Selective electrodes commercial devices

Also, discussions of a number of applications of Nafion are not included in this document and are, at most, mentioned within the context of a particular study of fundamental properties. A number of these systems are simply proposed rather than in actual commercial applications. Membranes in fuel cells, electrochemical energy storage systems, chlor-alkali cells, water electrolyzers, Donnan dialysis cells, elec-trochromic devices, and sensors, including ion selective electrodes, and the use of these membranes as a strong acid catalyst can be found in the above-mentioned reviews. 

Development of lithium selective electrodes (LiSE) and their application in clinical chemistry have been amply reviewed Several models of lithium ion specific electrodes are commercially available. The central problems in developing such sensing devices are their dynamic range, the accuracy and precision by which the signals are correlated to the concentration of the analyte and the selectivity towards that species, especially in relation to other alkali metal cations. Additional problems of practical importance are the times of response and recovery and the durability of the electrode in the intended service. 

Commercial solid-state potential measuring devices based on the type of op-amp described are often called pH or plon meters and are designed to work with glass pH electrodes, ion selective electrodes, and other indicator electrodes described earlier. Research quality plon meters have built-in temperature measurement and compensation, autocalibration routines for a three-point (or more) calibration curve, recognition of electrodes (so you do not try measuring fluoride ion with your pH electrode ), and the ability to download data to computer data collection programs. The relative accuracy of pH measurements with such a meter is about +0.005 pH units. Meters are available as handheld... 

Ion-selective electrodes (ISEs) are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H concentration in a solution. Other parameters that can be measured include fluoride, bromide, nitrate, and cadmium, and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. ISEs do have their limitations including lack of selectivity and sensitivity and problems connected with conditioning of electrodes. Detection limits for nitrate-N, for example, are typically 0.098mgl for commercial field devices and have chloride as a major interferent. 

A wide variety of membrane electrodes are available from commercial sources that permit the rapid and selective determination of numerous cations and anitms by direct potentiometric mea.suremcnts. Often, membrane electrodes are called ion-selective electrodes because of the high selectKity of most of these devices. 

In view of all the difficulties associated with liquid junction potentials, it is desirable to construct cells without liquid junctions. In practice this is possible more often than one might think. All that is needed is for the sample solution to contain an ion with constant activity which does not interfere with the ion-selective indicator electrode used, and which can itself be specifically sensed with a second ion-selective electrode. This second ion-selective electrode can then function as a reference electrode, since it fulfills the primary requirement of a good reference electrode, i.e. it has a constant equilibrium Galvani potential at the interface electrode/solution. With such a set-up problems of a technical nature arise only if the resistance of the ion selective electrode connection with the reference electrode jack is too large. In this case an ion-meter with a high-ohmic differential input must be used. Such devices are already sold commercially (see Fig. 43). 

With the exception of the Corning-476200 acetylcholine bromide-selective liquid membrane electrode, there are no commercially available bio-sensors which use ion-selective electrodes to indicate biochemically important compounds (such as proteins, antigens, hormones, enzymes and their substrates). Nonetheless, there is a large number of such devices which can be easily self-made, so that it is appropriate to briefly discuss them here. For further details the reader is referred to the excellent review articles by Rechnitz [173] and Moody and Thomas [174]. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Many ion-selective interfaces have been studied, and several different types of electrodes have been marketed commercially. We will examine the basic strategies for introducing selectivity by considering a few of them here. The glass membrane is our starting point because it offers a fairly complete view of the fundamentals as well as the usual complications found in practical devices. 

Potentiometric detectors typically measure the potential difference (A ) across a membrane, which originates from the difference in analyte concentration in the eluent versus an internal reference solution. The most common potentiometric measuring device is a pH electrode, in which a glass membrane responds to hydronium ion concentration in the test solution. Other ion-selective, or indicator, electrodes are also available commercially. The attribute of an indicator electrode to be highly selective for a particular species is also its drawback, in that a different electrode is needed for each type of ion. Halides and sulfates can be monitored using silver/silver salt and lead/lead salt electrodes, respectively [50]. 

The most common RF ion trap is a Paul trap [42], a 3-D quadrupole device in which ions are confined in a small volume of typically a few tens of millimeters [2] between a hyqterbolically shaped inner surface of a ring electrode and two end-cap electrodes, also of hyperbolic shape (Fig. 1). Elach end-cap electrode has a central hole for loading and ejection of irais. As these traps are compact, commercially available, and allow mass-selection of stored ions, they have become an increasingly popular technically simple solution for cryogenic ion spectroscopy. Paul traps have several drawbacks for cold-ion spectroscopy, however inefficient ion injection an intrinsically limited ability to cool ions low storage volume and inconvenient optical access to the ions by laser beams. 

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