Title of Invention: CROSS-LINKED ENZYME MATRIX AND USES THEREOF Abstract: An electrochemical sensor system and membrane and method thereof for increased accuracy and effective life of electrochemical and enzyme sensors. Detailed Description: FIELD OF THE INVENTION The present invention is related to the field of electrochemical sensors, particularly enzyme-electrode sensors, and to the regeneration or maintenance of the functional properties of the membranes of such sensors. BACKGROUND OF THE INVENTION In a variety of clinical situations it is important to measure certain chemical characteristics of the patient's blood such as pH, hematocrit, the ion concentration of calcium, potassium, chloride, sodium, glucose, lactate, creatinine, creatine, urea, the partial pressure of O2, and CO2, and the like. These situations range from a routine visit of a patient in a physician's office to monitoring of a patient during open-heart surgery. The required speed, accuracy, and other performance characteristics vary with each situation. Typically, electrochemical sensor systems which provide blood chemistry analysis are stand-alone machines or are adapted to be connected to an extracorporeal shunt or an ex vivo blood source such as a heart/lung machine used to sustain a patient during surgery. Thus, for example, small test samples of ex vivo blood can be diverted off-line from either the venous or arterial flow lines of a heart/lung machine directly to a chamber exposed to a bank of micro-electrodes which generate electrical signals proportional to chemical characteristics of the real time flowing blood sample. Electrochemical sensor systems are analytical tools combining a chemical or biochemical recognition component (e.g., an enzyme) with a physical transducer such as a platinum electrode. The chemical or biochemical recognition component is capable of selectively interacting with an analyte of interest and of generating, directly or indirectly, an electrical signal through the transducer. Electrochemical sensor systems play an increasing role in solving analytical and clinical problems, and find applications in the field of medical diagnostics. The selectivity of certain biochemical recognition components makes it possible to develop electrochemical sensors which can accurately detect certain biological analytes even in a complex analyte mixture such as Whole blood. Despite the high degree of selectivity of certain biochemical recognition components, the selectivity of such sensors as a whole may nonetheless be compromised by the presence of certain biological interferents (e.g. ascorbic acid, uric acid, acetaminophen, cysteine, etc.) which can directly interact with the physical transducer if they are not prevented from doing so. Accuracy and precision of electrochemical sensor systems with biochemical recognition compounds is also compromised by residual levels of analyte remaining in the sensor from a prior sample affecting the analysis of the following sample. SUMMARY OF THE INVENTION One objective of the present invention is to provide a system and method for increasing the accuracy and effective lifetime of an electrochemical sensor. Polymerization of electropolymerizable monomers into an inner polymeric membrane on the electrochemical sensor forms an interference rejection membrane. This inner polymeric membrane functions to protect the electrochemical sensor from the fouling or interference by compounds in the sample and thus increase the accuracy that is lost by the fouling degradation of the membrane or by interference by analyte compounds from the sample. In one aspect of the present invention, an electrochemical sensor includes at least one electrode, and a composite membrane. The composite membrane includes an outer layer, an enzyme layer, and a restorable inner layer. The inner layer is in contact with at least one electrode and includes a polymerizable membrane. The outer layer of the composite membrane may include a compound selected from the group consisting of polyurethane-based compounds, polyvinyl-based Compounds, silicone elastomer-based compounds, and polycarbonate-based compounds. In one embodiment, the enzyme layer of the electrochemical sensor includes a H2O2 generating enzyme, such as glucose oxidase or lactate oxidase, for example. In another embodiment, the enzyme layer includes one or a combination of several enzymes, such as a mixture of glucose oxidase, lactate oxidase, creatininase, creatinase, and sarcosine oxidase. In one embodiment, the electrochemical sensor further includes a restored surface on the inner layer wherein the surface is restored by polymerized monomer. The inner layer of the electrochemical sensor may include a compound selected from the group consisting of benzothiophene, phenylenediamines, and dihydroxybenzenes. In one aspect of the present invention, an electrochemical sensor cartridge, includes an electrochemical sensor card, at least one electrochemical sensor, and a reservoir containing an electropolymerizable monomer solution in fluid communication with the electrochemical sensor card. In an embodiment of the present invention, the electrochemical sensor cartridge may include an electrochemical sensor card that includes at least one composite membrane. In another embodiment, the electrochemical sensor cartridge may include a composite membrane with a restorable inner layer. In an embodiment of the present invention, the electrochemical sensor cartridge includes at least one calibration solution reservoir in fluid communication with the electrochemical sensor card. In another embodiment the electropolymerizable monomer solution may be combined with the calibration solution in a single reservoir. In another embodiment of the present invention, the electrochemical sensor cartridge includes electropolymerizable monomer solution in the calibration solution wherein the concentration of the monomer is in the range of about 1-100 mM. In another embodiment, at least one of the electrochemical sensors of the electrochemical sensor cartridge comprises an enzyme electrode sensor. In another embodiment the electrochemical sensor of the electrochemical sensor cartridge is formed on an electrode composed from a material selected from a group consisting of platinum, gold, carbon or one of their modified structure. In another embodiment the electrochemical or includes an electropolymerizable monomer selected from a group consisting of benzothiophene, phenylenediamines, and dihydroxybenzenes. In another embodiment the electrochemical sensor is selective for a hydrogen ion, carbon