Selectivity measurements, coated-wire

Among other factors its performance depends on the -> membrane properties. Its potential is equal to the potential across the membrane. It can be measured with high - input impedance voltmeters versus an outer -> reference electrode present in an outer - electrolyte solution. In some cases - inner reference electrode and inner electrolyte solution can be eliminated, see for example -> coated-wire electrodes. This term is very often used as synonym of - ion-selective electrode. 

Fig.l ISEs (a) The solid membrane (b) liquid membrane (c) coated-wire electrode (d) field-effect transistor. The analyte solution is in contact with the ion-selective layers potentials for A-C are measured using an external reference electrode, while the drain current, 7,, is monitored using a current-to-voltage converter in D. 

Compared with separation-based techniques, potentiome-try is a difficult technique to use to detect multiple analytes because of the selectivity of the ion-selective electrode. Because of the influence of the separation voltage, it is difficult to measure the response of the electrode potential, which is correlated logarithmically to the concentration of analyte it has not yet been employed with microchip CE. Potentiometric detection has found application in conventional CE. Kappes and Hauser have fabricated a universal potentiometric sensor for CE by coating wire electrodes with a solid PVC membrane [10]. The sensor was of approximately the same diameter as the outside of the capillary and located at a distance of about 50 p.m from the capillary outlet. A reference electrode was located beside the detection electrode. These coated-wire electrodes with PVC membranes have been employed to determine alkali and alkaline earth metals, small inorganic anions, and lipophilic organic ions such... 

A potentiometric determination of saccharin was proposed by Fatibello-Filho et al. [86]. In this method, saccharin was potentiometrically measured using a silver wire coated with a mercury film as the working electrode. With this, the main difficulty was the presence of a precipitate (mercurous saccharinate) that could adsorb on tube walls and the electrode surface. To avoid these undesirable effects, a relocatable filter unit was placed before the flow-through potentiometric cell and a surfactant was added to the carrier solution (Figure 24.12). The same investigation team reported the construction and analytical evaluation of a tubular ion-selective electrode coated with an ion pair formed between saccharinate anion and toluidine blue O cation incorporated on a poly(vinyl chloride) matrix [87]. This electrode was constructed and adapted in a FIA system. The optimum experimental conditions found were an analytical path of 120 cm, an injection sample volume of 500 pL, a pH of 2.5, a flow rate of 2.3 mL/min, and a tubular electrode length of 2.5 cm. 

In the interest of avoiding potential drift, these ion-selective coated wire electrodes should be conditioned in a solution about 0.1 M in the measured ion for about 15 minutes prior to the first use. Between measurements they can be kept in air, and after longer periods of non-use condition again in a 0.1 M solution for about 5 minutes. 

Ion-selective electrodes responsive to anionic detergents159 consist of a tip of a 0.7-mm-thick Pt wire, fused at the end to make a ball ca. 1.5 mm in diameter and dipped several times in a coating mixture after an initial conditioning by soaking it for 30 min in a 10 4 M solution of the anion to be measured, it is... 

Finally, we mention a novel transduction concept based on the heal evolved from a reaction such as combustion. Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (were evolved heat can be measured) was coated with a thin film of CoA1P04-5, the other was kept open as a reference 135] The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with a meandering Pt-wire resistor. This device was examined in the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm-range was observed. 

Fig. 32. Field emission microscope for adsorption studies. A—gas bottle B—break off seal C—inverted ionization gauge (also serves as selective getter) D—Granville-Phillips valve E—ionization gauge F—grounding rings G—double Dewar H—emitter assembly (tip mounted on hairpin support wire, equipped with potential leads for measuring resistance) I—anode terminal J—willemite screen settled onto tin-oxide conductive coating K—ground glass port L—trap.

Chromosorb. The coated support was packed in stainless steel columns (0.5 cm id, about 0.8 m long) and served as the stationary phase in IGC. The stationary phases were contacted with selected vapours at very high dilution and described procedures were followed [6, 11] to measure the net retention volume, V , of the vapours probing the deposited solids. A Perkin-Elmer Sigma-2 chromatograph with hot wire detector was used. All determinations were in the range 30 - 60 °C, and values of V were measured in at least triplicate, with a reproducibility better than 4%. 

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