Abstract:
The invention includes a method and apparatus for a chromatography system having a chromatographic column for separating gases in a mixture from one another and an electrochemical gas sensor coupled to the chromatographic column for detecting a gas being emitted from the column. The electrochemical gas sensor further includes a substrate having a surface for depositing electrodes thereon, an ionomer membrane in contact with the surface, an electrode in contact with the surface, and an opening in the ionomer membrane in a location proximate to the electrode for permitting a gas to diffuse through the opening to simultaneously contact the electrode and the ionomer membrane within the opening.

Description:
PRIORITY APPLICATION  
       [0001]     This application is a divisional patent application of co-pending U.S. patent application Ser. No. 10/345,608 for a “Method and Apparatus for Enhanced Detection of a Specie Using a Gas Chromatograph,” filed Jan. 16, 2003, which is a continuation in part of co-pending U.S. patent application Ser. No. 09/443,875 for a “Film Type Solid Polymer Ionomer Sensor and Sensor Cell” filed Nov. 19, 1999, now issued as U.S. Pat. No. 6,682,638. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a gas chromatograph and detection unit for detecting minute quantities of a gas specie.  
       BACKGROUND OF THE INVENTION  
       [0003]     Gas chromatography is essentially a method of separation of mixtures of substances into their individual components. In a typical analysis of a sample by a gas chromatograph, the sample is introduced into a chromatographic column together with a carrier gas. At the end of the column the individual components are more or less separated in time. Generally, detection of the gas provides a time-scaled pattern which, by calibration or comparison with known samples, indicates the constituents of the test sample.  
         [0004]     Separation of the sample usually occurs within the column upon interaction with the stationary phase, whereby the distribution coefficients of the elements may cause the separation. Typically, the constituents of a test sample in a carrier gas are adsorbed and desorbed by a stationary phase material in a column. Polarity may also play a role in separating the components from one another over time. Differences in polarites may cause the components to attach to the stationary phase at different intervals. Elements exiting the column are typically detected by a detector and the results are usually charted, often resulting in a chromatogram.  
         [0005]     For gases that may have difficulty being detected by the detector, the system for the gas chromatography may optionally include a reactor, which generally heats the desired gas with a reactant to form a detectable compound. The reactant may be a gas, liquid, or solid and varies according to the desired gas to be detected. Typical reactants include air, hydrogen, and oxygen. A detectable compound is one that generally provides an electrical signal detectable by the detector.  
         [0006]     Typical detectors for measuring the gases exiting the column include mass spectrometers and electrolytic conductivity detectors. Other detection systems include thermal conductivity, flame ionization and argon detectors. Electrolytic conductivity detectors usually provide an electrical signal that is functionally related to the presence of a selected element.  
         [0007]     Electrolytic conductivity detectors are known for investigating the properties of electrolytes in solutions. Such devices typically include electrode surfaces with a continuous phase liquid electrolyte therebetween. These detectors may entail measuring a difference in resistance in the electrolytic material before and after the gas exiting the column enters the detector and is absorbed by the electrolytic material. If the gas was mixed with a reactant in the reactor, the reactant may also be absorbed in the electrolytic material before providing a detectable electrical signal. A possible disadvantage of the conductivity detector is that absorption by the electrolytic material takes time, which lengthens the detector&#39;s response time. The disadvantage may be exacerbated if both the gas and reactant need to be absorbed. Another possible disadvantage is the limited accuracy of the detector. Because the gas is detected indirectly, where the difference in resistance of the electrolytic material may indicate the type and/or concentration of the gas, a standard of deviation in the measurement error between the electrolytic material measurement and correlation from this measurement to the gas may negatively affect accuracy.  
         [0008]     A typical conductivity detector is described in U.S. Pat. No. 4,440,726 to Coulson and shown in  FIG. 1 . As shown, an electrolyte, reactant gas, and gas exiting from the column enter the capillary. Electrodes  24  and  28  are placed in the electrolyte solution which may measure the difference in resistance.  