dioxide, oxygen, sodium ion, potassium ion, ionized calcium, chloride, hematocrit, glucose, lactate, creatine, creatinine or urea. In yet another embodiment, the electrochemical sensor includes a electropolymerizable monomer that is a derivative of phenylenediamine. In another aspect of the present invention, an electrochemical sensor system includes an electrochemical sensor card including at least e electrochemical sensor, wherein the electrochemical sensor includes at least one polymeric membrane. The electrochemical sensor system also includes an electrochemical sensor apparatus that is in electrical contact with the electrochemical sensor card. The electrochemical sensor apparatus is configured to measure electrical signals from the electrochemical sensor card and is capable of providing an electrical potential to the electrochemical sensor for the polymerization of the electropolymerizable monomer solution to the polymeric membrane. The electrochemical sensor system also includes a reservoir containing an electropolymerizable monomer solution in fluid communication with the electrochemical sensor card. The electropolymerizable monomer solution is polymerized to the polymeric membrane by the electrical potential provided by the electrochemical sensor apparatus. In an embodiment of the present invention, the electrochemical sensor cartridge may include an electrochemical sensor card that includes at least one composite membrane. In another embodiment, the electrochemical sensor cartridge may include a composite membrane with a restorable inner layer. In an embodiment, the electrochemical sensor system further includes a calibration solution in a reservoir in combination with an electropolymerizable monomer solution. The concentration of the electropolymerizable monomer solution is in the range of about 1-100 mM. In another embodiment, the electrochemical sensor system includes at least one enzyme electrode sensor. In yet another embodiment, the electrochemical sensor system includes an electrochemical sensor that is selective for a compound selected from a group consisting of hydrogen ion, carbon dioxide, oxygen, sodium ion, potassium ion, ionized calcium, chloride, hematocrit, glucose, lactate, creatine, creatinine or urea. In yet another embodiment, the electrochemical sensor system includes an electropolymerizable monomer that is selected from a group consisting of benzothiophene, phenylenediamines, and dihydroxybenzenes, of which the concentration of the electropolymerizable monomer solution in the calibration solution is 1400 mM. In another embodiment, electrochemical sensor system includes an electrochemical sensor apparatus capable of providing an electrical potential for at least the partial removal of interfering agents in the polymeric membrane. In another embodiment, electrochemical sensor system further includes an outer membrane and an enzyme layer, in which the enzyme layer is in contact with the outer membrane and the polymeric membrane. In another aspect, the invention relates to accelerating the recovery of the electrochemical sensor during the rinse process following exposure to a sample so that the recovery time of the electrochemical sensor system in a shorter time period. The reduction in recovery time is accomplished by removing interfering agents from the polymeric membrane layer. Residual concentration of substrates for the enzymatic reaction and the products of the enzymatic reaction after exposure of the electrochemical sensor to a sample, are examples of interfering agents. Another example of interfering agents is the residual concentration of the electropolymerizable monomer in the polymeric membrane after exposure of the electrochemical sensor to the electropolymerizable monomer solution. The removal of interfering agents from a polymeric membrane is accomplished by providing an electrochemical sensor including an electrode and a composite membrane, the composite membrane including at least one polymeric membrane, an electrical source in electrical contact with said electrode, and by applying an electrical potential to the electrode sufficient to cause at least a portion of the interfering agents in the polymeric membrane contact with the electrode to be removed. In one embodiment, the electrical potential is in a range of about 0.1 to 0.8 V versus the on-board reference electrode and is applied for a range of time from about 10 to 200 seconds. In another embodiment, the electrical potential is about 0.4 V versus the on-board reference electrode and is applied for about 50 seconds. In another aspect, the invention relates to the method of restoring the functional properties of an electrochemical sensor. The method includes providing an electrochemical system, which includes an electrochemical sensor card including at least one electrochemical sensor. The electrochemical sensor includes an electrode and a composite membrane, the composite membrane including at least one polymeric membrane. The electrochemical sensor system also includes an electrochemical sensor apparatus in electrical contact with the electrochemical sensor card. The electrochemical sensor apparatus is configured to measure electrical signals from the electrochemical sensor card and to provide an electrical potential to the electrochemical sensor. The electrochemical sensor system also includes a reservoir containing and electropolymerizable monomer in a solution in fluid communication with the electrochemical sensor card. The electropolymerizable monomer solution is polymerized to the polymeric membrane by the electrical potential provided by the electrochemical sensor apparatus. The method of restoring the functional properties of an electrochemical sensor also includes contacting the electrochemical sensor with the solution and applying an electrical potential of sufficient strength and sufficient duration to cause at least a portion of the electropolymerizable monomer in the solution to polymerize onto the polymeric membrane. In an embodiment, the method of restoring the functional properties of an electrochemical sensor includes adding the electropolymerizable monomer to a calibrating solution to form the electropolymerizable monomer solution. In one embodiment, the electrical potential comprises a range of about 0.1 to 0.8 V versus the on-board reference electrode and is applied for a range of time from about 30 seconds to 1 hour. In another embodiment, the electrical potential comprises about 0.5 V versus an on-board reference electrode and is applied for about 3 minutes. In an embodiment, the method of restoring the functional