Electrodes as detectors

The GECE sensors were used for lead determination in real water samples suspected to be contaminated with lead obtained from water suppliers. The same samples were previously measured by three other methods a potentiometric FIA system with a lead ion-selective-electrode as detector (Pb-ISE) graphite furnace atomic absorption spectrophotometry (AAS) inductively coupled plasma spectroscopy (ICP). The results obtained for lead determination are presented in Table 7.1. The accumulation times are given for each measured sample in the case of DPASV. Calibration plots were used to determine the lead concentration. GEC electrode results were compared with each of the above methods by using paired -Test. The results obtained show that the differences between the results of GECE compared to other methods were not significant. The improvement of the reproducibility of the methods is one of the most important issues in the future research of these materials. 

A few other types of anion sensors have been mentioned recently in the literature. Tetrathiofulvalene microcrystals immobilised at a platinum electrode displayed electrochemical properties that were affected by the presence of anions in solution, with some selectivity for anions such as bromide [ 145]. A flow-injection analysis system using anion-exchange columns for separation and polyaniline electrodes as detectors could detect dichromate down to 0.004 ppb and could be used for seawater samples [146]. 

Limoges, B. Degrand, C. Brossier, P. Blankespoor, R.L. Homogeneous electrochemical immunoassay using a per-fluorosulfonated ionomer-modified electrode as detector for a cationic-labeled hapten. Anal. Chem. 1993, 65, 1054-1060. 

UE Spichiger, A Fakler. Potentiometric microelectrodes as sensors and detectors. Magnesium-selective electrodes as sensors, and Hofmeister electrodes as detectors for histamine in capillary electrophoresis. Electrochim Acta 42 3137— 3145, 1997. 

J. F. Coetzee and C. W. Gardner, Jr., Determination of Sulfate, Orthophosphate, and Triphosphate Ions by Flow Injection Analysis with the Lead Ion Selective Electrode as Detector. Anal. Chem., 58 (1986) 608. 

H. Muller and V. Muller, An FIA System Using an Ion-Selective Electrode as Detector for the Catalimetric Determination of Molybdenum(VI) and Tungsten(VI). Anal. Chim. Acta, 180 (1986) 30. 

Figure 1. Schematic of static (A) and continuous flow (B) gas sensing system using solvent/polymeric ion-selective membrane electrodes as detectors.

Determination of IgG has been performed in a sandwich type Immunoassay using the recycling electrode as detector and both alkaline phosphatase and B-galactosidase as enzyme label. 

Coetzee, J.F. and Gardner, C.W., Determination of sulphate, ortho-phosphate, and triphosphate ions by flow injection analysis with the lead ion selective electrode as detector. Analytical Chemistry 58, 608,1986. 

In on-colnmn detectors, the working electrode is actually inserted into the tube used to make the column as illustrated in Figure IB. (Technically, these are not truly on-column detectors unless the stationary phase actually extends to the end of the tube however, for this discussion, on-column wiU refer to the case of the electrode inserted into the same tube used for the column.) The first on-column electrochemical detector was a 1 pm tip potentiometric, ion-selective electrode inserted into the ontlet of a 25 pm i.d. OTLC column [15]. The detection cell volume of 20 fL was proportional to the surface area of the electrode and the length of the stagnant diffusion layer from the electrode surface. Unlike the end-column detector, experiments demonstrated that the on-column detector did not cause a measurable loss in theorical plates. Little follow-up on use of micro-potentiometric electrodes as detectors has been reported. 

One of the limitations of using electrodes as detectors is that many electroactive substances do not react at the electrode surface easily, causing variations in response with time. This problem can be overcome by utilizing highly reversibly electroactive substances (known as mediators) as intermediates between the analyte and electrode. Some common mediators are ferrocene, phenazine methosulfate, and benzylviologen. Since different organic materials have different potentials where they react in the electrode, is necessary to choose a mediator that has an oxidation/reduction potential that is near the oxida-tion/reduction potential of the analyte. 

C Reticulated vitreous carbon flow-through electrode as detector No data 24 1-2 Pharmaceuticals 1979 153 ... 

Cano, M., B. Palenzuela,]. L. Avila, and R. Rodriguez-Amaro. 2007. Simultaneous determination of ascorbic acid and uric acid by using a PVC/TTF-TCNQ composite electrode as detector in a FIA system. Electroanalysis 19 973-977. 

The diamond electrode has proven useful in overcoming the limitations of conventional carbon electrodes as detectors for CE. Based on the attractive performance of BDD electrodes, one may expect more diverse analytical applications, including field-deployable diamond-based microchip detection systems for on-site bioanalysis and environmental monitoring. 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

Due to their response mechanism the polyion-selective electrodes are not sensitive to the small fragments of polyionic macromolecules. Thus, if an enzyme cleaves the polyionic molecule these sensors can be used for detection of enzyme activity. Polycation protamine is rich in arginine residues that make it a suitable substrate for protease-sensitive electrochemical assays. Real-time detection of trypsine activity was demonstrated with the protamine-selective electrode as a detector [38],... 

A disadvantage of the described reference electrode is its maintenance porous frits may dry out, Cl solution needs refreshment from time to time. Therefore the Hewlett-Packard HP 1049A Electrochemical Detector cell contains a solid state, in situ Ag/AgCl reference electrode as a standard with the refillable electrode described above as an option. 

Fig. 18b.1. Electrochemical cells and representative cell configurations, (a) Schematic diagram of a cell-potentiostat system, (b) Typical laboratory cell with Hg-drop electrode and drop knocker, (c) Voltammetric cell as detector at the end of a high-performance liquid chromatographic column, (d) A two-electrode (graphite) chip cell for biosensor development, (e) Three-electrode chip cells on a ceramic substrate for bioanalytical work.

Electrochemical detection is based on measurement of the current resulting from oxidation/reduction reactions of analytes at a suitable electrode as a function of the applied voltage. Since the level of the current is directly proportional to the analyte concentration, these detectors are used for both qualitative and quantitative purposes (127). 

F. Mizutani, Y. Hirata, S. Yabuki and S. Iijima, Flow injection analysis of acetic acid in food samples by using trienzyme/poly(dimethylsiloxane)-bilayer membrane-based electrode as the detector, Sens. Actuators B Chem., 91(1-3) (2003) 195-198. 

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