         [0009]     Similar to the conductivity detector, the mass spectrometer and other detection systems of gas chromatography have potentially limiting abilities to detect gas with a high degree of sensitivity. As mentioned in U.S. Pat. No. 6,165,251 to Lemieux et al., gas chromatography systems in general have insufficient sensitivity to measure amounts of volatiles in the parts per billion concentration range.  
         [0010]     What is desired, therefore, is a gas chromatography system having a detector with improved sensitivity. Another desire is provide a gas chromatography system that detects gas in the parts per billion concentration range. What is also desired is a gas chromatography system having an improved response time.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, it is an object of the invention to provide a gas chromatography system having improved detection capabiltities.  
         [0012]     It is another object of the invention to a gas chromatography system having a sensitivity in the parts per billion range.  
         [0013]     It is a further object of the invention to provide a gas chromatography system having an improved response time.  
         [0014]     These and other objects of the invention are achieved by provision of a chromatography system having a chromatographic column for separating gases in a mixture from one another and an electrochemical gas sensor coupled to the chromatographic column for detecting a gas emitted from the column. The electrochemical gas sensor further includes a substrate having a surface for depositing electrodes thereon, an ionomer membrane in contact with the surface, an electrode in contact with the surface, and an opening in the ionomer membrane in a location proximate to the electrode for permitting a gas to diffuse through the opening to simultaneously contact the electrode and the ionomer membrane within the opening.  
         [0015]     For instances where a gas emitting from the column is difficult to detect, such as a gas that is not electrochemically active, the gas may be oxidized and/or reduced by a reactor prior to entering and being detected by the electrochemical gas sensor. Hence, a reactor may optionally be placed between and coupled to both the chromatographic column and the electrochemical gas sensor for facilitating oxidation and/or reduction.  
         [0016]     The opening in the ionomer membrane may further extend from a first surface to a second surface of the ionomer membrane for defining walls to facilitate guiding the gas to the electrode. Additionally, the electrochemical gas sensor may include a housing for containing the substrate, the ionomer membrane, and the electrode. The housing may include a gas diffusion passage in a location proximate to the electrode and having fluid connection with the opening. In some embodiments, the gas diffusion passage is angled or misaligned with in relation to the opening.  
         [0017]     In some embodiments, the substrate may include at least one hole extending from a first surface to a second surface of the substrate for permitting moisture to diffuse through the at least one hole to contact the ionomer membrane.  
         [0018]     To enhance sensitivity, the ionomer membrane may be wetted with a solution or moisture. Hence, the sensor may include a reservoir containing the moisture or solution. In some embodiments, the reservoir is located adjacent to the substrate and on a side of the substrate opposite the ionomer membrane. In these embodiments, moisture from the reservoir diffuses through the at least one hole in the substrate to wet the ionomer membrane.  
         [0019]     To facilitate diffusion of the moisture from the reservoir through the at least one hole, the sensor may have a wicking material in contact with the second surface of the substrate for drawing moisture from the reservoir toward said substrate. In addition to, or instead of placing the wicking material in this position, the wicking material may be located in the at least one hole of the substrate.  
         [0020]     To reduce diffusion time for the moisture from the reservoir, the substrate&#39;s thickness may be reduced, wherein said substrate is a foil.  
         [0021]     In another aspect of the invention, a method for detecting a gas in a gas chromatography system is provided. The method includes the steps of eluting a gas from a chromatographic column, coupling an electrochemical gas sensor to the chromatographic column, and detecting gas in an approximately parts per billion range.  
         [0022]     In situations where the gas emitted from the chromatographic column is difficult to detect, the method may further include the step of oxidizing and/or reducing the gas. To carry this step out, the method may add a reactant to the emitted gas and provide a reactor to heat both the gas and the reactant during oxidation/reduction.  
