Electrodes Severinghaus membrane

Carbon dioxide devices were originally developed by Severinghaus and Bradley (59) to measure the partial pressure of carbon dioxide in blood. This electrode, still in use today (in various automated systems for blood gas analysis), consists of an ordinary glass pH electrode covered by a carbon dioxide membrane, usually silicone, with an electrolyte (sodium bicarbonate-sodium chloride) solution entrapped between them (Figure 6-17). When carbon dioxide from the outer sample diffuses through the semipermeable membrane, it lowers the pH of the inner solution ... 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

Severinghaus electrodes have found wide application in clinical analysis. It is pertinent to mention here that the general principle of permeation of the gas through a hydrophobic membrane followed by its detection (with or without its solvolysis) has been used with different types of internal sensors, for example, optical, ampero-metric, conductimetric, or a mass sensor. The choice of the internal sensing element depends on the circumstances of the application in which the gas sensor would be used, such as the required time response, selectivity considerations, complexity of instrumentation, and so on. 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

The theory of operation of the conductometric gas membrane sensor has been experimentally verified in detail for CO2 and SO2, and sensors for H2S and NH3 based on the same principle have also been made. The basic transport and equilibration processes are the same as in the Severinghaus electrode (Section 6.2.2). Upon entering the aqueous solution inside the cell, the gas dissociates to its constituent ions. Because each dissociated species contributes to the overall conductivity, the specific conductance A of the cell is... 

These both depend upon the use of membranes permeable to the gas but not to ions. The Severinghaus COa electrode (S5) uses a glass elec-... 

For measurements of partial pressures in tissues and in liquids such as serum or blood, sensors according to Severinghaus are used. A membrane separates the external liquid or gas phase from the electrolyte surrounding a glass electrode, the potential of which is measured against an Ag/AgCl electrode (see Sections 23.3 and 23.4). 

In 1954 Leland Clark demonstrated that a platinum cathode would measure the oxygen concentration of blood when it and a reference electrode were covered by an oxygen permeable membrane. Later in that same year Stow and Severinghaus showed that carbon dioxide could be estimated in blood with a glass electrode fitted with a gas permeable membrane. In the seventies the Huchs demonstrated that mechanical adaptations of these devices could be utilized to provide transcutaneous (non-invasive) measurement of arterial blood gas concentration if the skin area surrounding the sensor was heated to 44 - 45°C. 

Gas sensors and biosensors are obtained by fixing an auxiliary chemical or biochemical system over the ISE membrane. The analyte reacts with the auxiliary system with production or consumption of the ion that is sensed by the ISE. Two basic types of gas sensor, the Severinghaus electrode and the air-gap electrode, are described elsewhere in this encyclopedia. 

Compact gas probes In compact gas probes (combination electrodes), the ion-selective sensor is covered with a gas-permeable membrane enclosing a thin film of electrolyte containing the ion sensed (the Severinghaus-type probe. Figure 1). A gas from the test medium passes through the membrane, dissolves in the electrolyte film, and changes the activity of the ion sensed. The type of ion-selective sensor. 

Hgure 1 A simplified scheme of the CO2 membrane gas probe (Severinghaus type). 1, Detector body 2, indicator electrode (pH glass electrode) 3, reference electrode 4, internal electrolyte 5, gas-permeable membrane 6, medium analyzed and 7, voltmeter (pH-meter). (The components of the probe are not drawn to a real scale.)... 

Fig. 6 Schematic of typical potentiometric gas-sensing probe. Inner electrode is a glass membrane pH sensor for a conventional Severinghaus style device. Other ISEs can be used in a similar geometric arrangement to detect ionic forms of gases within the thin layer of the inner filling electrolyte solution at the distal end of the probe.

The oldest clinical appKcations of po-tentiometry involved blood gas analysis, and this method is still used to quantitate dissolved carbon dioxide a glass pH electrode is housed behind a membrane permeable to CO2, and a thin solution layer of hydrogen carbonate separates the pH electrode from the membrane. This device, called the Stow-Severinghaus electrode, remains unchanged in principle from its original version reported in 1958 [18]. The measurement is based on the hydrolysis of CO2 to HC03 andH+ in the thin solution... 

To date, the majority of enzyme-based potentiometric sensors do not involve detection with an ionophore-doped selective membrane and fall outside of the scope of this chapter. The same is also true for most Severinghaus-type gas sensors, where a gas-permeable membrane covers an inner solution in which the gaseous analyte is determined with an ISE. Most Severinghaus-type electrodes use a pH-sensitive glass electrode to monitor the pH of this inner filling solution. However, ammonia has been detected indirectly with an ammonium-selective ionophore-based ISEs upon protonation in that inner solution, and the use of other ionophore-based ISEs for the more selective detection both in enzyme-based and Severinghaus-type ISEs is readily conceivable. 

A NOx gas sensor constructed from a molecularly-imprinted nitrate-selective electrode has also been developed in our laboratory (30). This gas sensor is produced by placing the nitrate-selective electrode behind a gas-permeable membrane. Also, there is a buffer compartment present between the gas-permeable membrane and the ISE. As before, NOx species that pass through the gas-permeable membrane are trapped in the buffer compartment as NO2 and NOs . In this case, the electrode responds proportionately to the amount of nitrate in the buffer, rather than nitrite as in the NOx sensor described above. The gas sensor based on the N03 -selective electrode offers advantages over the Severinghaus arrangement similarly to the other aforementioned gas sensors (SO2 and NOx). Detection limits for this gas sensor were on the order of 1x10 M, with response characteristics being retained for over 80 days. This lifetime is consistent with lifetimes of the molecularly-imprinted nitrate-selective electrodes. 

As noted by Severinghaus, much of the instability of the Clark oxygen electrode is associated with gradual cold flow and loosening of its plastic membrane, which initially is tightly stretched over the cathode tip. With its membrane loose, the electrode responds slowly, shows increased current, and becomes sensitive to minor differences in hydrostatic pressure. Even in its... 

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