CA1320249C - Polarographic oxygen sensor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The cathode sub-assembly of a polarographic sensor comprises an elongate housing with a longitudinal opening therethrough and with noble metal wires extending completely through the opening, the wires held symmetrically in the longitudinal opening by an extensive plug of glass which is solidified in situ in sealing contact with the wires and with the wall of the longitudinal opening. A suitable tubular anode is provided through which the cathode sub-assembly protrudes, and a flexible membrane of an inert material, which is permeable to gas and impermeable to water vapour and other than gaseous dissolved solutes, is secured in sealed relationship over the anode and the cathode, with a film of electrolyte bathing the anode and the cathode and contained by the membrane.

Description

` 1 ~32~2~9 FIELD OF TE~E INVENTION
The present invention relates to polarographic sensors and to improved methods of making such sensors.
Although this inventiQn is particularly useful for sensors to be used chiefly for the measurement of oxygen, it will be recognized by those skilled in the art that most of the underlying principles and discussions will pertain equally to other types of polarographic sensors as well, and accordingly the present invention is not limited to oxygen sensors.
BACKGROUND OF THE INVENTION
The measurement of partial pressures and concentrations of oxygen is of great importance in many areas of biology, medicine, (bio)chemistry, (bio)physics and engineering. This measurement can be accomplished with various degrees of accuracy and resolution using a wide variety of instrumentation and methodology, from simple manometric and chemical assays to high resolution gas chromatography, neutron activation and positron emission tomography. Techniques used to measure oxygen content in solids ~which will not be discussed) differ ~ub~tantially from those used to measure oxygen content in llquids and gases, and for the last two phases, the technlques can vary substantially depending on the physLcal and chemical environment o~ the sample.
For temperatures in the biological range ~e.g.
0-50C) it is often necessary to measure and/or control oxygen concentrations both in the gas and the liquid phase and thi~ type of measurement can only be made with a membrane covered polarographic sensor of the type disclosed in United States Patent No. 2,913,386, issued November 17, 1959 to Leland C. Clark, Jr., and hereinafter referred to as a "Clark" sensor. In this type of sensor, oxygen is reduced by a noble metal cathode, whose potential is fixed with respect to a reference anode (often Ag-AgCl), both elements of the cell in an aqueous electrolyte and enclosed by a gas-permeable membrane.
Unfortunately, many scientific studies involve reactions ` 2 132~2~9 which are modulated by very small concentrations of oxygen and both the resolution and the response time of typical Clark sensors are often inadequate in monitoring such reactions (C. J. Xoch, Oxygen Effects in Radiobiology.
In: Hyperthermia (Hl Bicher & DF Bruley, eds) Adv. Exp.
Med. & Biol., Vol. 157, 123 - 144, Plenum Press, New York, 1983).
One can easily list several criteria of importance in determining the overall quality of a sensor.
Some of the criteria are quantitative in that absolute numbers can be assigned to them:
la) Sensitivitv: the magnitude of the response produced by a given partial pressure of oxygen ~e.g. 0.5 nanoamp~ ~nA] per kilopascal lkPa] of oxygen partial pre55ure [P02]). The current generated by the sensor will be directly related to the amount of oxygen consumed by the ~ensor - see 8a below for importance - and below a certain value, the current becomes increasingly difficult to measure accurately ~state-of-art circuitry allows the measurement of currents below 1 picoamp only with great difficulty and cost).
2a) Minimum Value of "Zero" Current the equilibrium value of response as a function of some reerence value for a sensor in an environment of zero oxygen (e.g. 0.1% of response in air).
3a) Stabilitv of "Zero~ Current 5 the change in re~pon~e of a sen~or in an environment o zero oxygen per unit time ~can include linear, exponential and random component~).
4a) Noise: the random fluctuations in response of a sensor in high or low concentrations of oxygen ~e.g.
% of air value or % of scale).
5a~ Stabilitv: the relative change in response ~i.e. current) of a ~ensor per unit time in a constant environment ~P02, ionic strength, temperature) ~e.g.-0.5~/hr).
6a~ ~inearitY: the range of oxygen partial pressures over which the relationship between oxygen 3 1 3 ~
partial pressure and sensor response is linear to some specified accuracy (it is common for sensors to deviate from linearity at very high and at very low partial pressures of oxygen).
7a) Response Time: the time for a given percentage change in response after a step change in oxygen partial pressure in the external environment of the sensor (usually l/e of step value). An ideal sensor would approach the new value exponentially with a single time constant but most sensors have two or more associated time constants, the relative importance of which depends upon the absolute values of oxygen partial pressure before and after the step change. Usually the most stringent test involves a step change in oxygen partial pressure from a high value ~i.e ambient air) to zero.
8a) Stirri nq Requirements in Liquids: the requlred velocity of the liquid of interest to minimize the response differential of the sensor between gas and gas-equilibrated liquid. This parameter should be expressed both as liquid-sensor velocity to achieve a given percentage of the gas phase response, and as maximum percentage decrea~e when the ~ensor is added to an unstirred solution which wa~ in equilibrium with the gas phase.
Other aspects of sensor de~ign can be just as important for practical usage but are inherently qualitative in nature.
9a) Shieldina EfficiencY: the relative i~olation of the performance of a sensor and its associated electronic amplifier in the environment of other laboratory equipment and large moving charged bodies ~people).
10a) Structure: the aspects of shape, size and materials of sensor constructlon which may enhance or limit its range of applications. For example, there would be no point in having a sensor which could mea~ure oxygen accurately at low levels but which could not be sealed into a closed volume without leakage or presence of air 4 132a2~s bubbles etc. It is equally important that the sensor body have a stable shape (e.g. plastics swell in water) and that this shape be suitable for fitting into a variety of standard vessels (e.g. many commercial sensors have their active tip as the widest part of the sensor, ma~ing it impossible to insert into standard fittings without disconnecting the sensor or having to reapply the membrane). An often overlooked factor is the contamination of the bulk of the sensor by oxygen and other chemicals from previous environments and conversely the contamination of samples by materials from the bulk of the sensor. For example if one is attempting to monitor an oxygen dependent reaction in a small closed volume, the plastic body of a sensor can give off large quantities of dis~olved oxygen.
lla) Consistency of ResPonse to Chanqes in Environment: the consistency of sensor response to changes in sensor environment such as pressure (i.e. one would hope to obtain the same response with 1% oxygen plus 99% nitrogen at lOl kPa pressure, as with 100% oxygen at 1.01 kPa pressure). In addition, the temperature dependence of the sensor should be reproducible and without hysteresis. Often one is reguired to monitor oxygen concentrations in an extremely humid environment and this poses rather severe problems for the electrical connections to the sensor, because large leakage currents can result from damp connections and cable.
12a) Electronic Amplification Requirements ~see also 5a): the current gain necessary to accurately convert the oxygen-derived sensor current to a form auitable for monitoring purposes - usually a full scale output of the order of 1 volt. Sensors available at the present time have a very broad range of sensitivities (lO-1-10-7 A/kPa P02). One ugually obtains the highest sensitlvities at the expense of some other quality factors, Since many experiments require measurements of the order of O.Ol kPa of oxygen partial pressure, the resolution of the amplification system must be at least as 5 132324~
sensitive as l0-12 A and preferably even better for the less sensitive devices. Thus, the development of significantly better sensors requires state-of-the-art electronic measurement techni~ues.
13a) Interference: the response of the sensor ; to any other than the desired molecule which can diffuse through the membrane and react with the redox-processes ongoing. Several gases have been reported to do so with polarographic oxygen sensors (CO2, N2O, H2S, SO2, NO, NO2) but it may be possible to reduce such interference by proper mechanical and electro-chemical design.
Commercial prior art polarographic oxygen sensors include those manufactured by Beckman Instruments lModel 325814 - no longer available, and patented under aforesaid United States Patent No. 2,913,386, issued November 17, 1959 to Leland C. Clark, Jr.), Yellow Springs Instruments ~Model 5331, disclosed in United States Patent 3,406,109 issued October 15, 1968 to Everett W. Mlloy), -and Diamond Electrotech ~formerly Transidyne General Corporation-Model 730).
One can much more easily appreciate the concepts of response time, zero current and noise by measuring as a function of time the response of an oxygen sensor after ~ts environment is rapidly changed from a high oxygen ; 25 partial pressure ~i.e. air) to zero. Such measurements, using the best responders of several each of the sensors described above, have shown an inltial rapid decrease in current to 2 - 10~ of that in air, but then a much slower decrease, taking many hours, to a minimum value of 0.2--~ 2S of that in air. Some of the poorer responders had zero currents several fold higher and more in keeping with i their actual specifications, Others had zero currents which remained large and variable.
Furthermore, it Was ound that the magnitude of the zero current was not independent of the previous 6 132~249 environment o~ the ~ensor (C. 1. Koch and J. Xruuv, Measurement of very low oxygen tensions in unstirred liquids. Analyt. Chem. 44 1258-1263, 1972), and this probably points to one source of the zero current, namely dissolved oxygen in the body, typically plastic, of the sensor itself. Under some circumstances one can measure small quantities of oxygen in the presence of a large zero current if one is able to continually switch back and forth between the gas to be monitored and a true "zero oxygen" gas but this is seldom possible. Such switches for measurements in liquids are virtually impossible.
The highly variable nature of the zero current and the other response parameters led the present inventor to believe that flaws in available sensors were expressing themselves to varying degrees, and that perhaps a much improved sensor would be possible if these flaws could be identified and corrected. Unfortunately, the flaws have been "over" identified in the literature, since changing almost any aspect of the operation of a sensor changes its response to some degree. Therefore, the approach taken was to re-examine the basic aspects of design and operation of polarographic sensors, to see if flaws could be predlcted in their performance and to devise construction ~ethod~ to correct or minimize these flaws.
The electrochemical processes have not been descrlbed quantitatively but there ls certainly general agreement about the basic anode and cathode reactions tsee Irving Fatt: "Polarographic Oxygen Sensors", CRC Press, Cleveland 1976; D.J.G. ~ves and G.J. Janz: "Reference Electrodes", Academic Press, New York, 1961).