         [0023]     In another aspect of the invention, a method for detecting a gas in a gas chromatography system is provided, including the steps of eluting a gas from a chromatographic column, providing a substrate having a surface, depositing an electrode on the surface, contacting the electrode with an ionomer membrane, providing an opening in the ionomer membrane in an approximate area of the electrode, introducing the gas into the opening toward the electrode, and simultaneously contacting the gas with both the electrode and ionomer membrane.  
         [0024]     The method may further include the step of providing a housing and a gas diffusion passage in the housing in the approximate area of the electrode. Both the opening in the ionomer membrane and the passage in the housing guide gas toward the electrode as it is introduced into the sensor. The opening and passage, by varying their length and/or girth, may control the gas as it passes through the opening and gas diffusion passage toward the electrode.  
         [0025]     Similarly, the method may also provide at least one hole in the substrate for diffusing moisture from a reservoir through the at least one hole to the ionomer membrane and control the amount of moisture passing through the at least one hole by varying its size.  
         [0026]     The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  depicts a conventional detector cell for electrochemical detection in accordance with the prior art.  
         [0028]      FIG. 2  depicts a chromatography system in accordance with the invention.  
         [0029]      FIG. 3  depicts a method for providing the chromatography system of  FIG. 2 .  
         [0030]      FIG. 4  depicts one embodiment of an electrochemical gas sensor used for the chromatography system of  FIG. 2 .  
         [0031]      FIG. 5  depicts another embodiment of an electrochemical gas sensor used for the chromatography system of  FIG. 2 .  
         [0032]      FIG. 6  depicts an exploded view of the electrode shown in  FIGS. 4 and 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]      FIG. 1  depicts a conventional detector cell for electrochemical detection. Cell  10  is typically placed at the exit of a gas chromatographic column and is usually formed from glass or other similar dielectric material. A first capillary extends horizontally from left to right across the cell  10 . The first capillary  12  has an entrance  14  for an electrolyte solution and an exit  16  for the solution. A second capillary  18  intersects the first capillary  12 , approximately midway between the entrance  14  and the exit  16  at right angles, to define a reaction zone  20  between the intersection  22  and the exit  16  from the first capillary  12 . Coiled electrode  24  is positioned longitudinally in the reaction zone  20 , along with the first capillary  12  at right angles, between the first electrode  24  and the exit  16  from the first capillary  12 . A third capillary  26  intersects the first capillary  12  at right angles, between the first electrode  24  and the exit  16  from the first capillary  12 . A second coiled electrode  28  is positioned longitudinally in the third capillary  26 .  
         [0034]     In use of the cell  10  for making potentiometric measurements, the entrance of the first capillary  12  is connected to an electrolyte source  30 . The second capillary  18  is connected to a reactant gas source  32 . The third capillary  26  is connected to a solution source  34 , in order to establish a solution bridge between the first and second electrodes  24  and  28 . In practice, the reactant gas source  32  may be a reactor tube, such as a pyrolysis tube for preconditioning the reactant in a reducing atmosphere, such as hydrogen gas, which may also contain a catalyst, such as nickel; a pyrolysis tube for preconditioning of the reactant in an oxidizing atmosphere, such as oxygen or air, which may also contain a catalyst, such as platinum; a photoionization tube in which chemical compounds are photochemically decomposed, a gas chromatographic column through any of the above reactor tubes; a pyrolysis tube furnace into which discrete samples are introduced; or the like.  
         [0035]     With the elements of the detector cell  10  described so far, it is possible to carry out potentiometric measurements, utilizing the electrodes  24  and  28  to determine a resistance through the electrolyte solution supplied at entrance  14  and the reactant gas supplied through capillary  18 , when they react in the reaction zone  20 .  
         [0036]     Because a measurement is made of the electrolyte solution across electrodes  24  and  28 , the response time of cell  10  is limited to the time it takes for the specie to be absorbed and dispersed throughout the electrolyte solution so that the measurement is accurate. Measuring the solution before the specie has been absorbed and dispersed may result in a lower concentration of the specie in the solution than is actually present. This passage of time for proper absorption and dispersion increases cell response time and reduces sensitivity.  