properties of an electrochemical sensor further includes the step of applying an additional electrical potential of sufficient strength and sufficient duration to the electrode to cause removal of at least a portion of interfering agents in the polymeric membrane. In one embodiment, the electrical potential is in a range of about 0.1 to 0.8 V versus the on-board reference electrode and is applied for a range of time from about 10 to 200 seconds. In another aspect, the invention relates to the method for restoring the functional properties of an electrochemical sensor cartridge. The method includes the steps of connecting an electrochemical sensor cartridge that includes an electrochemical sensor to an electrochemical sensor apparatus. The electrochemical sensor includes an electrode and a composite membrane, which includes at least one polymeric membrane. The method further includes contacting the electrochemical sensor with electropolymerizable monomer solution from the cartridge, and applying an electrical potential of sufficient strength and sufficient duration to cause at least a portion of the electropolymerizable monomer solution to polymerize onto a polymeric membrane. In one embodiment, the method further includes adding an electropolymerizable monomer to a calibrating solution to form the electropolymerizable monomer solution. In a particular embodiment, an electrical potential is applied at a range of about 0.1 to 0.8 V versus the on-board reference electrode. The electrical potential may be applied for a range of time from about 30 seconds to 1 hour. In one embodiment, the method also includes applying an additional electrical potential of sufficient strength and sufficient duration to the electrode to cause removal of at least a portion of interfering agents in the polymeric membrane. In one embodiment, the electrical potential is in a range of about 0.1 to 0.8 V versus the on-board reference electrode and is applied for a range of time from about 10 to 200 seconds. In another aspect, the invention relates to a composite membrane for a biosensor. The biosensor includes an inner membrane layer, an outer membrane layer, and an enzyme layer. The enzyme layer includes a matrix that includes at least one enzyme, a cross-linking agent, and an enzyme stabilizer. In one embodiment of the present invention, the composite membrane includes one or more of the enzymes lactate oxidase, creatinase, sarcosine oxidase, and creatininase. In another aspect, the invention relates to a matrix for an enzyme sensor. The matrix includes lactate oxidase, a cross-linking agent, and a enzyme stabilizer. In one embodiment, the matrix forms a cross-linked matrix of proteins having enzymatic activity. The matrix may form an electrochemical electrode. The matrix may also include bovine serum albumin. Other inert proteins similar to bovine serum albumin may also be included. In another embodiment, one or more of the cross-linking agent present in the matrix may include a dialdehyde, glutaraldehyde, for example, a diisocyanato, 1,4-diisocyanatobutane, for example, and a diepoxide, 1,2,7,8-diepoxyoctane and 1,2,9,10-diepoxydecane, as examples. In another embodiment, the cross-linking agent present in the matrix is 1-10% glutaraldehyde by weight. In yet another embodiment, the cross-linking agent present in the matrix is 5% glutaraldehyde by weight. In another embodiment, the enzyme stabilizer present in the matrix may include one or more of the compounds, polyethyleneimine, polypropyleneimine, poly(N-vinylimidazole), polyallylamine, polyvinylpiridine, polyvinylpyrollidone, polylysine, protamine and their derivatives. In another embodiment, the enzyme stabilizer present in the matrix is 1-20% polyethyleneimine by weight. In another embodiment, the enzyme stabilizer present in the matrix is 5% polyethyleneimine by weight. In yet another aspect, the invention relates to a matrix for an enzyme sensor that includes creatinase, sarcosine oxidase, a cross-linking agent and, an enzyme stabilizer. In one embodiment, the matrix also includes creatininase. In one embodiment, the matrix font's a cross-linked matrix of proteins having enzymatic activity. The enzyme sensor may form an electrochemical sensor. In another embodiment, one or more of the cross-linking agent present in the matrix may include a dialdehyde, glutaraldehyde, for example, a diisocyanato, 1,4-diisocyanatobutane, for example, and a diepoxide, 1,2,7,8-diepoxyoctane and 1,2,9,10-diepoxydecane, as examples. In another embodiment, the cross-linking agent present in the matrix is 1-10% glutaraldehyde by weight. in yet another embodiment, the cross-linking agent present in the matrix is 5% glutaraldehyde by weight. In another embodiment, the enzyme stabilizer present in the matrix may include one or more of the compounds, polyethyleneimine, polypropyleneimine, poly(N-vinylimidazole), polyallylamine, polyvinylpiridine, polyvinylpyrollidone, polylysine, protamine and their derivatives. In another embodiment, the enzyme stabilizer present in the matrix is 1-20% polyethyleneimine by weight. In another embodiment, the enzyme stabilizer present in the matrix is 5% polyethyleneimine by weight. In yet another aspect, the invention relates to a matrix for an enzyme sensor including one or more of the enzymes, lactate oxidase, creatinase, sarcosine oxidase and creatininase, a cross-linking agent, and an enzyme stabilizer. These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. BRIEF DESCRIPTION OF THE DRAWING The foregoing and other objects, features and advantages of the present invention disclosed herein, as well as the invention itself, will be more fully understood from the following description of preferred embodiments and claims, when read together with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. FIG. 1 is a schematic diagram of the components of an electrochemical sensor apparatus including a sensor cartridge with a bank of sensors and a thermal block for accelerated hydration and calibration of the sensors. FIG. 2 illustrates a reverse frontal view of the sensor card, partly fragmentary, of a cartridge embodiment of the invention. FIGS. 3A-B illustrate cross-sectional views of an enzyme sensor. FIG. 4 illustrates an embodiment of a pO2 sensor. FIG. 5 illustrates a frontal view of the electrode card contained in one embodiment of the cartridge. FIG. 6 illustrates cross-sectional views of an ion sensor. FIGS. 7A-G illustrate the components of a thermal block assembly. DETAILED DESCRIPTION OF THE INVENTION The present invention provides electrodes and electrochemical sensor