Direct ion-selective electrodes

Few trace element species can be analysed in natural samples directly. Ion selective electrodes (ISE) allow measurements of metal ion activity (Cu2+, Cd2+) however, their use in the marine environment is limited due to low sensitivity and interference by Cl-. 

This phenomenon is explained by the fact that intravenous immunoglobulin increases the non-aqueous phase of the plasma, resulting in a relative loss of plasma water volume. Sodium is virtually restricted to serum water, so each volume of plasma measured will contain less sodium and be interpreted as hyponatremia. Using a direct ion-selective electrode avoids this problem. 

ISE(direct) ion-selective electrode without sample dilution... 

Figure 27-1 Predicted Influence of water content on sodium measurements for a lOOmmol/L NaCi solution by direct ion-selective electrode (tSE versus flame emission photometry or indirect ISE). Hatched areas represent nonaqueous volumes, which could consist of lipids, proteins, or even a slurry of latex or sand particles. (From Apple FS, Koch DD, Graves S, Ladenson JH. Relationship between d/rect-potent/ometric and flame-photometric measurement of sodium in blood. Clin Chem 1982 28 1931-5.)...

Directions are provided for constructing and characterizing an ammonium ion-selective electrode. The electrode is then modified to respond to urea by adding a few milligrams of urease and covering with a section of dialysis membrane. Directions for determining urea in serum also are provided. 

The direct deterrnination of fluoride using ion-selective electrodes has allowed analysis of fluorspar without the tedious distillation step (see... 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

The other method is less accurate but more rapid and involves direct Nessleri2ation of the sample for colorimetric deterrnination. Other colorimetric indicators with more sensitivity, such as indophenol, have been used in place of Nessler s reagent. Ion-selective electrodes have also found use in analysis for trace ammonia (93). 

Individual polyethers exhibit varying specificities for cations. Some polyethers have found appHcation as components in ion-selective electrodes for use in clinical medicine or in laboratory studies involving transport studies or measurement of transmembrane electrical potential (4). The methyl ester of monensin [28636-21 -7] i2ls been incorporated into a membrane sHde assembly used for the assay of semm sodium (see Biosensors) (5). Studies directed toward the design of a lithium selective electrode resulted in the synthesis of a derivative of monensin lactone that is highly specific for lithium (6). 

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

Many double-charged anions, such as sulfate, hydrophosphate, oxalate etc., are highly widespread in natural sources and at the same time lack any convenient technique for their determination. Therefore, development of ion-selective electrodes (ISEs), responsive to these anions, is of great practical importance. However, for a long time all attempts directed toward creation of such electrodes were unsuccessful (except for carbonate ISEs based on trifluoroacetylbenzene derivatives), and only in recent years this field has shown significant progress. 

Drawbacks of ion-selective electrodes ° Selectivity not always sufficient ° Direct read-out is less accurate with higher valent ions EMF-Drift may require frequent standardization... 

Fields of Application for ion-selective electrodes o Routine-analytical work in the laboratory (direct or as end-point indicators application frequency in industry 30%) o Clinical analyzers for Na+, K.+, Ca2+, pH, pC02, etc. o Process analyzers... 

Direct-reading meters suitable for use with ion-selective electrodes are available from a number of manufacturers they are sometimes referred to as ion activity meters. They are very similar in construction to pH meters, and most can in fact be used as a pH meter, but by virtue of the extended range of measurements for which they must be used (anions as well as cations, and doubly charged as well as singly charged ions), the circuitry is necessarily more complex and scale expansion facilities are included. They are commonly used in the millivolt mode. 

An example of a modem instrument of this type is the Coming Model 410 flame photometer. This model can incorporate a lineariser module which provides a direct concentration read-out for a range of clinical specimens. Flame photometers are still widely used especially for the determination of alkali metals in body fluids, but are now being replaced in clinical laboratories by ion-selective electrode procedures (see Section 15.7). 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

Although the speciation of some minor elements has been determined directly by experimental means (e.g., ion selective electrodes, polarography, electron spin resonance) most of our thinking about speciation is based on equilibrium calculations. Garrels and Thompson... 

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

Calcium ion-selective electrodes have recently been commercialized for the measurement of either total or ionized calcium Approximately 45 % of the calcium present in serum is bound to proteins, 5% is complexed to simple anions and 50% exists as the free ion. Traditionally, total calcium measurements have been made by releasing the protein bound fraction. An ion-selective electrode has now allowed the free (ionized) calcium to be measured directly. There has been much debate on the clinical significance of these measurements. The dependence of ionized calcium on pH must be considered. Samples must be either treated anaerobically, tonometered to a constant pH or have a correction factor applied. 

Ion-selective electrodes allow the measurement of ionic activity in diluted or undiluted whole blood, plasma or rum. The direct (undiluted) measurement may be preferred, since no sample pretreatment is necessary and the assay values are independent of hematocrit and amount of solids present. However, direct potentiometry by its very nature does not provide total concentration values similar to those obtained by flame photometry and indirect (diluted) potentiometry... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications as health disorders are usually correlated to ion activity. While most ISEs are used in vitro, the possibility to perform measurements in vivo and continuously with implanted sensors could arm a physician with a valuable diagnostic tool. In-vivo detection is still a challenge, as sensors must meet two strict requirements first, minimally perturb the in-vivo environment, which could be problematic due to injuries and inflammation often created by an implanted sensor and also due to leaching of sensing materials second, the sensor must not be susceptible to this environment, and effects of protein adsorption, cell adhesion, and extraction of lipophilic species on a sensor response must be diminished [13], Nevertheless, direct electrolyte measurements in situ in rabbit muscles and in a porcine beating heart were successfully performed with microfabricated sensor arrays [18],... 

This field is therefore at an exciting stage. Ion-selective electrodes have a proven track record in terms of clinical and biomedical analysis, with a well-developed theory and a solid history of fundamental research and practical applications. With novel directions in achieving extremely low detection limits and instrumental control of the ion extraction process this field has the opportunity to give rise to many new bioana-lytical measurement tools that may be truly useful in practical chemical analysis. 

Calcium can be directly determined with ion selective electrodes [74]. Especially for cations there are different ion selective materials available for fast and selective determination. 

Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

The techniques developed in enzyme immobilization have facilitated the development of enzyme electrodes and of novel enzyme -based, automated, analytical methods (l6,17,l8). Enzyme electrodes have resulted from the combination of an enzyme membrane and an ion-selective electrode they were used successfully to assay directly appropriate substrates. Enzyme columns or enzyme tubes, prepared in a conventional manner, were used as a specific auxiliary component in the indirect assay of substrates in many of the novel automated analytical procedures. 

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