         [0037]     Further, cell  10  provides a limited capability for detecting minute quantities of a specie in the sample. This limited resolution, or detection capability, may be due to the absorption and/or dispersion technique or inherent in conventional detector cells  10 .  
         [0038]      FIG. 2  depicts the system  50  for enhanced detection of a specie using a gas chromatograph in accordance with the invention. Similar to conventional systems for detecting a specie, a chromatographic column  52  is used to separate a sample of gas  58  into its respective components. The chromatographic column (“GC”)  52  is not germane to the invention.  
         [0039]     Upon exiting GC  52 , each specie is detected by an electrochemical gas sensor  66 , which is coupled to an end of GC  52  where the components, or species, are exiting GC  52 . The combination of sensor  66  being coupled to GC  52  provides a system  50  for detecting a specie having enhanced sensitivity and response time because sensor  66  provides numerous advantages over conventional detector cells  10 , as shown and described under  FIG. 1 .  
         [0040]     Sensor  66  reduces the need for a specie of the sample to be absorbed and dispersed in an electrolyte solution in order for an electrical measurement to be taken across electrodes in contact with the solution. Sensor  66  detects gas as the gas comes in contact with an electrode, thereby reducing response time and increasing sensitivity. Moreover, the resolution, or detection capability, of sensor  66  is typically in the range of parts per billion, which is generally more sensitive than conventional detector cell  10  shown in  FIG. 1 . Sensor  66  is more particularly described under  FIGS. 4 and 5 .  
         [0041]     System  50  further includes reactor  54 , which is placed between sensor  66  and GC  52 , for oxidizing and/or reducing a specie exiting GC  52  so that the specie may be detected by sensor  66 . Typically, a specie desired to be detected by sensor  66 , but which is not easily detectable, would be oxidized/reduced by reactor  54 . As shown in  FIG. 2 , reactor  54  oxidizes/reduces a desirable specie of gas by heating the specie together with a reactant gas  56  at a specified temperature. Reactor  54  is not used to oxidize/reduce all species exiting GC  52  but is merely used to oxidize/reduce the particular specie(s) desired to be detected.  
         [0042]     Species that are detectable by sensor  66  upon exiting GC  52  do not need to be oxidized/reduced. Hence, sensor  67  is coupled directly to GC  52 . Sensor  66  and sensor  67  have all of the same limitations and advantages but are merely positioned in varying locations, whereby sensor  66  is downstream of reactor  54  and sensor  67  is directly downstream of GC  52 . For the sake of simplicity, sensor  66  will be described hereinafter. Reactor  54  is not germane to the invention.  
         [0043]      FIG. 3  depicts a method  70  for detecting a specie of gas using a gas chromatograph system. Method  70  includes the step of eluting  74  a gas component from a gas chromatographic column  72  and determining  76  whether or not the eluted gas component is detectable. The gas component is usually at least one specie of the gas entering column  72 . Also, a user, operator, or computer system, such as a programmable logic controller or microprocessor, may make the determination as to the component&#39;s detectability.  
         [0044]     If the eluted component is determined be difficult to be detected by an electrochemical gas sensor, the component is then oxidized or reduced  78  by heating the component in a reactor to facilitate converting the component into an electrochemically active specie. During oxidation and/or reduction, method  70  adds  80  a reducing or oxidizing agent, or reactant  82 , such as oxygen or hydrogen. Upon being reduced and/or oxidized in the reactor, the gas component enters an electrochemical gas sensor for detection, the sensor being coupled  86  to the reactor. The gas component is then detected  88  by the sensor. The types of sensors that may be used in method  70  are more particularly described under  FIGS. 4 and 5 .  
