33,076-0 available from Aldrich Chemical Co.). The mineral oil strongly wet the membrane material resulting in a transparent film of solid consistency with no observable void volume. The membrane was removed from the liquid and blotted to remove excess liquid from the surface. A one centimeter diameter piece of tissue (KIMWIPES™ No. 34133, 1-ply tissue wiper) was placed on the front of the sensor working electrode. A one centimeter diameter sample of the diffusion barrier were mounted in front of the tissue-covered sensor working electrode.

The thus-formed rate limiting amperometric electrochemical sensor was tested in a 500 cc sample jar with exposure to 10 ppm hydrogen sulfide which was generated by dilution from 100 ppm or 500 ppm hydrogen sulfide in an air balance. Exposure was carried out for 10 minutes after a 10 minute equilibration period. The sensor was connected to a Keithley 197 A electrometer to monitor the signal during exposure. A load resistance of 100 K_ connected the working and counter electrodes. The result is shown in FIG. 2.

For Comparative Example 1, a sensor having no tissue or diffusion barrier on the working electrode was exposed to hydrogen sulfide in the same manner as the sensor of Example 1. The result is shown in FIG. 3.

The efficacy of the rate limiting properties of the element is clearly demonstrated by the approximately 80% reduction in the steady state signal compared to when no diffusion element is present, and thus demonstrating that the sensor is operating in the limiting current mode. As discussed above, to operate the sensor in such a mode is necessary both for consistent sensitivity among replicate sensors and also to provide signal and baseline stability.

Example 2 and Comparative Example 2 In Example 2, the rate limiting amperometric electrochemical sensor used in Example 1 was exposed to relative humidity of 10% for 10 minutes, 80% for 40 minutes and 10% for an additional 10 minutes. FIG. 4 shows the baseline change. In Comparative Example 2, the same humidity exposure was carried out as in Example 2 except the sensor had no diffusion barrier. The results are shown in FIG. 5.

Example 3 and Comparative Example 3

In Example 3, microporous polypropylene membrane material

(CELGARD™ 2400, available from Hoechst Celanese Corp.) having a thickness of 25 μm was imbibed with heavy white mineral oil (available from Aldrich

Chemical Co.) and mounted in front of the sensing electrode as in Example 1. The sensor was exposed to 5 ppm hydrogen sulfide at 22°C, 10% relative humidity and a flow rate of 10 liters per minute with a load resistance of 100 KΩ. FIG. 6 shows the sensor response. In Comparative Example 3, the sensor was subjected to the hydrogen sulfide as in Example 3 but without the diffusion barrier. FIG. 7 shows the sensor response. Again, the approximately 70% reduction in the steady state signal of Exampole 3 relative to Comparative Example 3illustrates the efficacy of this particular immobilized liquid membrane to act as a rate limiting element.

Example 4

In Example 4, the rate limiting amperometric electrochemical sensor prepared as in Example 3 and containing the diffusion barrier was modified such that the working electrode was biased +0.2 V anodically, vis-a-vis the counter electrode, to favor oxidation A load resistance of 200 KΩ was used in this case. FIGS 8 and 9 show the response curves and calibration of the sensor as a function of hydrogen sulfide concentration FIG 10 illustrates the stability of the sensor baseline and sensitivity to changes in the ambient humidity level. FIG. 1 1 illustrates the signal constancy for longer-term exposure to 5 ppm hydrogen sulfide. FIG. 12 illustrates the temperature stability of the baseline and sensitivity. FIG. 13 illustrates the stability of the steady-state signal to changes in linear flow velocity

Example 5 and Comparative Example 4

In Example 5, a portion of the microporous membrane prepared in Example 1 was imbibed with polypropylene glycol diol (625 molecular weight, available from Aldrich Chemical Co.)and mounted on a sensing electrode as in Example 1. The sensor was tested with 10 ppm hydrogen sulfide at 22°C, 10% relative humidity and 10 liters per minute flow rate. The response was monitored using a 100 KΩ load resistance. The steady state response to 10 ppm hydrogen sulfide was 3 mV.

