Severinghaus sensor

To detect CO2 in ambient air, the sensor is utilized as a novel design Severinghaus sensor. A sputtered thin film of iridium oxide is used as the pH sensing electrode. Iridium oxide, as well as several other metal oxides, has been shown to be pH sensitive [26], although the mechanism of this effect is not understood in detail. It is believed that the pH dependence is related to one of the redox couples of iridium oxide, such as... 

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. ... 

The determination of arterial blood gases is very important to monitor the gas exchange in critically ill patients. It provides valuable information about the patient s metabolism, gas exchange, ventilation, and acid-base homeostasis [2]. A reliable way for blood gas determination is the direct analysis of an arterial blood sample by point-of-care blood gas analyzers with implemented Clark and Severinghaus sensors to measure p02 and PCO2, respectively [3]. 

Figure 5 shows a scheme of a typical Severinghaus sensor for PCO2 measurements in blood. The major components of this sensor are a pH-sensitive combination glass electrode, a thin electrolyte layer (bicarbonate solution), and a polymer membrane permeable for CO2 [20]. If a flowing solution of blood sample passes the sample channel, CO2 permeates the membrane and dissolves in the thin electrolyte layer of bicarbonate solution. In this way the following equilibrium is influenced ... 

The constmction of the pH electrode can be seen as the main factor in terms of miniaturizing a Severinghaus sensor. It should be a small component of the overall sensor design exhibiting fast and reliable response characteristics. Known CO2 microsensors use pH electrodes with a liquid ion exchanger (LIX) membrane... 

Fig. 15 Left Calibration curve of a conventional Severinghaus sensor (SH) and a novel non-Severinghaus sensor measuring the potential difference between a pH glass electrode and a carbonate ISE as a function of log(pC02/Po) [46]. Reproduced from [46] with permission. Right Comparison of the response characteristics of both CO2 sensors - the SH and the new pC02 electrode arrangement. Measured in 0.1 M Tris-H2S04, pH 8.0 buffer solution for the following PCO2 (A) 0.0004 atm, (B) 0.0066 atm, (O 0.0655 atm [46]...

Vurek G.G., Feustel P.J., Severinghaus J.W., A fiber optic pC02 sensor, Ann. Biomed. 

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... 

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 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). 

Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

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... 

Figure 4-4 Schematic of Severinghaus style PCO sensor used to monitor CO2 levels in blood samples. (From S/ggard-Andersen O.Tfie add-base status of the blood. 4" ed. Eialtimore Williams Wilkins, 1974 172.)...

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... 

Amirkhanian V and Lee W I 1990 Mode mixing in fiber optic oximeter SPIE Optical Fibers in Medicine V vol 1201 (Bellingham, WA SPIE) pp 330-7 Lee WI and McCann B P 1993 Optical fibers for medical sensing SPIE Fiber Optic Sensors in Medical Diagnostics vol 1886 (Bellingham, WA SPIE) pp 138-46 Vurek G G, Fuestel P J and Severinghaus J W 1983 A fiber optic Pco sensor Ann. Biomed. Eng. 11 499-510... 

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