systems for measuring characteristics of aqueous samples including, but not limited to, blood, serum or other body fluids. Specifically, the invention is directed to such sensors in which the electrodes include an interference rejection membrane, which is the inner polymeric membrane of the composite membrane and is renewable in situ. The electrochemical sensor systems according to the invention have increased accuracy and precision and increased effective life spans. In preferred embodiments of the invention, the sensor system is adapted to measure the concentration or activity of blood gases (e.g., oxygen and carbon dioxide) ions (e.g., sodium, chloride, potassium and calcium), glucose, lactate, creatine, creatinine, blood pH and hematocrit. Definitions In order to more clearly and concisely point out and describe the subject matter which applicant regards as the invention, the following definitions are provided for certain terms used in the following description and claims. As used herein, the term “electrode” refers to a component of an electrochemical device which makes the interface between the external electrical conductor and the internal ionic medium. The internal ionic medium, typically, is an aqueous solution with dissolved salts. The medium may also comprise proteins in a stabilizing matrix. Electrodes are of three types, working or indicator electrodes, reference electrodes, and counter electrodes. A working or indicator electrode measures a specific chemical species, such as an ion. When electrical potentials are measured by a working electrode, the method is termed potentiometry. All ion-selective electrodes operate by potentiometry. When current is measured by a working electrode, the method is termed amperometry. Oxygen measurement is carried out by amperometry. Working electrodes may also function by including an enzyme as part of an enzyme layer that is part of a composite layer that is in close contact with the electrode. The enzyme, which is specific to a particular analyte, produces hydrogen peroxide, a by-product of the catalytic reaction of the enzyme on the analyte. Hydrogen peroxide is detected by the electrode and converted to an electrical signal. A reference electrode serves as an electrical reference point in an electrochemical device against which electrical potentials are measured and controlled. In one embodiment, silver-silver nitrate forms the reference electrodes. Other types of reference electrodes are mercury-mercurous chloride-potassium chloride or silver-silver chloride-potassium chloride. A counter electrode acts as a sink for the current path. As used herein, the term “sensor” is a device that responds to variations in the concentration of a given chemical species, such as glucose or lactate, in a sample, such as a body fluid sample. An electrochemical sensor is a sensor that operates based on an electrochemical principle and requires at least two electrodes. For ion-selective measurements, the two electrodes include an ion-selective electrode and a reference electrode. Amperometric enzyme electrodes additionally require a third electrode, a counter electrode. Moreover, enzyme sensors based on two electrodes, a working and reference electrode, are also common. As used herein, the term “ion selective electrode” generally refers to a silver wire coated with silver chloride in contact with a buffer solution containing a chloride concentration (the inner solution). The buffer solution is covered with a polymeric ion selective membrane that is in contact with the test solution. The ion selective membrane typically consists of a high molecular weight PVC, a plasticizer, an ionophore specific to a particular ion, and a borate salt. The surface of the polymeric membrane is in contact with the test sample on one side and the inner buffer solution on the other side of the membrane. As used herein, the term “dry electrochemical sensor” refers to the ion selective electrode, described above, and a reference electrode, described above. In the “dry chemical” embodiment, the ion-selective electrodes have the same configuration as described above, however, the inner solution containing chloride, is dried, i.e., dehydrated leaving a layer of dry salt. In order to function as an electrochemical sensor, the dried salt must be solubilized in water to obtain a buffer solution. As used herein, the term “enzyme electrode” generally refers to a composite membrane deposited on a metal electrode, comprising platinum for example. The composite membrane is at least three distinct layers including an outer polymeric membrane on the side of the composite membrane in contact with the sample that forms a protective layer, a middle enzyme layer that is located between the outer and inner layers, and an inner polymeric membrane closest to the metal electrode that forms the inner interference rejection membrane. The outer polymeric membrane, which is comprised of one or more polymeric compounds, generally functions to protect and maintain the structure of the middle enzyme layer and to control the diffusion of the analyte into the middle enzyme layer. The middle or enzyme layer comprises at least one protein species with enzymatic activity. The enzymatic activity may also be provided by compounds which include DNA, RNA, and carbohydrate, for example. The enzyme is stabilized in a matrix conducive to the activity of the enzyme. The inner or interference rejection membrane is a polymeric membrane that functions to insulate the wire electrode from compounds in the sample that interfere with the functioning and accuracy of the electrode. As used herein, the term “hydration” refers to the process of solubilizing the salts of a sensor's inner salt layer by the passage of water through the ion-selective outer polymeric membrane bounding one side of the inner salt layer, into the inner salt layer to form a solution. Hydration normally can be achieved by mere contact of the outside of the polymeric membrane and inner salt solution with an aqueous salt solution for a required duration. As used herein, “thermal cycling” is the process by which the temperature of an electrochemical sensor, soaked in an aqueous salt solution, is raised to a specified elevated temperature for a specified length of time, and then lowered. As used herein, the term “calibration” refers to the process by which the response characteristics of a sensor to a specific analyte are determined quantitatively. To calibrate a sensor, the sensor is exposed to at least two reagent samples, each reagent sample having a different, known concentration of an