In Comparative Example 4, a sensor having no diffusion barrier was tested in the same manner as the sensor in Example 5. The steady state response to 10 ppm hydrogen sulfide was 14 m V. This difference in response demonstrates the efficacy of this particular composition to act as a mass transport limiting device on the amperometric electrochemical sensor. Examples 6-10

In these examples, microporous membranes (CELGARD™ 2400, 25 μm thick, available from Hoechst Celanese Co.)were imbibed in solutions of heavy white mineral oil (available from Aldrich Chemical Co.) in xylene (boiling range 137-144°C, available from EM Science) in concentrations of 5, 10, 15, 20 and 25 percent xylene by volume, respectively. The imbibed membranes were blotted to remove excess liquid and the xylene was allowed to evaporate over 24 hours. The samples were then mounted onto hydrogen sulfide sensors prepared as in Example 1 and tested with 10 ppm hydrogen sulfide at 30% relative humidity and 23°C Each of the samples tested gave diffusion limited response of 25% or less of that obtained without a membrane on the sensor. No correlation was observed between the original percentage of xylene in the liquid phase and the diffusion limited sensor response

The various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention and this invention should not be restricted to that set forth herein for illustrative purposes only.

Claims

What is claimed is:

1. A diffusion rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ionic charge between electrodes, and a diffusion barrier coextensive with or covering one of said electrodes, said diffusion barrier being a porous membrane containing within the pores of the membrane a low vapor pressure liquid phase in which a gas to be detected is soluble.

2. The diffusion barrier of claim 1 wherein said porous membrane comprises polytetrafluoroethylene, polyolefin, polyamide, polyimide, polyester, polyether sulfone, polycarbonate, cellulosic polymers, polyvinyl chloride or polyvinylidene fluoride.

3. The diffusion barrier of claim 1 wherein said porous membrane without the liquid phase has a porosity of at least about 50%

4 The diffusion barrier of claim 1 wherein said porous membrane has a pore size of about 10 nm to 100 μm.

5. The diffusion barrier of claim 1 wherein said porous membrane has a thickness of about 2.5 μm to 2500 μm.

6. The diffusion barrier of claim 1 wherein said liquid phase is an oligomer or polymer.

7. The diffusion barrier of claim 1 wherein said liquid is mineral oil, polypropylene glycol or a silicone.

8. A method of sensing the presence of a gas comprising the steps of permitting passage of said gas through a diffusion barrier comprising a porous membrane containing within the pores of the membrane a non-evaporating liquid phase in which the gas to be detected is soluble, said gas contacting a sensing electrode which is proximate said diffusion barrier and causing said gas to be oxidized or reduced, generating ions, permitting ions to pass through electrolyte between electrodes, permitting ions to contact a counter electrode, said counter electrode supporting a reaction to balance the oxidation or reduction of the gas of interest, and quantifying the rate of gas reacted at said sensing electrode by determining the current flow in an external circuit connecting said electrodes.

9. A method of preparing a rate limited amperometric electrochemical sensor comprising the steps of providing a porous membrane, imbibing said membrane with a non-evaporating liquid in which a gas to be detected is soluble, and bringing said imbibed membrane proximate an electrochemical sensor.

10. A respirator comprising a facepiece defining a space covering at least the mouth and nose of a wearer, at least one air inlet port, at least one air outlet port, means for filtering one or more components from external air drawn into said space, means for detecting a gas in said space comprising a rate limited amperometric electrochemical sensor comprising two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ions between electrodes, a diffusion barrier coextensive with or covering one of said electrodes, and a signal emitting means operatively connected to said external circuit, said diffusion barrier being a porous membrane containing within the pores of the membrane a non-evaporating liquid phase in which a gas to be detected is soluble.

11. A supplied air respirator comprising a facepiece defining a space covering at least the mouth and nose of a wearer, at least one air inlet port, a supply of breathable air for transmission through said air inlet port, at least one air outlet port, means for detecting a gas in said space comprising a rate limited amperometric electrochemical sensor comprising at least two electrodes, an external circuit connected to said electrodes, an electrolyte capable of conducting ionic charge between electrodes, a diffusion barrier coextensive with or covering one of said electrodes, and a signal emitting means operatively connected to said external circuit, said diffusion barrier being a microporous membrane containing within the pores of the membrane a non-evaporating liquid phase in which a gas to be detected is soluble

12 A personal exposure indicator comprising the sensor of claim 1 and a signal emitting means operatively connected to said external circuit

13 An environmental exposure indicator comprising the sensor of claim 1 and a signal emitting means operatively connected to said external circuit.