At Cathode: 2 1 4e ~ 4H ~ 2H20 ~rll Note that H+ and other cations present in the electrolyte will be attracted to the cathode so that the "pH" near the oxygen reductive surface will be more acidic than the "pH"
of the electrolyte as mixed in bulk solution.

7 132D2~9 At Anode: 4Ag + 4Cl~ > 4AgCl + 4e~ [r2]

The converse will be true for the anode (i.e. OH- and anions attracted) but the surface area of the anode is usually so much larger than that of the cathode, that pH
changes would only be expected to affect the cathode reactions.
Although reactions rl and r2 are often given in de~criptions of polarographic sensors, there is not much evidence that they actually occur in this manner. For example, it is clear that almost all non-enzyme mediated ~and probably most enzyme-mediated) redox reactions occur in 1 electron steps. This means that the 4 electron reduction would have to proceed through 2-' or H02 , H2o2 and OH~:

O + e~ + H+ ~~~~~> 2 ' + H <--pKa=4.3--> H02 ~r3]
+
2 ' + e + 2H -----> H202 [r4a]
or 2 + 2H ----~ H202+02 ~r4b]

or +

H02 ~ e + H ~~~ ~ H22 ~r4c]

or 2 ' + H02. + H+ ~~~~~~ 2 + H22 lr4dl ~22 + e~ -----~ OH- + OH~ ~r5]

OH~ + e- ------> OH- ~r6]

Reactions r4a and r4b are likely to be extremely slow. Even reaction r5 is much slower than reaction r3. In fact, at relatively low polarizing voltages 8 ~32a2~9 (e.g. 0.55 volt) the response of a typical oxygen sensor (without its membrane) in a deoxygenated hydrogen peroxide solution is less than l/lOth that in a solution with the same concentration of oxygen (unpublished observation).
From inspecting these reactions, and noting that the electrolyte tends to become more basic, one might expect that the optimal pH for sensor operation might fall within the acid range. This is because the very slow reactions (r4a and r4b) must predominate at neutral and higher pH's.
However, in agreement with other investigators this inventor has found the most reproducible responses under basic conditions. Similarly, in agreement with Hahn et al. (C.E.W. Hahn, A.W. Davis & W.J. Albery, Electrochemical Improvement in the Performance of P02 Electrodes, ~esp. Physiol. 2S, ~09-133, lg~5) this inventor has sometimes found an improved and more stable zero current when a trace of catalase is added to the electrolyte. With the sensors according to the present invention, which already have a much reduced zero current, the presence of catalase can cause negative currents in the absence of oxygen. This may be due to a preferential build-up of superoxide which may actually be oxidized rather than reduced in the absence of oxygen. The point of thi3 discus8ion though is that an "improved" re~ponse of a particular sensor under certain specified conditions may re~ult from more than one interacting phenomena.
Thus, the improvement may not be found under other circumstances and its cause may not be ully under~tood.
The linearity of a Clark sensor depends on the condltion that the reduction of oxygen and its reduced products ~reactions r3 to r6) at the cathode maintains their concentration in the immediate vicinity of the cathode at essentially zero. Thus, since the oxygen gradient acro~s the membrane will be linear ~there is no oxygen consumption within the membrane), the flux of oxygen through the membrane will simply be proportional to the oxygen partial pressure in the environment external to the membrane ~external environment). The proportionality 9 ~32a249 constant must allow for the very different solubility and diffusion constants of the external environment, the membrane and the electrolyte. The flux of oxygen from the external environment can cause a decrease in partial pressure at the interface of membrane and external environment if the diffusion constant of the external environment is small enouqh and/or the flux is large enough. Thus in order to minimize the difference in sensor response between gas and gas-equilibrated liquid, it is desirable to decrease this flux by increasing the relative diffusion barrier of the membrane (i.e. thicker membrane or smaller diffusion constant) and/or decreasing the size of the cathode. There is a tradeoff involved however because these measures have the effect of reducing the sensitivity.
BRIEF DESCRIPTION OF THE DRAW~NGS
The invention will be readily understood from the following description of prior art and of a preferred embodiment thereof given, by way of example, with reference to the accompanying drawings, in which:
Figure 1 shows a view in longitudinal cross-section through a polarographic sensor representative of prior art ~ensors;
Pigure 2 shows a vlew taken in longitudinal cross-section through a polarographic sensor embodying the present invention;
Figure 3 shows a view in longitudinal cross-section through component~ of the sensor o Figure 2;
Figure 4 shows a view in longitudinal cross-3ection of a completed cathode sub-assembly;
Figure 5 shows a broken-away view of front and back ends of the components of Figure 3, with the addition of a completed anode and the active end of the cathode 3ub-as~embly of Figure 4 at the front, and with a connector a~sembly at the back;
Figure 6 shows a view in longitudinal cross-section through components of a cathode sub-assembly of 13~0~9 the sensor of Figure 2 during manufacture of the sub-assembly, and related clamping devices;
Figures 6a, 6b and 6c show views taken in cross-section along the lines a, b and c respectively of Figure 6; and Figure 7a, 7b and 7c diagrammatically illustrate successive steps in the application of a mem~rane to the sensor of Figure 2.
In Figure 1, there is illustrated a prior art polarographic sensor which is generally representative o~
the above-mentioned Beckman Model No. 325,814 and Yellow Springs Model No. 5331 sensors.
This reprasentative sensor comprises a platinum wire cathode 1 provided within a body of resin material in the Yellow Springs sensor or within a glass enclosure 2, which projects from one end of a housing 3 of plastic material, the cathode extending through a space 4 filled with air, in the above Beckman sensor. An anode 5 is provided in the form of a silver wire ~Beckman) block or film (Yellow Springs) and an O-ring 6 retains a membrane 7 covering the top of the sensor and enclosing a supply 8 of KCl.
Also, in Figure 1 potential sources of oxygen leakage, which occurs primarily along the seal of the cathode 1 through the glass enclosure 2 and the ~pace 4, through the housing 3 and along the ~ealing of the anode 5, are represented diagrammatically by arrows Al at the r~ght-hand side of Figure 1 and sources of electrolyte 10s8, which occurs primarily at the O-ring seal, the membrane 7 and into the housing 3, as indicated above, are represented diagrammatically by arrows A2 at the left-hand side of Figure 1. The Yellow Springs sensor ha~ its greatest diameter at the O-ring, so insertion Lnto leak-proof fittings is impossible after membrane assembly. The overall tip diameter i~ too wide to allow sufficient stretching of relatively thick and non-elastic Teflon membranes which can minimize electrolyte loss. Both sensors require machining of their plastic bodies to ll 13~249 ensure a leak-proof seal with, for example, Ultra-Torr ~ittings (manufactured by Cajon) because of im~erfections in the plastic casting.
A fundamental design decision of an embodiment of the present invention described in greater detail below was to devise methods which would isolate as completely as possible the cathode, the anode and their associated reactions/reaction products (reaction elements) from the influence of both internal (i.e. from within the sensor) and external contaminants or chemical and/or physical modulations.
Possibly the most important problem of sensor construction is the mechanical seal of the cathode within the sensor body itself. One can appreciate the magnitude of this problem by calculating the leakage involved with a typical engineering specification for a "hermetic" seal ~e.g. 5 x 10-9 cc per second per atmosphere (101 kPa) of differential pressure). If this amount of oxygen were leaking to the cathode from some source within or outside the sensor then the current from this "leakage" would be:
5 x 10-9 ~cc per sec per 101 kPa) x 1 ~mole per 22400 cc) x 6 x 1023 ~molecules per mole) x 4 ~electrons per reduced oxygen molecule) x 1 ~coulomb per 6 x 1018 electrons) - 10-7 amps per 101 kPa.
That is, i the external oxygen partial pressure to be measured were zero, but oxygen could leak to the cathode at the above rate, a "zero" current of 100 nanoamps would still flow. This i~ many times greater than the aerobic sensitivity of typical sensors with small ~<0.1 mm) diameter cathodesl To have a meaningful "zero"
current, the leakage rate of oxygen to the cathode from all source~ should be perhaps 100,000 times less than the engineering sta~dard of a leak-proof seal. Although some polarographic sensors with very large cathodes ~e.g.
Yellow Springs Model 5331) seal their cathode directly into a plastic, this technique will not work well with small cathodes because oxygen diffusion out of the plastic will lead to the above problems.

