Abstract:
An ion exchange membrane for use in sensors which measure dissolved gases. Disclosed is a method for using ion exchangers in gas permeable membranes to maintain optimal conditions in the electrolyte contained within the sensor. This can greatly extend the lifetime and improve the stability relative to sensors of similar size and electrolyte volume that have been constructed using prior art.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of provisional application Ser. No. 60/109,431 filed Nov. 23, 1998. 
     US Patents 
     U.S. Pat. No. 4,933,048 Lauks Application No. 156262 Filed Feb. 16, 1988 
     U.S. Pat. No. 5,212,050 Meir, et al. Application No. 568441 Filed Aug. 15,1990 
     U.S. Pat. No. 4,534,356 Papadakis Application No. 403,808 Filed Jul. 30, 1982 
     U.S. Pat. No. 4,409,980 Yano et al. Application No. 293304 Filed Aug. 17, 1981 
     European Patents 
     EP0496521 Tsukada et al. Application No, EP19920300305 19920114 
     Other References Cited 
     Janata, J. Principles of Chemical Sensors, Plenum Publishing, 1991, Chapter 4; Polarographic Oxygen Sensors Gnaiger, E, and Forstner, H. (eds.) Springer-Verlag, 
     Kesting, R. Synthetic Polymer Membranes, 2 nd  Ed, Wiley-Interscience, 1985 Ion Exchange Processes: Advances and Applications, A. Dyer, M. J. Hudson, P. A. Williams.(Eds.) Royal Society of Chemistry, 1993 
     Liquid Membranes: Theory and applications, Noble, R. D and Way, J. D (Eds.) ACS Symposium Series 347, American Chemical Society, 1987 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     REFERENCE TO MICROFICHE 
     Not Applicable. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to the measurement of dissolved gases in liquid samples by means of an amperometric sensor. Such sensors, known collectively as Clark cells, have been extensively described (Janata, J. Principles of Chemical Sensors, Plenum Publishing, 1991, Chapter 4; Polarographic Oxygen Sensors Gnaiger, E, and Forstner, H. (eds.) Springer-Verlag, 1983) and are widely used for the analysis of gases which are readily reduced or oxidized such as hydrogen sulfide, NO, CO, or oxygen. Clark cells consist of a gas permeable membrane enclosing an electrolyte in contact with a working and reference electrode. The gas crosses the membrane by diffusion, and is reduced or oxidized at the working electrode thereby creating a current flow. The stability and reliability of Clark cells depends on many factors. Of importance to this invention is the limitation on sensor lifetime imposed by the amount of electrolyte within the Clark cell. Since the oxidation and reduction processes consume components of the electrolyte, Clark cells are inherently prone to instability and limited lifetime due to the exhaustion of the electrolyte. This problem is particularly acute for small electrodes (microelectrodes). There have been many attempts in the past to address this shortcoming through mechanical means, for example European Patent No. EP0496521 proposes such a method. In addition, Clark cells are expensive to construct and require frequent maintenance and calibration. 
     BRIEF SUMMARY OF INVENTION 
     A practitioner skilled in the art will recognize that exhaustion of the electrolyte is a serious barrier to the construction of long lived and stable Clark cells. The invention describes a membrane with a combination of properties: selective permeability for the gas to be analyzed and an ion exchange capacity to allow discharge of the ionic byproducts of the redox reaction. A membrane formed in this manner will allow an amperometric sensor to function in a stable manner for a much longer period of time than a sensor that must rely on only the ions present in the original electrolyte volume. This in turn will allow someone skilled in the art to produce a very small AND long lived, stable sensor. 
     Ion exchangers for both cations and anions are well-known, either formulated as a functionalized polymer membrane (see for example Kesting, R. Synthetic Polymer Membranes, 2nd Ed, Wiley-Interscience, 1985) or as a liquid membrane containing an ion exchanger (see for example Ion Exchange Processes : Advances and Applications, A. Dyer, M. J. Hudson, P. A. Williams.(Eds.) Royal Society of Chemistry, 1993). Both types facilitate the transfer of an ion across the membrane in exchange for a counter-flow of another ion. For example, quaternary ammonium salts in liquid membranes will facilitate the exchange of chloride ions flowing in one direction across the membrane with a flow of bromide ions moving in the opposite direction. 