         [0045]     An example of reductive pyrolysis in a reactor is as follows:
 
 RX+H   2   →HX+RH 
 
X=S, P, Cl
 
         [0046]     An example of oxidative pyrolysis in a reactor is as follows:
 
 RX+O   2   →XO+CO   2   +H   2   O 
 
X=N
 
         [0047]     The corresponding reaction at the sensing (SE) and counter electrodes (CE) during oxidation and/or reduction is as follows: 
        Sensing Principle at Sensor 
            At SE: H 2 S+4H 2 O→H 2 SO 4 +8H + +8e −      2O 2 +8H + +8e − →4H 2 O    
               
 
         [0051]     If the eluted component is determined to be detectable without being oxidized/reduced, the gas component enters the electrochemical gas sensor directly upon exiting column  72 . Hence, the electrochemical gas sensor is coupled  84  to column  72  without a reactor between the sensor and column  72 . The gas component is then detected  88  by the sensor. The types of sensors that may be used in method  70  are more particularly described under  FIGS. 4 and 5 .  
         [0052]      FIG. 4  shows an electrochemical gas sensor of copending U.S. patent application Ser. No. 09/443,875, which may be used as sensor  66 . As shown, electrochemical gas sensor  66  includes substrate  111 , electrode  103 , and ionomer membrane  105 . Gas enters and exits sensor  66  through the inlet and outlet as shown. A portion of the gas entering sensor  66  diffuses through diffusion hole  120  and contacts electrode  103 , which detects the type of gas present in sensor  66 . As stated above, for the purposes of simplicity, sensor  67  will not be described but is understood to include the limitations of sensor  66 .  
         [0053]     To enhance sensitivity to sensor  66 , a reservoir  109  is provided containing electrolyte solution to wet ionomer membrane  105 . As shown, reservoir  109  and, therefore, the electrolyte solution is in contact with inomer membrane  105 . Because reservoir  109  is located on a same side of ionomer membrane  105  as diffusion hole  120 , a length of diffusion hole is typically at least as long as a height of reservoir  109 .  
         [0054]      FIG. 5  depicts another embodiment of electrochemical gas sensor in accordance with the invention. Sensor  66 ′ includes substrate  132 , ionomer membrane  134 , and electrode  138 . Gas enters sensor  66 ′ through inlet  142  and is detected after diffusing through diffusion hole  144  to contact electrode  138 , which is in contact with ionomer membrane  134 . Gas exits sensor  66 ′ through outlet  146 . It is understood that the gas may flow in a reversed direction where outlet  146  is the inlet and inlet  142  is the outlet.  
         [0055]     To enhance the sensitivity of sensor  66 ′, ionomer membrane  134  is wetted by solution  152 , which is contained in reservoir  156 . In  FIG. 4 , electrolyte solution and reservoir  109  were placed on the same side of substrate  111  as electrode  103 . Although the electrolyte solution wetted ionomer membrane  105  to enhance the sensitivity of sensor  66  in the same manner as solution  152  enhances the sensitivity of sensor  66 ′, reservoir  109  being on the same side of substrate  111  inhibits a length L of diffusion hole  120  from being reduced, which would reduce gas diffusion time and thereby improve sensor sensitivity. As shown in  FIG. 4 , length L could not be shortened more than height H of reservoir  109 . Therefore, time required for gas to diffuse from the inlet through diffusion hole  120  to contact electrode  103  was difficult to reduce due to the length L of diffusion hole being of a minimum dimension not less than the height H of reservoir  109 .  
         [0056]     Sensor  66 ′ of  FIG. 5  overcomes this disadvantage by wetting ionomer membrane  134 , via hole  136  in substrate  132 , with solution  152  located on a side of substrate  132  opposite from electrode  138 . Because of the position of reservoir  156 , length L′ can be shortened, thereby reducing gas diffusion time and improving the sensitivity of sensor  66 ′. The more length L′ is reduced, the faster the response time of sensor  66 ′. In some embodiments, length L′ is less than 1.4 mm. In other embodiments, length L′ is less than 1 mm. In further, embodiments, length L′ is less than 0.5 mm. In still further embodiments, length L′ is less than 0.1 mm. In fact, length L′ or a thickness of ionomer membrane  134  may be reduced until it is flush with or below a surface of electrode  138 . In some embodiments, diffusion hole  144  is eliminated because length L′ is flush with or below a surface of electrode  138 . All that is required is for ionomer membrane  134 , of any length L′, to be in contact with electrode  138  so that gas entering through inlet  142  provide a desired gas/ionomer membrane/electrode interface.  