12 1~2~2~9 A somewhat less stringent seal of the anode may be necessary since oxygen which leaked past this seal would have t~ diffuse along the thin film of electrolyte and would have the opportunity to diffuse out of the membrane. However, it should be noted that oxygen leakage past the anode could be important if the sensor were monitoring the oxygen partial pressure in a small volume, since this would cause the equivalent of an overall leakage into the monitored volume. Other requirements for the anode have not been studied carefully. Silver/silver chloride is often used since silver chloride is insoluble in water and hence its activity does not change with continued sensor use. This stability, while generally acknowledged (Irving Fatt: ~Polarographic Oxygen Sensors", CRC Press, Cleveland 1976) has been aisputed by Clark ~L.C. Clark, Electrolytic sensor with anodic depolarization. United States Patent No. 3,380,905, i~sued April 30, 1968 to L.C. Clark, Jr.,) A general feature that i8 often mentioned is that the anode should have an area of at least 50 times that of the cathode.
However, it may be that much larger anodes are desirable and many o~ the leakage characteristics previously ldentified for the cathode may also apply to the anode.
one problem that exists with available sensors is that the anode and it~ connecting wire, usually a linear or spiral wire adjacent to or around the cathode,do not provide a contlnuous electrical shield of the cathode. This problem has been addressed in sensors with an all-metal body, but this metal represents a major contamination problem since even stalnless steels are attacked by many of the commonly uced salts. Furthermore, the metal body can cause ground loop or other electrical isolation problems ~ince most apparatus is already grounded.
Since the active tip of the sensor has a voltage imposed between conducting elements ~the anode and cathode) the resistivity of the Qensor body should be as high as pQssible. Although the bulk resistivity o~ many otherwise suitable materials ~e.g. epoxies) can be very 13 ~32~9 high, the surface resistivity is often not specified and can be greatly influenced by the aqueous electrolyte. It is pointed out that in order to measure currents in the picoamp range, and with an anode-cathode voltage of greater than 0.5 volts, the resistivity of all components of the connectors, cable and sensor would have to be greater than 500,000 megohms. This value, while realizable under some conditions in practice, is certainly beyond the specifications of all components presently available except perhaps for Teflon and some electrical sealing glasses. Bulk resistivities of epoxies and other plastics vary by many orders of magnitude but typically are in the range of lOlO - lOl7 ohm cm. However, most components and materials are specified for operation at high voltages ~ l volt) where all kinds of conduction are possible. For polarographic sensors operating at voltages of roughly 0.5, only conduction-band conduction and electrochemical redox processes are possible and of course the dominant redox process is the desired reduction of oxygen. Therefore, some desirable features of the design of a high quality sensor are:
lbl a near perfect seal of the anode and cathode is preferable:
2b) extremely high resistivity materials are preferable throughout, especially at the active tip of the seneor which 1s covered by electrolyte; and 3b) a method should preferably be provided for adequate ~hielding of the high-impedance cathode.
The requirements of the electrolyte film are contradictory. On the one hand, the electrolyte must pass the sensor current via ionic flow without any appreciable drop in potential. If this were not the case then the anode/cathode potential would change with current, leading to non-llnearities in current vs. oxygen partial pressure.
This implies that a relatively thick film of electrolyte ~and/or high dissolved salt concentration) with low resistivity would be required. On the other hand, the cathode must be able to reduce all of the oxygen in its 14 Lc~i~32~9 vicinity as rapidly as possible, to allow a fast response time to changing oxygen partial pressures, and this suggests the need for as thin a film as possible. It is likely that a thin film would also be required to confine the products of the reduction reactions (r3 to r6) so that reduction and/or dismutation will always be near completion. Thus:
4b) Some of the one-electron reductions may need to be catalyzed and may even be partially reversible or reactions r4 to r6 may be chemical dismutations rather than electrochemical reductions.
5b) The cathode should preferably be small to allow the theoretical operating condition of zero oxygen partial pressure at the cathode surface, but of a ~ufficient size to allow reasonable sensitivity; and 6b) The shape of the anode and electrolyte film should preferably be made to allow condition (5b), yet minimize the cathode-anode resistance.
Another aspect of the sensor's construction which is desirable for maintaining the electrolyte composition ~and hence the sensor response) constant is the method of containment of the membrane and electrolyte.
Thi~ subject has been the subject of several papers and patent~ ~see United States Patents 3,a87,194; 3,575,836;
3,445,369; 3,577,332; 4,325,797 and Irving Fatt:
"Polarographic Oxygen Sensors" CRC Press, Cleveland 1976).
The electrolyte volume should be large enough to prevent significant changes in composition (i.e. pH and Cl ) during the lifetime of operation between membrane changes. The gas-permeable membrane should preferably be impermeable to water vapor to prevent dessication of the electrolyte, particularly when operating at low hydrostatic pressure or in dry gases. A membrane which has suitable properties of electrolyte retention and gas permeability may become mechanically flawed during application, which typically involves stretching a planar material over a rod-shaped sensor. Improved methods to do 80 while radially supporting (C. J~ Koch and J. Kruuv, 13202~9 Measurement of ~ery Low Oxygen Tensions in Unstirred Liquids. Analyt. Chem 44 125a-1263, 1972) and tensioning (J.A. Porter and A.F. Dageforde, Membrane Tensioning Means and the Use Thereof. United States Patent ~o~ 3,887,194, issued June 3, 1975 to Porter et al.) the membrane have been described. The present invention also includes a simple pre-stretching procedure which is important for relatively non-elastic membranes.
Electrolyte can also be lost due to an incomplete seal of the membrane against the body of the sensor. Okun et al. suggested many years ago that a plastic, cylindrical membrane retainer would be superior to the o-rlng which is typically used by most manufactured sensors ~United States Patent No. 3,227,643, issued January 4, 1966 to D. A. Okun et al.). A recent patent by ~ale (J. M. Hale, Membrane Mounting Method and Membrane-Enclosed Amperometric Cell. United States Patent No.
4,325,797, issued April 20, 1982 to Hale et al.) suggests the use of a precision, non-elastic double-tapered membrane retainer but this device is stated not to be useful for relatively non plastically deformable membranes such as Teflon ~trade-mark).
Similarly electrolyte can be lost through water absorption by the body of the sensor itself ~many plastics wlll absorb a 9ubstantial percentage of water). Water can also be lost via a slow diffusion along the surface roughness o~ the sealing area and conversely other molecules may be able to enter the electrolyte.
I~ the sensor's response is to depend only on the oxygen partial pressure, and not the hydrostatic gas pressure, then the membrane should be stretched and held in place tightly enough to resist deformation at low or hlgh external pressures, particularly if the external pressure drops below the vapor pressure of the electrolyte. Sensor instabilitles caused by changes in pressure increase if small gas bubbles develop in the volume normally filled by electrolyte.