analyte. The responses, i.e., signals, measured by the sensor, relative to the concentrations of the analyte in the two different reagent samples, serve as reference points for measurements of the analyte in samples having unknown concentrations of the analyte. Referring to FIG. 1, the electrochemical sensor system 8 employs a sensor assembly, rally indicated at 10, incorporating a plurality of electrodes adapted to make electrical measurements on a sample; such as a blood sample, introduced to the sensor assembly 10. Blood samples to be analyzed by the system are introduced through a sample inlet 13a. Blood samples are obtained by, for example, phlebotomy or are derived on a periodic basis from an extracorporeal blood flow circuit connected to a patient during, for example, open heart surgery. Blood samples may be introduced into the sample inlet 13a through other automatic means, or manually, as by syringe. The blood samples may be introduced as discrete samples. The electrochemical system 8 including a number of essential components as heretofore described in a preferred embodiment of the present invention is contained in a disposable cartridge 37. A cartridge of a similar type is set forth in detail in U.S. Pat. No. 4,734,184, the entirety of the specification incorporated by reference herein. In one embodiment of the invention, the electrochemical sensor system 8 incorporates in the cartridge 37 at least two prepackaged containers 14, and 16, each containing a calibrating aqueous solution having known values of the parameters to be measured by the system. For purposes of reference, the solution contained within the prepackaged container 14 will be termed Calibrating Solution A, the solution contained within the prepackaged container 16 will be termed Calibrating Solution B. In another embodiment of the invention, the electrochemical system 8, illustrated in FIG. 1, includes a third prepackaged container 23 containing Calibrating Solution AO. Each of the prepackaged containers 14, 16 and 23 contain a sufficient quantity of its calibrating solution to allow the system to be calibrated a substantial number of times before the prepackaged container becomes empty. When one or more of the containers 14, 16 and 23 containing the calibrating solutions are empty, the cartridge containing prepackaged containers 14, 16 and 23 must be replaced. In a particular embodiment of the invention, the Calibrating Solution AO contains electropolymerizable monomer. Electropolymerizable monomer such as m-phenylenediamine may be included in the calibrating solutions at a concentration in a range of about 1 to 100 mM, preferably about 15 mM. In another embodiment of the invention, a solution of electropolymerizable monomers is contained in a prepackaged container (not shown) separate from the prepackaged containers 14 and 16 for the calibrated solutions at a concentration in a range of about 1 to 100 mM, preferably about 15 mM. Referring to FIG. 1, in one embodiment the prepackaged container 14 is connected to the input of a multi-position valve 18 through a flow line 20, and the prepackaged container 16 is connected to a second input of the multi-position valve 18 through a flow line 22. In yet another embodiment, the container 23 is connected to a third input of the multi-position valve 18 through a flow line 25. Another container 17 contains a rinse solution and is connected to the input of the multi-position valve 18 through a flow line 21. In yet another embodiment, the rinse bag 17 is eliminated and one of the calibration solutions A or B is used as a rinse solution, as well. The output line 12 is the output of the multi-position valve 18 and is connected to the sample input line 13 through a stylus 11. Depending upon the position of the valve 18, the input lines 20, 21, 22, 25 or air is open to the valve 18. Similarly, when the stylus is in a normal position (position 11b) of the sample input line 13b, line 12b is open to the sample input line 13b and allows passage of the calibrating, or rinse solution, or air through the sample input line 13b to the sensor assembly 10 through line 24, facilitated by the operation of a peristaltic pump schematically adiaz@example.org. However, in a sample accepting mode (13a), line 12 is separated from the sample input line (position 12a) and the sample is introduced directly to the sensor assembly 10 through line 24, facilitated by the operation of the peristaltic pump 26. The cartridge 37 also includes a container 28, for a reference solution. The container 28 is connected to the sensor assembly by a flow line 30. The system further includes a container 32 for waste, which receives the blood samples, the calibrating solutions and the reference solution after they have passed through the sensor assembly 10, via a flexible conduit 34 that has input from the sensor assembly 10. Both the waste flow conduit 34 and the reference solution flow line 30 consist of or include sections of flexible walled tubing that pass through the peristaltic pump 26. The pump 26 compresses and strokes the flexible sections of the flow lines 30 and 34 to induce a pressured flow of reference solution from the container 28 to the electrode assembly 10 and to create a negative pressure on the waste products in flow line 34 so as to draw fluids, including the fluids with the polymerizable monomers, in the flow line 24 through passages in the electrode assembly 10 past the membranes of the sensors. This arrangement, as opposed to the alternative of inducing positive pressure on the blood and calibrating solutions to force them through the electrode assembly 10, avoids the imposition of unnecessary and possibly traumatic mechanical forces on the blood sample and minimizes possibilities of leaks in the electrode assembly 10. Cartridge 37 also contains a sensor card 50 which provides a low volume, gas tight chamber in which the sample, such as a blood sample, calibration solution, or monomer-containing solution, is presented to one or more electrochemical sensors, i.e., the pH, pCO2, pO2, Na+, Ca++, glucose, lactate, creatine, creatinine and hematocrit sensors, together with the reference electrode collectively indicated as sensors 10, are integral parts of the chamber. Chemically sensitive, hydrophobic membranes typically formed from polymers, such as polyvinyl chloride, specific ionophores, and a suitable plasticizer, are permanently bonded to the chamber body. These chemically sensitive, hydrophobic membranes, described below in detail, are the interface between the sample or calibrating solutions and the buffer solution in contact