     For the purposes of this invention, the membrane can be formed in a number of ways. One way, is as a supported liquid membrane (SLM) (see Liquid Membranes: Theory and applications, Noble, R. D and Way, J. D (Eds.) ACS Symposium Series 347, American Chemical Society, 1987). In such membranes a porous support polymer contains a solvent imbibed in the pores. An ion-exchanger is dissolved in the solvent thereby creating the SLM. A related SLM is a solvent-polymer membrane in which the solvent plus ion exchanger acts as a plasticizer for a polymer. For example a poly(vinyl chloride) film containing a phthalate plasticizer and an ion exchanger can create an ion exchange membrane. A third type of ion exchange membrane can be formed by modification of a polymer backbone to incorporate ion exchange groups directly linked to the polymer. Films of such materials can be cast or fabricated in a form suitable for this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 . Is a graphical representation of the processes embodied in the current invention. It presents an idealized oxygen sensor and shows how the ion exchange membrane would keep the interior electrolyte replenished from the environment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the performance of many gas sensors can be improved using the principles outlined in this invention, the description of the invention will focus on a sensor for dissolved oxygen. A Clark cell for this system consists of a inert cathode, typically gold or platinum, and a reversible anode such as silver/silver chloride. Oxygen reduction at the cathode produces hydroxide ions that build up in the vicinity of the cathode. As current flows, there is a concomitant depletion of chloride ions in the vicinity of the anode. These concentration changes alter the stability of the sensor, and as the electrolyte is consumed, will ultimately inactivate the sensor. FIG. 1 shows, in diagrammatic form, how the invention alleviates these problems. This problem has been central to Clark cells since their inception and is mentioned as a problem in many earlier patents (See U.S. Pat. No. 5,212,050). 
     The invention describes an anion exchanger within the membrane which is capable of removing the hydroxide ions produced and capable as well of supplying the required chloride ions to the inside of the cell. The external solution being analyzed acts as a sink for hydroxide ions and a source of chloride ions in this system. For example, dissolved oxygen in seawater could be analyzed with such a device. The invention will ensure a longer working lifetime and greater stability of the sensor over time, when measured against sensors produced by the current state of the art with equivalent electrolyte volumes. The medium in which the gas is being measured need only have sufficient chloride ions for the exchange mechanism to operate and sufficient capacity to absorb hydroxide ions, opening the analysis of dissolved gases to a wide range of possible substances, including foodstuffs and biological fluids. 
     For the oxygen sensors described above, the chemical nature of the anion exchanger is not critical provided that the rate of anion exchange is sufficient to maintain the electrolyte composition within the sensor. Suitable anion exchangers may include, but are not restricted to: 
     ammonium salts and quaternary ammonium salts (R1R2R3R4N + X − ) 
     guanidinium salts (R1R2NC + (NR3R4)NR5R6X − ) 
     cationic metal complexes (L n M m+ X m− ) 
     other cationic anion exchangers 
     where R, R1,R2,R3,R4,R5,R6 are hydrogen, alkyl, aryl, or other groups sufficient to create a high affinity of the exchanger for the membrane phase, X −  is an anion such as chloride, bromide, acetate etc., and L is a ligand for the metal M m+  such that the complex has a high affinity for the membrane phase. 
     One preferred version of an oxygen sensor using this invention is based on a SLM fabricated from a microporous Teflon membrane with a high-boiling solvent such as ortho-nitrophenyl octyl ether or dioctyl adipate imbibed in the pores. A related version uses a high molecular weight poly(vinyl chloride) plasticized with ortho-nitrophenyl octyl ether or dioctyl adipate. The membrane is cast from a solution of the polymer and plasticizer in a volatile solvent such as tetrahydrofuran or trifluroethanol. Either type of membrane can be fabricated with 1-5 wt % of added anion exchanger preferably drawn from the list of types disclosed above. 
     A preferable embodiment of the invention is to form the ion exchange membrane so as to be a “thin film”. By this means the response of a sensor using such a membrane can be significantly improved. The effect of “thin” membranes on sensor performance has been discussed frequently in the patent literature (For example, see U.S. Pat. No. 4,409,980). 
     Membranes such as those described by the invention can be applied to form an amperornetric sensor with a large number of geometries and construction methods. The invention is capable of being applied in place of existing gas permeable membranes which otherwise lack the ion exchange capability to improve the lifetime and stability of an existing sensor. The invention can also be applied to “solid state” micro-fabricated sensor systems to more fully take advantage of the low electrolyte volumes required. This could, for instance, take the form of a membrane cast in place over a suitable electrolyte for the oxygen sensor which has in turn been placed over a pair of electrodes so as to form a “solid state” sensor. Such a sensor would have the ability to be stored “dry” to be “wetted” before use (See U.S. Pat. No. 4,933,048). Other prior art pertaining to permeable polymeric membranes can be found in U.S. Pat. No. 4,534,356 where a bound electrolyte is discussed but does not include ion exchangers or the regulation of the internal electrolyte composition by such exchangers.