~` 16 1 ~2a2~9 A ~inal technical problem involving the actual use of commercial oxygen sensors is that after membrane application, the sheet of membrane is trimmed around the membrane retainer (typically an 0-ring). This trimming S process leaves a "ruffled skirt" of residual membrane which traps air bubbles and makes the sensor tip difficult to clean. The trapped air bubbles limit accuracy when using small samples or low oxygen concentrations since a given volume of oxygen-containing gas contains about 40 times more oxygen molecules than a similar volume of gas-equilibrated water. In some cases, plastic caps attached (usually via a screw thread) over the sensor tip have been used to cover the residual membrane (United States Patent No. 3,445,369, issued May 20, 1970 to J.A. Porter et al.), but these caps inevitably contain trapped gas and/or liguids and increase the total amount of plastic material exposed to the solution to be measured. A similar trapping would be found for the precision non-elastic retainer described above (United States Patent No.
4,325,797, issued April 20, 1982 to Hale et al.). In summary, the containment of the electrolyte should preferably allow for:
7b) a water and ion tight seal to allow operation at reduced pressures, in dry gases or in solutions of significantly different composition than the electrolyte ~e.g. pH, osmolality, ion content, presence of ~olvent~)~
8b) counteracting absorption of the electrolyte by the ~ensor body;
9b) sufficient volume to maintain the electrolyte concentrations ~pH, Cl-) constant;
lOb) sufficiently strong membrane and seal to resist plastic deformation due to pressure changes in the external environment and a method to eliminate any res~dual membrane distal from its retainer which could prevent easy cleaning and which could trap gas bubbles;
and 1~2~2~9 llb) minimi7ation of air or liquid traps, or materials which could cause cross-contamination between sequential measurements.

It is an object of the present invention to provide a polarographic sensor having an improved isolation of the polarographic reactions ongoing in said sensor either from modulating influences caused from sources internal to said sensor or from sources in the external environment of said sensor and an improved isolation of the possibly modulating effect of said sensor on said external environment.
According to an aspect of the present invention, there i6 provided a polarographic oxygen sensor comprising:
a first elongate housing having a longitudinal passage therethrough; a second elongate housing having a second longitudinal passage therethrough, said first elongate housing being received within said second longitudinal passage; a generally annular anode disposed about an end of said first elongate housing at a front end of said sensor; a cathode extending within said longitudinal passage; a glass seal formed in situ within said longitudinal passage and defining an extended seal between ~aid cathode and an interior surface of 6aid longitudinal passage; an electrolyte operatlvely di~posed at said front end of said sensor in electrical contact wlth ~ald anode and sald cathode; a flexible membrane of an lnert materlal which i~ permeable to gas and impermeable to water, water vapour and ions or other solutes; and means for securing said membrane in sealed relationship over said anode and said cathode at said front end of said sensor.
According to the present invention there is provided, in a polarographic sensor, a cathode sub-assembly made from an elongate housing with a longitudinal opening therethrough and with noble metal wires extending completely through ~aid opening, said wires held symmetrically in said longitudinal opening by an extensive plug of glass which is solidified in situ in sealing contact with said wires and with the wall of said longitudinal opening. Preferably, the ,~

17a 132~249 elongate housing, the cathode and the sealing glass have at least similar coefficients of thermal expansion. A suitable tubular anode is provided through which said cathode sub-assembly protrudes, and a flexible membrane of an inertmaterial, which i8 permeable to gas and impermeable to water vapour and other than gaseous dissolved solutes, is secured in sealed relationship over said anode and said cathode, with a film of electrolyte bathing said anode and said cathode and contained by said membrane.
Advantageously, the cathode sub-assembly is enclosed by a second elongate housing portion which incorporates the above-mentioned anode, hereinafter referred to as the combined anode/body sub-agsembly. This second housing is also made of inert, electricàlly non-conductive material and with an elongate opening therethrough.
A suitable material for both elongate housings has been found to be a ceramic manufactured by Corning under the Trade Mark Macor which, further, offers the advantage of 18 13232~9 enabling precision machining of the elongate housings t~ be effected.
In a preferred embodiment, the longitudinal opening of the end of the anode/body sub-assembly which will become the active or measuring tip of the completed sensor, is convergently tapered to a frusto-conical shape receiving in sealed engagement therewith a complementary, precision-machined outer convergently tapered surface on the active end of the cathode sub-assembly said sealed engagement being mediated by an epoxy adhesive.
The exterior surface of the active end of the body housing has a precision-machined convergently tapered frusto-conical shape fitted into sealed engagement with a precision-machined internal surface on the anode.
lS In a preferred embodiment of the invention, an electrically conductive coating is provided internal to the anode/body sub-assembly for providing a connection to the anode, the conductive coating extending around both ends of the body housing and shielding the cathode. This coating advantageously compromises a fired, thick film composite of precious metal and glass.
Furthermore, ln a pre~erred embodiment of the invention, the sen~or has, at the active end thereof, a tip portion having a cylindrical outer sur~ace and peripheral serratlone formed in the cylindrical outer sur~ace, and wherein the ~ensor ~urther comprises cylindrical retaining mean~ ~itted over the cylindrical outer sur~ace in co-axial relation~hip therewith and retaining the membrane in clamped relationship around the cylindrical outer ~ur~ace between the cylindrical retaining means and the cylindrical outer surface.
The membrane has a periphery which is entirely contained between the cyllndrlcal outer ~ur~ace and the cylindrical retalnlng means to ¢ounteract the retention o~ trapped gases at the membrane perlphery.
Re~erring now to the polarographic oxygen sensor lllustrated in Figure 2 o~ the accompanylng drawings, which 132~2~9 shows a preferred embodiment of the present invention, it is first pointed out that the design of this sensor was effected taking into account the following eight principle objectives:
(1) minimization of plasitcs throughout:
5~2) an improved seal of the cathode electrode which at the same time allowed versatility with respect to number of cathode elements, materials, size and shape:
(3) a pure silver anode of very great area compared with that of the cathode, which totally surrounded and shielded the cathode, and which would not allow leakage of oxygen from internal or external sources;
(4) a body which allowed a complete electrostatic shield of the cathode:
(5) a body with excellent structural stability and which was biological and chemically inert;
(6) a body which could be precisely machined to fit standard-taper leak-proof fittings;
(7) a leak-proof membrane retention system re~istant to changes caused by variations in external environment and with no residual membrane (which can trap air bubble~ and increase the difficulty of cleaning); and ~8) a modular design which allowed flexibility of ¢onstru¢tlon, testlng o~ key components and straightforward a~embly.
25It may be appreciated that although some of the above principles have been previou~ly identified, their actual implementation hae often been compromised. Even with these compromi~es, many patented devices have never been produced commercially. This undoubtedly stems from the extraordinarily complex machining and as~embly requirements of typical, previous sensors and a further ob~ective of the present design was to reduce this complexity wlthout compromising the other ob~ectives.
The preferred embodiment o~ the sensor, as described below, comprises several component parts or sub-assemblies, namely an anode/body sub-assembly, shown as its component parts in Figure 3, a cathode sub-assembly, shown in Figure 4, l3~a2~