with the inner (silver/silver chloride) electrode. In one embodiment of the invention, referring still to FIG. 1, included in the cartridge 37, are three solutions that allow for calibrations at high and low concentrations for all parameters except hematocrit, which calibrates at one level. In one embodiment, the cartridge 37 also includes the rotor-for-sample inlet arm 5, the pump tubing 24, 30 and 34, the sampling stylus 11, a waste bag 32, the reference solution container 28, the rinse solution container 17, calibration solution containers 14, 16 and 23, the check valve 33, and tubes 12, 20, 21, 22 and 25. Blood samples that have been analyzed are prevented from flowing back into the sensor card 50 from the waste container 32 due to the presence of a one-way check 33 valve in the waste line 34. After use in the system 8, the cartridge 37 is intended to be discarded and replaced by another cartridge. Referring to FIG. 1, sensors are available as a bank of electrodes 10 fabricated in a plastic card 50 and housed in the disposable cartridge 37 that interfaces with a thermal block assembly 39 of a suitably adapted blood chemistry analysis machine. The thermal block assembly 39 houses the heating/cooling devices such as a resistive element or a Peltier-effect device, a thermistor 41 to monitor and control the temperature, the electrical interface 38 between the sensors in the plastic card 50 and the microprocessor 40 through the analog board 45. The analog board 45 houses analog-to-digital and digital-to-analog converters. The signal from the electrode interface 38 passes through the analog-to-digital converter, converted into digital form for the processor 40 to store and display. Conversely, the digital signals from the processor 40, for example, the polarization voltage for oxygen sensor, go through the digital-to-analog converter, converted into an analog form and fed to the sensors for control, through the electrode interface 38. The electrochemical sensor system 8 is formed upon insertion of the cartridge 37 into the electrochemical sensor apparatus. Upon insertion, the sensor card 10 fits into the heater block assembly 39, described in detail below, and the heating/cooling assembly regulated by the microprocessor 40 cycles the temperature of the sensor card 50 and the solution in contact with the sensors inside the sensor card 50 through a specific temperature for a specified duration. The heater block assembly 39 is capable of rapid heating and cooling by, for example, a thermoelectric device applying, for example, the Peltier-effect, monitored by a thermistor 41, all controlled by the microprocessor 40. The sensors connect to the electrode interface 38 which select one of the plurality of electrical signals generated by the sensors and passes the electrical signal to the microprocessor 40 in the machine through an analog-to-digital converter into the analog board 45 where it is converted from analog to digital form, suitable for storage and display. Referring to FIG. 1, the electrode assembly 10 has a number of edge connectors 36 in a bank which allow it to be plugged into a female matching connector 38 so that the electrodes formed on the assembly 10 may be connected to microprocessor 40 through the analog board 45. The microprocessor 40 is connected to the multiport valve 18 via a valve driver 43 by a line 42 and to the motor of the peristaltic pump 26 via a pump driver 45 by a line 44. The microprocessor 40 controls the position of the sample arm 5 through arm driver 15, and the position of the valve 18 and the energization of the pump 26 to cause sequences of blood samples and calibrating solutions to be passed through the electrode assembly 10. When the calibrating solutions from, for example, containers 14, 16 and 23 are pumped into the electrode assembly 10, the electrodes forming part of the assembly make measurements of the parameters of the sample and the microprocessor 40 stores these electrical values. Based upon measurements made during the passage of the calibration solutions through the electrode assembly 10, and the known values of the measured parameters contained within the calibrating solution from containers 14, 16, and 23, the microprocessor 40 effectively creates a calibration curve for each of the measured parameters so that when a blood sample is passed through the electrode assembly 10 the measurements made by the electrodes can be used to derive accurate measurements of the parameters of interest. These parameters are stored and displayed by the microprocessor 40. The microprocessor 40 is suitably programmed to perform measurement, calculation, storage, and control functions such as differences in electrical potential across one or more electrodes. Calibrating Solutions In one embodiment of the invention a composition of calibrating solution A used for second point calibration, prepared at 37° C. and at atmospheric pressure tonometered with 9% CO2 14% O2 and 77% Helium gas, is as follows: pH 6.9 organic buffer; pCO2=63 mmHg; pO2=100 mmHg; Ne=100 mmol/L; K+=7 mmol/L; Ca++=2.5 mmon; glucose=150 mg/dL; lactate=4 mmol/L; creatine=0.5=mmol/L; creatinine=0.5 mmol/L; surfactant and inert preservative. In another embodiment of the invention a composition of calibration solution B used for one-point calibration and rinse, prepared at 37° C. acid at 700 mmHg absolute pressure tonometered with 27% O2, 5% CO2, and 68% Helium as, is as follows: pH 7.40 organic buffer; pCD2=34 mmHg; pO2=180 mmHg; Na+=140 mmol/L; K+=3.5 mmol/L; Ca+=1.0 mmol/L; surfactant and inert preservative. In yet another embodiment of the invention a preferred composition of calibration solution AO for low level oxygen calibration and in situ regeneration of the inner polymeric membrane for the enzyme sensors contains aqueous solution of Na+, K+, Ca++ salt; 15 mmol/L of m-phenylenediamine, 20 mmol/L of sulfite, surfactant and inert preservative; organic buffer, pCO2. The reference solution contains AgNO3=1 mmol/L; KNO3=1 mol/L; surfactant. The compositions of the A and B calibrating solutions are chosen so that for each of the characteristics measured by the system a pair of values are obtained that are spaced over the range of permissible values that are measured by the system, providing a balanced 2-point calibration for the instrument. The AO calibrating solution is chosen for low level oxygen calibration and regeneration of the inner polymeric membrane in the glucose, creatine, creatinine and lactate sensors. The A and B calibration compositions are prepared by premixing all of the constituents in a certain order starting with