The invention set forth in this specification pertains to new and improved disposable, electrochemical sensing devices. More specifically it pertains to sensing devices or sensors which are used in conjunction with appropriate electronic apparatus in order to determine the content of certain specific ions or gases in various different fluids.
In many different medical, ecological and industrial circumstances it is necessary or desirable to determine whether or not a specific ion, ions of a group of closely related ions, a gas or gases of a group of gases of a closely related character are or are not present within a fluid and, if such an ion, gas or group of either is present, the amount of the latter which is present in the fluid. Thus, for example, in the medical field it is often necessary to determine the potassium, oxygen or other content of blood. Similarly, in monitoring various industrial processes it is frequently necessary to determine if a specific ion or gas is present in a solution or in a specific atmosphere such as ambient air, and, if it is, the quantity of the same which is present. Such determinations are needed and used in so many different areas of technology that it would be extremely time consuming to attempt to even suggest them all in this document.
Although determinations as are indicated in the preceding paragraph can be made in different manners in many cases whenever it is reasonably possible it is highly preferable to make such determinations electrochemicaly using ion or gas selective electrodes. The reasons for this are considered to be primarily related to the ease with which electrochemical measurements can be made, the comparatively low costs involved in making such measurements, the relative reliability of electrochemical analysis, and the fact that normally only comparatively limited equipment is required in order to make measurements to determine if an ion, a gas or a group of related ions or gases is present in a fluid, and, if so, the quantity of the latter present in the fluid.
Such equipment is closely related to that used in electrochemically making common pH measurements. It normally consists of a device having two separate electrochemical half cells, means for connecting the half cells by a sample of the fluid which is to be analyzed with the device and of an item of electronic equipment for making measurements in connection with the half cells and for indicating the results of such measurements. Although the physical structures of the cells used in a specific apparatus usually vary normally both cells used include a housing having an internal cavity, an electrode extending into the cavity and an electrolyte in the cavity in communication with the electrode. A half cell as described normally includes a flow control means such as a barrier, membrane or other structure for restricting flow between the electrolyte in the half cell and the means connecting it with the other half cell with which it is used.
The flow control means in one of the half cells is always of such a character as to permit essentially only movement of the ion or ions to be measured in or through such means or to permit a specific reaction or reactions with or absorption of the gas or gases to be measured in or at the surface of such means. Such a half cell is commonly referred to as a "sensing" cell or half cell. In a device employing two half cells as described the other or non-sensing half cell used normally employs a different flow control means than the sensing half cell. This latter half cell is utilized to provide a reference measurement for use in analyzing a corresponding measurement obtained using the sensing cell. Because of this the second cell in a device as described may be referred to as a "reference" cell or half cell.
As a result of its function the flow control means used as a part of the sensing half cell is quite critical to electrochemical sensing devices of the type to which this invention pertains. Ion or gas determinations using an electrochemical sensing device as indicated in the preceding discussion can be made only when an appropriate flow control device having a character as indicated in the preceding paragraph is available. As a consequence of this the flow control structure in a sensing half cell is restricted to an available membrane or barrier which is desirable in making a desired analysis. Fortunately advances in technology have resulted in and are resulting in membranes and barriers which are useful for an apparently ever increasing series of electrochemical determinations or measurements.
For some measurements it is considered necessary or preferable to use solid state membranes or barriers. The latter have been bodies of single crystals; they have also been bodies formed of particles such as small crystalline or other particles by such techniques as pressing such particles into a single, unitary mass. In some cases such particles have been physically held together with and/or within an inert or essentially inert binder. These membranes or barriers have also been prepared by forming polymer gels including one or more polymers and a fluid having desired ion selective or gas reactive or absorptive properties. It is common for the electrolyte in such half cell to contain the same ion or ions which are present in the membrane or barrier.
Fortunately an understanding of the invention does not require a detailed consideration of all of the different membranes or barriers capable of being used in devices in accordance with the invention. Neither does it require a detailed consideration of all of the diverse constructional details of known sensing and reference half cells of cells and of the various electrolytes employed in such cells in making determinations such as can be made using the devices of the present invention. However, an understanding of the reasons as to why the present invention is needed does require a brief, further discussion of prior devices which are related to the devices of the present invention.
Such prior devices are not considered to be completely satisfactory in meeting present and contemplated future needs for electrochemical sensing devices or sensors for one or more of a series of different reasons. Because of the diversity of such prior devices it is impractical to attempt to indicate all of the limitations and disadvantages of them in this document. Frequently such prior devices or sensors have been relatively complex and as a result of this undesirably expensive. Often such prior devices are comparatively difficult to use.