a connector sub-assembly shown as part of Figure 5, and a membrane retainer and membrane, shown in Figure 7.
Additionally, Figure 6 provides a detailed view of the steps involved in the sealing of the cathode wires into the cathode sub-assembly.
In the following description, the orientation of the elements of the sensor are described with reference to an active tip or front end of the sensor at the left, and to the connector or back end of the sensor at the right, as shown in the drawings.
Referring now in greater detail to Figures 2 and 3, the sensor is indicated generally by reference numeral lo and comprises an elongate ceramic housing or outer sleeve 22, coated on its interior surface and opposite end surfaces 36, 34 with an electrically conductive film 35 and with a solid silver anode 24 mounted at the front end, the housing 22 having an axial opening or passage extending therethrough.
The passage extends (from the center) backwardly with a cylindrical surface 26 and forwardly with a left convergently tapered or frusto-conical surface 28 extending to the front end of the sub-asaembly. The electrical conductive film 35 comprises a thick film composite made from precious metal and gla~ (e.g. Thick Film Sy~tems Multifire 3580), which is ~ired in sealing contact with ~urfaces 26, 28, 32 and 34 by placing the coated housing 22 in a furnace at a temperature of 750C for a period o~ thirty minutes. At its external ~ront end the cerami¢ housing 22 is previously machined to provide a ~orwardly convergent, tapered ~rusto-conical outer sur~ace 36.
An anode 24 is made from a rod of 801 id pure silver, with an internal frusto-conical surface 38 being machined into the back end of the rod and being dimensioned B0 as to be exactly complementary to the fru~to-conically tapered outer surface 36 o~ the ceramic housing 22. Also, a ~orwardly convergent ~rusto-conical opening 40 is machined through the anode, this taper ~orming a continuation of the ~orward passage 28 on the interior o~ the hou~ing 22.

21 132~249 After being carefully cleaned, the anode is etched in hot, O.lN nitric acid and is then secured to the housing 22, by high quality epoxy, the components being held together under pressure during cure of the epoxy material to form an anode~body sub-assembly. This pressure results in a stable mechanical fixture of these components, and in addition ensures good electrical contact between the anode 24 and the metal film 35 on the end face 32 of the housing 22.
After attaching the anode to the body, the anode is machined to the same diameter as the cylindrical snoulder 42 of the housing and peripheral serrations 25, 27 (see Figure 5) are added to the anode to improve the seal of the membrane by its retaining sleeve (described below).
~he housing 22 has two other externally machined lg surfaces which will be described in more detail below; near the ~ront end, a forwardly tapered frusto-conical surface 44, and at the back end a backwardly tapered frusto-conical surface 46.
The cathode sub-assembly, indicated generally by reference number 50 in Figure 4, is made from a smaller elongate ceramic housing 51 whose exterior surface has a cylindrical back end portion 52 and an externally frusto-con~cally tapered ~ront end portion 54. An axial opening or pa~age therethrough follows the contour of the external shape wlth a back cylindrical internal portion 56 and a forwardly convergent tapered front internal portion 58. A pair of cathode wlre~ 60 extend through the passage from the ~ront end of the houslng and are connected by solder 64 to a Teflon-covered multi~trand wire 66.
In the ~ront end o~ the passage 58 the cathode wlre~ 60 are embedded in a glass seal 62, which fills the space between the cathode wires 60 and the surface of the passage 58.
At the back end of the housing 51, the cathode wires 60, the Folder 64 and a length o~ the multistrand wire 66 are embedded in an epoxy material 67.