the buffer and ending with the sodium bicarbonate salt, then tonometering the solution with oxygen and CO2 mixed with helium to produce the desired level of pCO2 and pO2. The AO calibration solution is prepared with a slight difference in procedure. The salts with the exception of sodium sulfite, m-phenylenediamine and sodium bicarbonate are added to water and the solution is tonometered with helium to bring the pO2 to less that 30 mmHg. Then, the remaining salts are added to the solution and the final mixture is tonometered with mixture of pCO2 and helium to produce the desired pCO2 level. At least one electropolymerizable monomer is added to at least one of the calibrating solutions, solution AO in container 23 for example. The absence of dissolved oxygen in the AO solution, due to presense of sulfite ion, allows for a longer shelf life of electropolymerizable monomer in the AO solution because dissolved oxygen will oxidize the electropolymerizable monomer and thus render the monomer incapable of polymerizing. The electropolymerizable monomers m-phenylenediamine for example, may be included in a calibrating solution at a concentration in a range between about 1 to 100 mM, preferably 15 mM. The electropolymerizable monomer may be included in the cartridge 37 in a separate reservoir. The temperature and pressure at which the calibrating solutions are prepared and their method of packaging must be such as to preclude the possibility of dissolved gases going out of solution in the container, which would affect the concentration of gases in the calibrating solutions, and to minimize the tendency for gases to permeate through even the most impermeable materials practically obtainable. The calibration solutions are packaged with the solutions completely filling the containers, so that there is no head space, by evacuating the containers prior to filling in a manner which will be subsequently described. By filling the calibration solution into the evacuated flexible wall container 14, 16, 23 at elevated temperatures and subatmospheric pressure, the solution will not have any tendency at a lower use temperature to outgas and thus produce gas bubbles in the container. Were outgassing to occur, the concentrations of the gases in the solution would be affected, creating an inaccuracy in the calibration of the instruments. Similarly, the calibration solutions must not be packaged at too low a pressure i.e., not below about 625 mm of mercury, because the absorptive capacity of the solution for gases conceivably increases as the packaging pressure decreases and below that pressure value the absorptive capacity of the solution may be sufficiently high that it will tend to draw gases in through the slight inherent permeability of even the most gas impervious flexible packaging material, over long periods of time. Accordingly, a packaging pressure in the range of 625-700 mm of mercury is preferred. In one embodiment, a calibrating solution prepared at a temperature in excess of its intended use temperature so that at the lower temperature there is less tendency for outgassing of the dissolved gases. This cooperates with the reduced pressure packaging to minimize the possibility of outgassing. Calibration Solution A, B and AO are prepared at a temperature above its intended use temperature at a controlled pressure close to atmospheric pressure. Through use of elevated temperature (e,g., 37° C.) the solution may be prepared at about atmospheric pressure without any possibility of subsequent microbubbles within the container or gas transfer through the container when packaged in a zero head space flexible gas impervious container. The envelopes which form the calibration solution prepackaged containers 14, 16, 23 are formed, for example, of rectangular sheets, heatsealed at the edges and heatsealed at one corner to an inlet stem of the valve 18 which is used for filling purposes. In the preferred embodiment illustrated, the prepackaged containers 14, 16, and 23 and the prepackaged container lines 20, 22, and 25 are formed in a unitary cluster with the valve 18 so that gas phase dead space in the lines 20, 22, 25 is thereby avoided. In a preferred procedure for purging and filling the envelope bags, the envelope is first evacuated and then filled with the prepared solution. The bag is then shaken while the excess solution continually flows out of the bag. This process removes any residual gas bubbles from the bag. The solution is then sealed in the container. The calibration solutions in the prepackaged containers 14, 16, and 23 have excellent stability and a long shelf life. When at use temperature and atmospheric pressure there is no possibility of any outgassing from the liquid to form gas bubbles within the prepackaged containers 14, 16, and 23. Reference Solution The reference solution disposed in prepackaged container 28 is employed in the electrode assembly 10 as a supply source to a reference electrode to provide a liquid junction and thereby isolate the reference electrode from the varying electrochemical potential of the calibrating solution or the blood in a manner which will be subsequently described. In a preferred embodiment, the solution is 1 mol/L potassium nitrate and 1 mmol/L silver nitrate solution. The solution also contains a surfactant such as Brij 35. The solution is packaged in a sealed flexible container with no head space. Electrode Assembly Referring to FIG. 1, during operation of the pump 26, the electrode assembly 10 receives a constant pulsating flow of the reference solution via line 30 and sequential, intermittent pulsating flows of either the blood sample or one of the calibrating solutions via line 24. The assembly also provides a corresponding output of its waste products to a waste collection bag 32 via line 34. Referring to FIG. 2, by way of example, the electrode assembly 10 in a preferred embodiment consists of a structurally rigid rectangular card 50 of polyvinylchloride having a rectangular aluminum (or other suitable material) cover plate 52 adhered to one of its surfaces. Cover plate 52 closes off the flow channels 56 formed in one surface of the card 50 and also acts as a heat transfer medium for hydrating the sensors by thermal cycling, described below, and to maintain the fluids flowing through the electrode assembly 10, and the electrodes themselves, at a constant temperature during calibration and during measurement of relevant parameters in patient sample. This may be achieved by measuring the temperature of the plate 52 and employing a suitable heating or cooling element e.g., a Peltier-effect