Electrode drift problems

Miniaturized catheter-type ISE sensors, such as the implantable probe shown in Figure 5-20 represent the preferred approach for routine clinical in-vivo monitoring of blood electrolytes. For these intravascular measurements the reference electrode is placed outside die artery (in die external arm of die catheter), tints obviating biocompatability and drift problems associated with its direct contact with the blood. 

A novel microhotplate design was proposed to overcome the CMOS operating temperature limit and to avoid polysilicon-induced drift problems. A cross-sectional schematic of the device is shovm in Fig. 4.11. Instead of using a polysilicon resistor as temperature sensor, a platinum temperature sensor is patterned on the microhotplate. The Pt-metallization process step was used to simultaneously fabricate the electrodes and the temperature sensor. The CMOS-Al/Pt contacts are located off the membrane... 

While there have been several reports on the use of the above type catheters (see Section 4.4.1.), in vivo measurements with ISE-type devices are not yet routine. Problems regarding electrode drift when in prolonged contact with whole blood, the effect of blood clots, the use of appropriate... 

The causes of electrode drift, e.g., unstable liquid junction, temperature gradients, high impedance, non-equilibrium system, electrode malfunction, etc., are numerous and varied. In general, replacement of one or both of the electrodes will remedy the problem, otherwise, sample instability or interfering sample constituents should be considered as a possible source of the difficulty. 

Back corona is caused by the electrical breakdown of gas in the dust layer. This breakdown produces positive ions, which drift toward the negative discharge electrode. The presence of ions with opposite polarity causes a reduction in the particle-charge and -collection efficiency. To avoid this problem, several methods are used. These include... 

Solutions in a conductance cell are often stirred to hasten salt dissolution, to promote solution mixing, or to prevent temperature gradients. Some workers observe an upward drift in measured resistances of unstirred solutions 12-17) while others report a downward drift unless the unstirred solution is mixed by shaking of the cell immediately before the measurement9-18-26>. The magnitude of this change is often 0.1 % or more. The effect has not been observed in other cases 8>10). The source of this problem has been variously attributed to temperature variations, electrode adsorption effects and solvent impurities, although the problem has not been analyzed in detail. In all but one of the above cases 12> the resistance of the stirred solution was taken as the true value. 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

Common problems for pH measurements include potential or temperature drift, breakage of the electrode when it is handled, sluggish pH response, low value for the slope, and so on. Most pH meter manufacturers have provided quite intensive troubleshooting instructions in their instrument manual. Table 15.2 is a summary of some common solutions. 

The fluoride ion selective electrode is the most popular means of fluoride ion determination after sample destruction by any method but it does have limitations. It can be used either directly to measure the fluoride potential6 or as an end-point detector in a potentiometric titration with a lanthanum(l II) reagent as titrant.4,7 Problems can be experienced with potential drift in direct potentiometry, especially at low fluoride ion concentrations. Titration methods often yield sluggish end points unless water miscible solvents are used to decrease solubilities and increase potentia 1 breaks and sulfate and phosphate can interfere. End-point determination can be facilitated by using a computerized Gran plotting procedure.4... 

Unfortunately, due to technical difficulties, reference electrodes have not been widely employed in fuel cell measurements. The primary cause is the geometric restriction imposed by the thin solid electrolyte (for example, a typical electrolyte thickness is about 50, um for a Nation 112 membrane). Additional factors such as the shape and position of the reference electrode must be taken into account in order to obtain reliable and meaningful results. In reality, the requirements for a reference electrode are less stringent than expected, since the main problem is the drift of the reference potential during measurement, which can result in large error due to the strong potential dependence of the impedance. 

The second problem concerns an understanding of the sharing of transport duties (e.g., the carrying of current) in pure liquid electrolytes. In aqueous solutions, it was possible to comprehend the relative movements of ions in the sense that one ionic species could drift under an electric field with greater agility and therefore transport more electricity than the other until a concentration gradient was set up and the resulting diffusion flux equalized the movements when the electrodes were reached. In fused salts, this comprehension of the transport situation is less easy to acquire. At first, it is even difficult to see how one can retain the concept of transport numbers at all when there is no reference medium (such as the water in aqueous solutions) in which ions can drift. 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

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