The external tapered front end surface 54 of the h~si~g 51 o~ the cath~de su~-assem~y ~0 has a shape which is exactly complementary to the front portion 28 of the conductive-film-covered interior passage in the housing 22 and the opening 40 in the anode. The cathode sub-assembly is fixed into this passage using an epoxy adhesive (see Figure 5).
The back or right hand end of the housing 22 is formed with a backwardly convergent, tapered or frusto-conical outer surface 46, which is fitted in tight engagement (see Figure 5) with a complimentary internal surface 82 formed in an end cap 80, which serves to retain a SMA or Sub-Minax coaxial receptacle 84 (of e.g. Amphenol 901-190 or 27-12;
Trade Marks) relative to the housing 22.
The coaxial receptacle 84 is slidingly received in a cylindrical axial opening 86 in the end cap 80, which is provided at its back end with an inwardly directed annular flange 88 for retaining the receptacle 84 which is fastened to thè end cap by a lockwasher and nut 90. A nickel-plated helical compression spring 92 i6 provided between the plated back surface 34 of the housing 22 and the front surface 94 of the coaxial receptacle 84 and serves to make an electrical conne¢tion between the receptacle's outer sur~ace ~ground) and the conductive ~ilm 35 which in turn makes an electrical connectlon with the anode 24. The central terminal 96 of the receptacle 84 i~ connected to the cathode wires via the Teflon coated wire 66.
The interengaged frusto-conically tapered surfaces 54 and 28 of the cathode sub-assembly and anode/body sub-assembly and the sur~aces 38 and 36 of the anode 24 and housing 22 are precision-machined and are fitted together with an epoxy material o~ high electrical resi~tivity and strength provided therebetween as an adhesive. Becau~e o~ the precleion-machining o~ the~e tapered sur~aces, and their con~eguential tight interengagement, this epoxy material is mostly extruded ~rom between the components as the components are ~oined together, ~o that only a trace o~ the epoxy 23 ~202~9 material remains after the assembly of these components.
Furthermore, these precision machined surfaces provide the additional advantage of accurate centering of the components of the sensor, which is of assistance in the final shaping of the completed sensor.
In the manufacture of the present sensor, it was determined that a method of fabricating a highly effective cathode wire seal was very desirable. Prior art sensors had employed cathodes made either by melting a glass tube around lo a platinum wire, or by making glass coated wire by melting a glass bead along a length of wire using conventional glass-blowing techniques. These prior art procedures, however, have led to imperfect seals and to a resulting structure which was very fragile and/or difficult to shape.
Furthermore, because of the inconsistencies in the shape of such prior art sensors, particularly when employing the melting of a glass tube around a platinum wire, the incorporation of a prior art cathode into a complete sensor had to depend on the generous use of epoxy adhesive or of other plastic material for joining the cathode to its anode and to the body of the sensor.
In contrast thereto, in the manufacture of the present ~eneor, and ln order to eliminate the above problems o~ the prior art cathode fabrication and incorporation into a ¢omplete ~en~or, it was ~ound de~irable to allow both ma¢hlnlng and ~haping o~ a cathode ~ub-a~sembly and to devise a more con~i~tent ~ealing process.
To that end, the present cathode sub-assembly is made u~lng the housing 51 shown in Figure 4 and Figure 6 and in the following referred to generally as the cathode housing, o~ machinable ceramic Macor~Trade Mark) to which the cathode wire~, made of platinum, could be mechanically secured to enable the cathode housing to be placed into an oven where a tube of electrical sealing glass was caused to melt and ~low to make the final glass seal 62 shown in Figure 4.
To minimize stress o~ the ~inal seal, and even though the ceramic, the sealing glass and the cathode wires ~32~2~9 have very similar thermal expansion coefficients, it was important to keep the cathode wires in a central, symmetric position. For the same reason of minimizing stress, it was also found desirable to exclude most interior sharp edges which could initiate fracture planes.
Thus the front or left end of the ceramic cathode housing 54 of the cathode sub-assembly has, at the beginning of the manufacture of the cathode sub-assembly, a restricted hole 68 at the end of the tapered internal passage 58 (Figure 6) sc that the cathode wires 60, which in the preferred embodiment comprise platinum and have a diameter of 0.075mm are held centrally with respect to the internal passage 58.
The cathode wires 60 are bent radially outwardly from this hole 68 at the front tip of the cathode housing and are then bent backwardly along the outer surface of the cathode housing 51 as shown in Figure 6, and held in place by a tightly fitting ceramic cap 70.
~ he back or right end of the cathode housing 51 is provided at the exterior thereof with a loosely fitting ceramic cap 72, which again holds the cathode wires centrally with respect to the cathode housing because of a small hole 74 therein and which extends along a substantial portion of the cathode housing 50 as to increase the total weight and stability o~ the cathode housing 51 and to increase the tension in the cathode wires 60. Ends 61 of the cathode wires 60 are employed to support the housing 51 in a vertical position during the sealing operation.
As can also be seen in Figure 6, a tube 76 of ~ealing glass ~such as Corning # 8940: Trade Mark) i8 provided in the right or back end portion of the internal passage 56 of the cathode housing 51.
During the sealing operation, which is effected by suspending the cathode housing in the above-described manner in a furnace maintained at a temperature of approximately 950C for a period of three hours, the glass is caused to melt and to ~low downwardly to the ~ront or lowermost end of the internal passage 58 of the cathode housing, where this glass, 132~2~19 in solidified form, forms the glass seal 62 (shown in Figure 4) in sealing contact with the surface of the internal passage 58 of the cathode housing 51 and the cathode wires 60. The restricted opening 68 of the cathode housing and the tight fitting ceramic cap 70, coupled with the relatively high viscosity of the molten glass, do not allow substantial leakage of the molten glass out of the tip of the cathode housing during the sealing operation. The glass seal 62 is annealed ~y reheating to 423C and rec~oling before any further operations are performed.
The end front portion of the cathode housing, including the cap 70 and the restricted terminal opening 68, is then removed with a diamond saw and ground to provide the front or left tip of the cathode sub-assembly as shown in Figure 4.
Before use of the sensor, the anode 24 is carefully cleaned, with a soft camel-hair brush and a fine, polishing grade of alumina, using a weak ammonium hydroxide/detergent solution ~Gllowed by thorough rinsing and a final cleaning with ethanol. The composition of the electrolyte used is empirically determined, and typically contains 0.8% KCl (or 0.75% NaCl + 0.03% KC1), 20mM TAPS buffer 0 pH 8.5, with 1 mM DTPA ~dlethylene-triaminepentaacetic acid, a chelating agent) and 0.1% NP-40 (a non-ionic detergent) as a wetting agent. The actual electrolyte compo~ition has not been ~ound to be very critical and is chosen to be similar to that of solutions in which the sensor 10 is normally used. To improve both long and short term stability o~ the sensor 10 (thought to arise ~rom a more con~istent reference potential of the Ag/AgCl anode) the sensor 10 is connected to it~ appropriate polarizing voltage (typically 550 mV, anode positive with respect to cathode) and operated in ~resh electrolyte without its membrane (de~cribed below) ~or about 1 hour. The ~ensor current under these conditions is about ten times that with the membrane in place.
In order to ~it the membrane 18 to the sensor, the membrane 18 i8 firstly secured as shown in Figure 7a. More ~32~2~9 particularly, the membrane is clamped against an 0-ring 102 by means of a pair of annular clamps 104, which are tightened together by means of knurled nuts 106 on bolts 108 extending through the annular clamps, in order to clamp the o-ring 102 and the membrane 18 between the inner marginal edge portions of the clamps 104. It will be observed from Figure 7a that the 0-ring 102 is located beneath the membrane 18. The membrane 18 is then pressed downwardly at its center by a pressure member 110, which may conveniently take the form of a #00 silicone rubber stopper or the like, the membrane being cleaned with alcohol before thus being stretched and some of the alcohol being left on the membrane, as indicated by reference numeral 118, to act as a lubricant during such stretching. The stretched portion 120 of the membrane 18 is then applied over the sensor tip as described below.
The anode of the sensor 10 being cleaned and preconditioned as described above, the thread of the connector (shown in Figure 5) is gently screwed into a weighted base 112 (Figure 7b). A very small amount of silicone grease 12 is then applied around the side of the anode 25 and filling the peripheral serrations 25, 27, and electrolyte 14 is added to the tip o~ the anode, as shown in Figure 7b.
The clamped membrane 18 is then wiped dry of any ~alcohol and inverted, as shown in Figure 7c, with the pre~tretched portion of the membrane 120 stretched over the tip of the sensor 10 in such a manner that the membrane 18 adheres tightly against the first two to three millimeters of the length of the anode (i.e. past the frontmost serration 25 ln the anode's periphery).
While maintaining a downward pressure on the ¢lamped membrane, a tight-fitting retaining sleeve 16, made from a cylindrical sleeve o~ Teflon (Trade Mark) about 6 mm long and with a 0.75 mm thick wall, is then pushed over the anode 60 that its leading edge ls ~ust past the first serration 25 of the anode. This serration ¢auses the sleeve 16 to be me¢hanically stable at this positlon even though the stretching of the retaining sleeve and the further stretching o~ the membrane ,~.,~, 132~249 caused by the pushing on of the retaining sleeve 16 generates a tensile force which would otherwise tend to push the sleeve 16 back off.
The mechanical stability of the retaining sleeve po~ition at the first serration 25 of the anode allows the membrane to be released from the clamps and substantially trimmed. The nuts 106 of the membrane retaining clamps are loosened, allowing the release of the membrane 18 which is then trimmed close to the retaining sleeve 16 using fine iris scissors. The retaining sleeve 16 is then pushed further over the anode and its second and third serrations 27 to cover completely both the anode and the trimmed peripheral edge of the membrane. This two-step procedure allows no residual membrane edqe portion to remain externally to the retaining lS sleeve 16 and the modest flexibility of said retaining sleeve allows it to follow tightly the contours of the anode periphery thus retaining virtually no trapped gas space. This solve~ a di~ficult problem with the use of oxygen sensors in critical applications where any residual membrane external to it~ retainer would trap gas bubbles or contaminants. As de~cribed above, this problem almost alway~ exists with prior art commer¢ial sensors, regardless of the method of contalnment of the membrane.
The combined ef~ect of the ~ilicone grease, the ~erration~, the long membrane retaining sleeve and the relatively thick membrane ~0.05 mm Teflon rilm with no sur~ace flaw~) lead to an extraordinarily effective ~eal of the electrolyte.
Operation The completed sensor was connected to a sensitive current-measuring instrument at a polarizing voltage of 0.55 volts. It was sealed into an air-tight aluminum vacuum chamber ~C. J. Xoch, The Ef~ect of Oxygen on the Repair of Radiation Damage to Cells and Ti~sues. In "Advance~ in Radiation Blology" (Eds J. Lett & H. Adler) Academic Pre~, New York, ~, pp 273-315, 1976) by way of an Ultra-Torr fitting (Ca~on). The sensitivity in air at one atmosphere of 132~2~9 pressure was established as approximately 5 x lo-9 amps (nA) at room temperature. Since air contains about 20.9% oxygen, and 1 atmosphere represents 101.3 kilopascals (kPa) this translates to about 0.25 nA/kPa of oxygen partial pressure.
When the sensor was switched from air (oxygen content 210,000 parts per million [ppm]) into a nitrogen atmosphere (oxygen content as analyzed by Union Carbide, less than 5 ppm) the current dropped exponentially with an initial response-time of 20 seconds to an absolute value corresponding to a value less than 100 ppm over the next 45 minutes. The stability of the zero current was equivalent to le~s than 20 ppm / day and the noise was of the order of 2 ppm.
The stability o~ the reading in continuous exposure to air at constant temperature was better than -2% per day a~ter an initial operating time of 1 hour. In sealed containers with solutions held at constant temperature, the response of the ~ensor was constant within measurement limits ~+/- 0.2%). This excellent ~tability is attributable to the sealing technigue of the membrane. A measure of this seal was that the aerobic response of the sensor was not affected by prior continuous exposure to a high vacuum, (less than 0.015 kPa) for 2 weeks.
The above-described test of stability after exposure to high vacuum and some of the following experiments 2S were performed by inserting the sensor, using an Ultra-Torr fitting ~Ca~on) into a leak-proof chamber connected via a manifold to a vacuum pump, nitrogen tank, and precision pressure gauge (C.J. Koch, The E~ect of Oxygen on the Repair of Radiation Damage to Cells and Tissues. In "Advances in Radiation Biology" (Eds J. Lett & H. Adler) Academic Press, New York, 8, pp 273-315, 1976).
Another experiment was u~ed to determine whether the sensor could operate independently of hydrostatic pressure. The response of the sensor, initially in air, was monitored a~ter evacuating the chamber to guite a high vacuum (approximately 1 kPa), waiting for sen~or eguilibrium, and then refilling the chamber with pure nitrogen. The 29 ~320249 equilibrium response was the same in the partial vacuum as it was following the readmission of pure nitrogen. As mentioned earlier, this is what would be expected if the sensor's response was only sensitive to oxygen partial pre~sure and was independent of actual hydrostatic pressure.
The linearity of the sensor was tested by filling the chamber with 30% oxygen and then every 5 minutes subjecting it to a "gas change". Each gas change was made by evacuating the chamber to 31.9 kPa pressure (as measured by the precision pressure gauge), then refilling the chamber to 101 kPa with pure nitrogen. As long as the vacuum gauge reading was reproducible, the oxygen partial pressure should decrease by the same proportion (roughly 0.32) with each gas change, and therefore a plot of log sensor current vs. number of ga~ changes should be a straight line (C.J. Koch, Measurement of Very Low Oxygen Tensions in Liquids: Does the Extrapolation Number For Mammalian Survival Curves Decrease A~ter X-irradiation Under ~ypoxic Conditions. In: Proc. 6th LH Gray Con~erence ~1974) Institute of Physics, London, England, pp 167-173, 1975). The re~ponse of the aensor was not di~erent than the predicted exponential relationship over ~our decade~ o~ oxygen partial pressure if the absolute zero current ~approximately 50 ppm in this experiment) was ~ubtracted ~rom all of the readings.
~he dependence on ~tirring ~peed of the sen~or'~
re~pon~e in a ga~-equilibrated liquid was low. The sensor wao placed above a stirred liquid, using a small vial ~itted with an Ultra-Torr ~itting (Ca~on). The sensor could be lowered into the liquid without causing any change in the ga~
pha~e oxygen value because o~ the O-ring seal of the fitting.
The re~ponse was essentially unchanged in a ~lowly stirred, ga~-equilibrated liquid and, as would be expected, this re~ponse was not dependent on the ab~olute oxygen level. For a completely unstirred liquid, the re~pon~e decreased by less than 2~ at room temperature.
~ he temperature re~ponse o~ the sensor in dry gas wa~ about 2.9% per degree C. TG measure thiB~ the sensor waB