device and thermistor 41 to maintain the temperature of the plate 52 at a desired temperature. Referring to FIG. 2, a reference solution is introduced to a well 64, formed in the surface of the substrate 50 in the same manner as the other flow channels 56 and similarly covered by the metal plate 52. The reference solution flow line 30 passes through an inclined hole in the well 64. The well 64 is connected to the output section 34 of the flow channel 56 through a very thin capillary section 66 formed in the surface of the plastic substrate 50 in the same manner as the main flow channels 56. The capillary channel 66 is substantially shallower and narrower than the main flow channel 56; its cross section is approximately 0.5 sq. mm. Reference fluid pumped into the well 64 by the pump 26, via a line 30 (see also FIG. 1), fills the well, and is forced through the capillary section 66 where it joins the output stream of fluid passing through the main flow channel section 56 and then flows with it to the waste bag 32. The combined influence of its higher density described above and the capillarity of the flow channel 66 serves to minimize any possibility of calibrating solution or blood passing downward through the channel 66 to the well 64 and upsetting the electrochemical measurements. As a blood sample or calibration solution quantity introduced into the flow channel 24 passes through the flow channel 56 to the output section 34, it passes over a number of electrodes as illustrated in FIG. 2. Referring to FIGS. 1 and 2, the heat plate 52 abuts and forms one wall of the sample channel 56. The heat plate 52 is in contact with the Peltier-effect device of the thermal block assembly 39 described below. The thermal block assembly 39 is capable of changing and controlling the temperature of the heat plate 52 between 15° C. and 75° C. The temperature change and control is monitored by a thermistor 41 and regulated by the microprocessor 40. An internal digital clock of the microprocessor 40 controls time and can switch on and switch off the thermal block assembly 39 according to a preset program. Thus, microprocessor 40 controls the thermal block assembly 39, regulating the temperature setting and the duration of each set temperature of the heat plate 52. The Electrodes The order of assembly of the electrodes given below is only by way of example and is not intended to be limited to the order provided. The Hematocrit Electrode Pair Referring to FIG. 2, a pair of gold wires 98 and 100 form electrodes for determining the hematocrit (Hct) of a sample based on its conductivity. The wires make contact with printed circuit edge connectors 102 and 104, respectively, also illustrated in FIG. 5. The Oxygen Sensor Referring to FIG. 2, the next sensor in the flow channel 56 is the oxygen sensor 70 with a three electrode configuration, also illustrated in FIG. 4. The Potassiums Calcium and Sodium Ion Sensing Electrode Next up the flow channel is a sodium sensing electrode 78, followed by a calcium sensing electrode 86 and a potassium sensing electrode 90 including an active membrane and a staked silver wire and an associated edge connector. The pH Electrode Referring to FIG. 2, next along the flow channel 56 is a pH sensing electrode 94 also illustrated in FIG. 6 which includes a membrane 148 and a silver wire 87 staked or press-fitted through the thickness of the plastic 50 into the flow channel 56. Referring to FIG. 6, joined on the opposite side of the flow channel 56 is a pad printed conductor section 88 (also see FIG. 5) that forms an edge connector. The nature of this pH electrode will be subsequently described in detail. The Carbon Dioxide Electrode Referring to FIG. 2, the next electrode 93 along the flow channel 56 measures the dissolved carbon dioxide in the blood or calibrating solution and works in combination with the pH electrode 94. The Lactate Electrode Referring to FIG. 2, next along the flow channel 56, lactate electrode 92 functions by measuring by-products of an enzymatic reaction of lactate oxidase on lactate. The lactate oxidase present in the enzyme layer oxidizes the lactate producing hydrogen peroxide, which is detected by the electrode of the lactate sensor. The Glucose Electrode Referring to FIG. 2, a glucose electrode 91 is the next electrode, which like the lactate electrode 92 functions by the detection of hydrogen peroxide produced by an enzymatic reaction in the enzyme layer. The enzyme, glucose oxidase, specifically oxidizes glucose and produces hydrogen peroxide, a compound detected by the electrode of the glucose sensor. The Creatine and Creatinine Electrodes: Measurement of creatinine in a blood sample requires two electrodes. One electrode measures the total concentration of creatinine and creatine and the other electrode measures the concentration of only creatine. The concentration of creatinine is determined by subtraction of creatine from the combined creatine and creatinine concentrations. Referring to FIG. 2, the next two electrodes, creatinine 116 and creatine 118, which like the glucose electrode 91 and lactate electrode 92, function by detection of H2O2 produced by enzymatic reaction in their respective enzyme layers. In the creatinine electrode 116, the enzyme layer includes a mixture of three enzymes: creatininase, creatinase and sarcosine oxidase. This enzyme mixture specifically oxidizes creatinine and creatine and produces H2O2 in the following cascade reaction. In the creatine electrode 118, the enzyme layer includes a mixture of two enzymes: creatinase and sarcosine oxidase. This enzyme mixture specifically oxidizes only creatine and produces H2O2 in the following cascade reaction: The Ground The ground 105 illustrated in FIG. 2, is a silver wire inserted through the substrate 50. A ground serves as a common electric reference point for all electrodes. The ground may also serve as a counter electrode for the amperometric sensor system. The Reference Electrode As illustrated in FIG. 2, two silver wires 106 are staked through the thickness of the plastic substrate board 50 into the reference solution well 64 to act as the on-board reference electrode. Use of two silver wires 106 which are electrically connected assures continuous contact between the silver wire and the reference solution in the presence of air bubbles. Air bubbles may form in the reference channel as a result of degassing the reference solution at the elevated temperature of the sensor control. A printed circuit element 108, also illustrated in FIG. 5, extends along the
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