         [0057]     As a result of the reduced length L′ of sensor  66 ′, the response time of sensor  66 ′ is less than approximately 2 seconds, more preferably less than approximately 1 second, and most preferably less than approximately 0.5 seconds. In some embodiments, the response time is less than approximately 0.1 seconds.  
         [0058]     To further enhance sensitivity, a thickness of substrate  132  is reduced to improve wetting by solution  152 . Substrate  132  is of an electrically non-conductive material for providing a surface upon which electrode  138  is placed. Optionally, substrate  132  is a thin foil having insulative, or electrically non-conductive, properties, such as Kapton or any other material. The foil is not metallic or conductive. The foil may also be flexible as compared to ceramic or glass. The thickness of the foil, or substrate  132 , is generally less than approximately 4 mils and preferably less than approximately 1 mil. The thinner substrate  132 , the faster ionomer membrane  134  is wetted and this positively affects sensor response time. Therefore, as the thickness of substrate  132  approaches 0 mils, the response time is further reduced.  
         [0059]     Optionally, in some embodiments, sensor  66 ′ may include wicking material  154  to facilitate or enhance wetting of ionomer membrane  134  by solution  152 . Wicking material  154  is typically of a material that absorbs liquid, such as a sponge. Hence, as shown in  FIG. 5 , wicking material  154  will draw solution  152  upwardly from reservoir  156  toward ionomer membrane  134 .  
         [0060]     As shown, reservoir  156  and substrate  132  are separable from one another where wicking material  154  is placed between reservoir  156  and substrate  132 . In other embodiments, wicking material is placed within reservoir  156  and reservoir comes in contact with substrate  132 . In further embodiments, substrate  132  and reservoir  156  are made not separable from one another but are formed as one unit. Wicking material  154  may optionally be used with any of these embodiments of reservoir  156  and substrate  132 .  
         [0061]     As shown in  FIG. 5 , substrate  132  further includes at least one hole  136  extending from a first surface  162  of substrate  132  to a second surface  164  of substrate  132 , thereby forming a thru-hole, for permitting solution  152  to pass, or diffuse, through at least one hole  136  to contact ionomer membrane  134 . In the embodiments where substrate  132  is a foil, or a thin non-conductive material, wicking material  154  would be positioned in a closer relationship to ionomer membrane  134  than where substrate  132  is of a thick material. Where substrate  132  is a foil, solution  152  absorbed by wicking material  154  would more easily wet ionomer membrane  134 . Optionally, wicking material  154  would be in contact, through at least one hole  136 , with ionomer membrane  134 . In some embodiments, wicking material  54 , in addition to or instead of being between substrate  132  and solution  152 , is placed within at least one hole  136 .  
         [0062]     To further facilitate wetting of ionomer membrane  134  by solution  152 , or optional wicking material  154 , a plurality of holes  136  are placed in substrate  132 . It is understood that hole  136  is of any diameter, length, shape, or dimension. Also, the more holes  136  in substrate  132 , in any location, the better ionomer membrane  134  is wetted. Hence, the hole  136  or plurality of holes  136  may act as a form of wetting control to ionomer membrane  134 , as too much wetting or too little wetting negatively affects sensitivity. Moreover, hole  136  may be, in addition or instead of being round, a square shaped or polygonal shaped hole. Hole  136  may further be a slit or aperture of any kind. All that is required of hole  136  is that it provides a passage from first surface  162  to second surface  164  so that solution  152  diffuses through hole  136  to contact ionomer membrane  134 .  