132~2~9 simply allowed to equilibrate at various temperatures in a refrigerated air-flow incubator with the temperature measured by a precision mercury thermometer. The Arhenious plot of the data showed an extremely linear relationship with an activation energy of 5 kilocalories per mole (i.e. thè sensor response doubled with a temperature~increase of approximately 25C) A major problem exists with many prior art sensors due to interference from chemicals other than oxygen. Some such chemicals, like C02, can affect the pH of the electrolyte and others can (also) participate in the redox reactions of the sensor itself (oxidizing or reducing gases). Still other chemicals, if they can diffuse through or around the membrane barrier, can make the sensor response completely unpredictable. A common example is to have reducing agents present and to be monitoring the oxygen consumption of the auto-oxidation reactions. An equally common result is to observe anomalous currents (i.e. with respect to the oxygen derived current) and/or gross changes in the appearance of the ~ilver anode (due to silver-sulfide for example). The response of the sensors in the present embodiment doe~ not change with C02, is only minimally sensitive to N20 (commonly u~ed in radiation chemistry and in medicine) and i8 not in~luenced by reducing agents in monitored solutions.
Thus the present sensor exhibits unprecedented accuracy and utility. Many problems associated with the Clark-type sensor have been eliminated or greatly reduced and it now appear~ ~easible to make additional improvement~ and to test various aspects of the design in an analytical manner.
The operating specifications of the sensor of~er a particular improvement in the mea~urement of extremely low oxygen partial pre~ures or in any situation requiring an extremely stable re~pon~e. These improvements should allow detailed study o~
the many processe~ which are very sensitive to small absolute values Or, or changes in, oxygen concentration. In addition, the excellent seal o~ the relatively thick (0.05 mm) Te~lon membrane, allowing exposure of only ceramic and Teflon to a 31 ~3~2~9 measured solution, allows the sensor to operate in many hostile environments. Thus, the sensor will operate at pH
extremes and in many organic and inorganic solvents, thus suggesting many new applications for the measurement of oxygen partial pressures. The operating specifications for the above-described embodiment of the present invention and several commercial sensors are summarized in Table 1.
The sealing methodologies described above for the cathode wires 60 are quite versatile. The number of cathode elements can be changed (from one to four wires have been sealed with success). The size of the cathode wires can be varied to suit a particular requirement (O.025-0.25 mm diameter Pt has been used by the present inventor with an optimal diameter appearing to be 0.075 mm for the membrane used).
It has often been stated, without clearcut experimental evidence, that cathodes other than platinum (e.g.
gold) could have electrochemical advantages in polarographic sensors. The described method of melting glass into a heat resistant, machinable container has been used to seal cathode wires such as gold into the ceramic cathode housing even though there is a tremendous mismatch of thermal expansion coe~icient~. 8imilarly, it is po~ible to obtain annular cathode~ and anodes by using different types of thick film preciou~-metal/gla~s ~ired oomposite~. It should be possible to include electrically separate element~ of combinations o~
the above to provide sensors with internal references, temperature sensing etc. Until now this type of versatility ha~ only been possible with semiconductor technologies (Thin Film Electrochemical Electrode and Cell. United States Patent No. 4,062,750, issued December 13, 1977 to J.F. Butler).
However these more 30phisticated approaches have not yet been attempted because the specl~lcations o~ the present devices already represent a many ~old improvement over commercially avallable devices.
Similarly, the use o~ inert yet machinable ceramic throughout allows for the custom shaping o~ the completed 132~2~9 sensor and its adaptation to all sorts of unusual physical situations. The chemical inertness of all exposed surfaces (ceramic and Teflon) has been found to be particularly advantageous in oxygen measurements using tissue culture and ultrapure chemical environments. The assembled sensors can be sterilized and/or cleaned by short or long-term exposure to pH extremes or solvents without effects on subsequent operation (the present inventor typically uses 70~ ethanol at pH2). The sensors have been used to measure oxygen consumption in heavily irradiated solutions and some have been exposed to several megarads of ionizing radiation without any degradation in response - these doses can cause the embrittlement of many plastics.
The enhanced sensitivity and stability of the pre~ent 6ensors will allow the detailed analysis of oxygen dependent reactions which are beyond the reach of current technology. It i5 also possible to consider improvements in the many other uses for polarographic devices. Even non-polarographic uses for such devices are possible. For example, the present sensors can be used to measure air pre~ure with a linear range of four orders of magnitude.

