Carbon dioxide Severinghaus sensors

The original design was developed specifically for sensing of carbon dioxide (Severinghaus, 1965), but the principle on which these gas sensors operate is general. The key element is a small compartment into which the gas can penetrate through a semipermeable membrane (Fig. 6.28). 

One could immobilize the urease layer on top of a Severinghaus electrode for CO2 or NH3 (Section 6.3.2) and use the device as an enzymatic-potentiometric gas sensor. The primary disadvantage of such an arrangement would be its slow response time. A more direct way is through the detection of the ionic species resulting from the hydrolysis of ammonia and carbon dioxide. 

The Severinghaus-type carbon dioxide sensor measures CO2 concentration by monitoring the resultant pH change when ambient CO2 dissolves into an internal electrolyte to form bicarbonate ions ... 

In addition to slow diffusion of the CO2 gas through the gas-permeable membrane, conventional Severinghaus-type carbon dioxide sensors suffer from an additional rate limitation, namely slow reaction kinetics. The hydration reaction of CO2... 

In this section the basic design and operation of these types of sensors are described. Examples of such sensors in common use are the Clark oxygen electrode [10] and the Severinghaus carbon dioxide electrode [11]. 

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. 

The gas-sensing probe for carbon dioxide (CO2) is the original Severinghaus sensor. The dynamic range of this determination is rather limited and the limit of detection is 10 mol 1 as the concentration of atmospheric CO2 dissolved in water is 1-3 x lO moll. ... 

Figure 19.9 shows schematically the so-called Severinghaus carbon dioxide sensor (sometimes also called electrode) and illustrates its mode of operation. Main constituents of the sensor are a thin polymer membrane, the hydrogen carbonate containing electrolyte solution, a thin hydrophilic spacer sheet soaked with the electrolyte solution and a pH sensor. CO2 permeates from the gaseous or liquid specimen through the membrane into the electrolyte film in the spacer until equilibrium between the CO2 partial pressure on both sides of the membrane has been established. During measurement virtually no CO2 is consumed. 

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