         [0063]      FIG. 6  depicts an exploded view of the electrode shown in  FIGS. 4 and 5 . As shown, to enhance sensitivity of sensor  66  or  66 ′ in some embodiments, a thin film  234  of electrolytic material, which may be the same material as the ionomer membranes of  FIGS. 4 and 5 , may optionally be placed on electrode  103  or  138  to increase the area of contact between the ionomer membrane  105  or  134 , electrode  103  or  138 , and gas to include the surface of electrode  103  or  138 . Gas diffuses throughout film  234 , which is in contact with the surface of electrode  103  or  138 . As a result of the increased contact area, the sensing area is increased and response time is minimized. Gas diffuses faster through film  234  when film  234  has a minimal thickness. Hence, the thinner film  234  is, the faster the response time is for sensor  66  or  66 ′.  
         [0064]     Without film  234 , the interface in the approximate area of electrode  103  or  138  would be substantially smaller, limited to an area where ionomer membrane  105  or  134  comes in contact with electrode  103  or  138 . This contact area would generally be a linear contact point defining an approximate circumference of electrode  103  or  138 .  
         [0065]     In some embodiments, film  234  has a thickness less than 2 micrometers. Ideally, film  234  should be as thin as possible to maximize sensor response time and sensitivity. Hence, sensor  66  or  66 ′ may further comprise film  234  having a thickness of less than 1 micrometer. A film having such reduced thickness permits faster gas diffusion and, thus, faster response times. Film  234  is an electrolytic medium, which includes all the limitations of ionomer membrane  105  or  134  and may be, but need not be, the same material as ionomer membrane  105  or  134 .  
         [0066]     Film  234  is in a solid state or dry electrolyte for it has more structural integrity than liquid state electrolyte, thereby permitting a consistently uniform thickness over electrode  103  or  138 . This enhances sensor repeatability and facilitates functionality for liquid state electrolyte would be difficult to maintain in a fixed position on the surface of electrode  103  or  138 .  
         [0067]     Optionally, the response time of sensor  66  or  66 ′ may further be improved by reducing the size of the inlet and outlet of each sensor  66  or  66 ′. In this effort, the gas is more concentrated while inside the sensor due to there being less internal volume for the gas to disperse. Less dispersion and a more concentrated gas generally results in a more easily detected gas and, therefore, reduced response time of sensor  66  or  66 ′. Hence, the volume in which gas may disperse is reduced. Such dispersion is generally referred to as axial dispersion because the dispersion is approximately along the axis containing a center point of sensor  66  or  66 ′. In some embodiments, the inlet and outlet have a diameter of approximately 1 mm. The inlet and outlet need not be round but may be of any shape so long as gas may be injected into and extracted from sensor  66  or  66 ′. Such shapes include 3 sided, 4 sided, or polygonal geometries.  
         [0068]     Optionally, as shown in  FIG. 6 , sensor  66 ′ of  FIG. 5  may also include cover  150  on ionomer membrane  134  for minimizing the vaporization or evaporation of electrolyte solution  152  as solution  152  is absorbed and passed upwardly through ionomer membrane  134 . Cover  150  is in contact with the surface of ionomer membrane  134  opposite from substrate  132 . Cover  150  does not block any portion of either diffusion hole  144  or electrode  138  because doing so would hinder gas detection and negatively affect sensor sensitivity. Cover  150  is not needed for sensor  66 ′ to operate properly and may be eliminated entirely from sensor  66 ′. For embodiments where sensor  66 ′ includes cover  150 , it is understood that the length L′ of the diffusion path is the height of both ionomer membrane  134  and cover  150 . For embodiments where sensor  66 ′ does not include cover  150 , length L′ is the height of membrane  134 .  
         [0069]     Sensor  66 ,  67  is preferably for detecting exiting a gas chromatograph column. However, sensor  66 ,  67  may also be attached to a liquid chromatograph column provided a vaporizer is placed between column  52  and sensor  66 ,  67 .