TAB~B 1 - CHARACTER~TIC~ OF FOUR OXYGEN 8BN80R8 Parameter Sensor Numberl #1 #2 #3 #4 5 Sensitivity2 (nA/kPa) 0.07 16.0 0.5 0.25 Linearity 2 2 --- 4 (decades-measured) Stability (+%/hr) 0.5% NS NS 0.1 Min. Zero Current (nA) <0.1 <3 NS <0.003 10 Temperature Coef.(%/C) 4 4 5 2.9 Response Time3 2/Z 3/2 4/1.5 20/4 (Sec/decades of response) Polarizing Voltage (V) 0.75 0.8 0.7 0.55 15 Membrane Thickness (mm) 0.025 0.013 0.025 0.05 Membrane Material4 PP T,PP PE T
Membrane Retainer5 0 0 0 TC
Body Material~ A E G/SF C
Electrolyte7 3K/D KS 0.9N 0.9B/Ch/D
20 Storage Dry Dry Dry Wet/Dry Notes 1. #1 iB Beckman 325814, #2 is Yellow Springs 5331, #3 i5 Diamond Electrotech 730, #4 is preaent invention.
2. Abbreviations: 1 nA(nanoamp)-109 amps, kPa~kilopa~cal) 0.01 atmospheres, NS means not ~peclfied.
3. Range unspeci~ied. Values quoted are measured using membranes supplied by manu~acturer.
4. PP is polypropylene, PE iB polyethylene, T is Te~lon.
5. 0 is O-ring, TC is Te~lon cylinder.
6. A is acrylic, E is epoxy, G/SF is glass/silver ~oil, C is ceramic.
7. 3K iB 3% KCl, D iB detergent, KS is saturated KCl, 0.9N is 0.9% NaCl, 0.9B is 0.9% NaCl bu~er and Ch i~ chelator.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A polarographic oxygen sensor comprising:
a first elongate housing having a longitudinal passage therethrough;
a second elongate housing having a second longitudinal passage therethrough, said first elongate housing being received within said second longitudinal passage;
a generally annular anode disposed about an end of said first elongate housing at a front end of said sensor;
a cathode extending within said longitudinal passage;
a glass seal formed in situ within said longitudinal passage and defining an extended seal between said cathode and an interior surface of said longitudinal passage;
an electrolyte operatively disposed at said front end of said sensor in electrical contact with said anode and said cathode;
a flexible membrane of an inert material which is permeable to gas and impermeable to water, water vapour and ions or other solutes; and means for securing said membrane in sealed relationship over said anode and said cathode at said front end of said sensor.

2. A polarographic sensor as claimed in claim 1, wherein said glass seal extends within said longitudinal passage from one end of said passage a substantial distance towards the other end thereof, said glass seal, said cathode, said housing, and said anode cooperating to prevent leakage of gas through said housing to said electrolyte.

3. A sensor as claimed in claim 1, wherein said means for securing said membrane comprises a generally cylindrical retainer adapted to be fitted over said front end of said sensor in co-axial relationship with said housing so as to clampingly retain said membrane at said front end of said sensor, said retainer extending over the peripheral edge of said membrane to counteract retention of trapped gases or liquids at said membrane periphery.

4. A polarographic sensor as claimed in Claim 1, wherein a film of sealant is provided between said membrane and an exterior surface of said sensor.

5. A polarographic sensor as claimed in Claim 1, further comprising at least one circumferential recess formed near said front end of said sensor to facilitate installation of said enclosing means over said front end of said sensor.

6. A polarographic sensor as claimed in Claim 5, wherein said at least one circumferential recess is formed in an exterior surface of said anode.

7. A polarographic sensor as claimed in Claim 1, wherein said first elongate housing is composed of a machinable ceramic material.

8. A polarographic sensor as claimed in claim 1, wherein said first elongate housing extends forwardly of said anode at said front end of said sensor.

9. A polarographic sensor as claimed in Claim 1, wherein said second longitudinal passage includes a tapered portion and said first elongate housing includes a tapered outer surface, said tapered portion and said tapered outer surface being precision-machined to frusto-conical shapes whereby said first elongate housing is securely and sealingly retained within said tapered portion of said second longitudinal passage.

10. A polarographic sensor as claimed in Claim 1, wherein said first and second elongate housings are composed of a machinable ceramic material.

11. A polarographic sensor as claimed in Claim 1, wherein said second housing and said first housing are sealed together by an adhesive.

12. A polarographic sensor as claimed in claim 1 or 7, wherein said machinable ceramic material is MACOR
(registered trade-mark).

13. A polarographic sensor as claimed in claim 1, wherein the external shape of said second housing is machined into a predetermined configuration depending upon the use to which said sensor is to be put.

14. A polarographic sensor as claimed in claim 1, wherein said second elongate housing further comprises an electrically conductive layer disposed on a wall of said second longitudinal passage, said electrically conductive layer surrounding and electrically shielding said cathode.

15. A polarographic sensor as claimed in claim 14, wherein said electrically conductive layer extends onto an exterior surface of said second housing so as to form an electrical contact with said anode.

16. A polarographic sensor as claimed in Claim 14 or 15, wherein said electrically conductive layer comprises a fired, thick film composite of precious metal and glass.

17. A polarographic sensor as claimed in Claim 3, wherein a film of sealant is provided between said membrane and a cylindrical outer surface of said sensor.

18. A polarographic sensor as claimed in Claim 3, wherein at least one circumferential recess is formed around said sensor front end within said cylindrical retaining means to facilitate installation of said membrane and said cylindrical retaining means over said sensor front end.

19. A polarographic sensor as claimed in Claim 3, wherein a film of sealant is provided between said membrane and a cylindrical outer surface of said sensor and wherein said at least one circumferential recess is formed in a cylindrical outer surface of said anode.

20. A polarographic sensor as claimed in Claim 1, wherein said tapered portion and said tapered outer surface are precision-machined to frusto-conical shapes.

21. A polarographic sensor as claimed in Claim 1, wherein said outer housing and said inner housing are made of a machinable ceramic material.

22. A polarographic censor as claimed in Claim 20, wherein said outer housing and said inner housing are sealed together by an adhesive.

23. The polarographic sensor of claim 21, wherein said ceramic material is MACOR (registered trade-mark), and wherein the external shape of said outer housing comprising said ceramic material is machined into a predetermined configuration depending upon the use to which said sensor is to be put.

24. In a polarographic sensor according to claim 1, the improvement comprising an electrically conductive layer formed on the wall of said passage for forming an electrical connection to said anode, said electrically conductive layer extending around and electrically shielding said cathode.

25. A polarographic sensor as claimed in Claim 24, wherein said electrically conductive layer comprises a fired, thick film composite of precious metal and glass.

26. A polarographic gas sensor comprising:
an elongate sensor housing having a longitudinal opening extendinq axially therethrough from one end thereof to the other;
a tubular anode fixedly mounted to said housing at one end thereof, coaxially with said housing and in fitting relationship therewith, said housing being extended at said one end to pass through said anode and extending forwardly thereof;
a cathode extending through said longitudinal opening to said one end;
a flexible membrane of an inert material which is permeable to the gas to be determined and impermeable to water, water vapour and ions or other solutes;
means securing said membrane to said sensor peripherally near said one end in sealing relationship over said housing, said anode and said cathode;
an electrolyte sealed between said membrane and said housing, anode and cathode; and sealing means sealing said cathode within said elongate opening, said sealing means comprising a glass seal formed in said elongate opening in sealing contact both with said cathode and the wall of said opening, and extending within said opening from said one end a substantial distance therethrough towards the other end thereof, said sealing means, said cathode, said housing, and said anode cooperating to prevent leakage of gas through said housing to said electrolyte.

27. A polarographic sensor as claimed in claim 26, wherein said elongate sensor housing comprises an inner housing having said longitudinal opening therein, and an outer housing having a longitudinal passage extending therethrough for coaxially receiving therein said inner housing in a fitting relationship.

28. A polarographic sensor as claimed in claim 27, wherein an end surface of said outer housing contacts said anode, and said end surface and said longitudinal passage are coated with an electrically conductive coating means forming an electrical shield around said cathode, and an electrical connection to said anode.