Electrochemical detectors potentiometric

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (42). In this case, the content of the immunoassay reaction vessel is injected to an appropriate flow system containing an electrochemical detector, or the electrode can be inserted into the reaction vessel. Remarkably low (femtomolar) detection limits have been reported in connection with the use of the alkaline phosphatase label (43,44). This enzyme catalyzes the hydrolysis of phosphate esters to liberate easily oxidizable phenolic products. 

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

Electrochemical detectors measure the current resulting from the application of a potential (voltage) across electrodes in a flow cell. They respond to substances that are either oxidizable or reducible and may be used for the detection of compounds such as catecholamines, carboxylic acids, sulfonic acids, phosphonic acids, alcohols, glycols, aldehydes, carbohydrates, amines, and many other sulfur-containing species and inorganic anions and cations. Potentiometric, amperometric, and conductivity detectors are all classified as electrochemical detectors. 

Potentiometric, voltammetric, or polarographic electrochemical detectors are useful for the quantitation of species that can be oxidized or reduced at a working electrode. These detectors are selective, sensitive, and reliable, but require conducting mobile phases free of dissolved oxygen and reducible metal ions. A pulseless pump must be used, and care must be taken to ensure that the pH, ionic strength, and temperature of the mobile phase remain constant. Working electrodes are prone to contamination by reaction products with consequent variable responses. 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

Another detector, which has found considerable application, is based on the changes in the refractive index of the solvent that is caused by analyte molecules. In contrast to most of the other detectors listed in Table 32-1, the refractive index detector is general rather than selective and responds to the presence of all solutes. The disadvantage of this detector is its somewhat limited sensitivity. Several electrochemical detectors that are based on potentiometric, conductometric, and voltammetric measurements have also been introduced. An example of an amperometric detector is shown in Figure 32-9. 

Electrochemical detectors are sometimes divided into the groups of potenliomctric. amperometric, and conductometric detectors, that is, according to the three parameters of electric measurements. Potentiometric detectors measure voltage, amperometric measure current, and conductometric measure resistance. 

The order of presentation of the electroanalytical methods will be direct potentiometry with ion-selective electrodes, potentiometric titrations, voltammetry/polarography, polarisation titrations (amperometric and potentiometric), conductometry/coulometry and electrochemical detectors. 

K. Brunt, Comparison Between the Performances of an Electrochemical Detector Flow Cell in a Potentiometric and an Amperometric Measuring System Using Glucose as a Test Compound. Analyst, 107 (1982) 1261. 

Electrochemical detectors can be classified according to the three fundamental parameters of voltage or potential (V), resistance (R), or current (i). These terms are related via Ohm s Law, which is V = I R. Electrochemical detectors are considered to be conductometric, potentiometric, amperometric or coulometric detectors. Conductometric detection has been discussed earUer in this chapter and there is only Umited discussion in this section. Coulometric detection is not commonly used and is discussed only briefly. 

Potentiometric detectors are among electrochemical detectors the most widely favored in flow techniques on account of their easy operation, direct applicability to underivatized analytes and extremely wide dynamic concentration ranges, which often span several orders of magnitude. Their recordings consist of peaks the potentials of which are proportional to the logarithmic concentration of analyte provided an ionic strength buffer (ISAS or TISAB) is used as carrier. 

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. 

Electrochemical detectors of the waU-jet type (Fig. 9.6, left) are simple, robust and flexible. Working and counter electrodes (if necessary, the reference electrode as well) are positioned in a large solution reservoir. The disc-shaped working electrode is in plane with one of the walls of this reservoir. In front of it, a fine nozzle is arranged. The carrier solution is ejected as a sharp jet directed towards the electrode area. The jet washes away quickly residues of older solution from the electrode surface and generates a powerful convection. Thus, a relatively high value of the diffusion-limited current is achieved with an amperometric operation, as is a fast response if operated potentiometrically. The virtual dead volume of wall-jet detectors is extraordinarily smalL It cannot be derived from the cell geometry, but must be determined experimentally by calibration. 

Most electrochemical detectors, such as amperometric and potentiometric detectors, are surface detectors. They respond to substances that are either oxidizable or reducible and the electrical output results from an electron flow caused by the chemical reaction that takes place at the surface of the electrodes (Rao et ah, 2002 Mehrvar and Abdi, 2004 Trojanowicz, 2009). Successful operation of a surface detector requires a reproducible radial concentration distribution. There are several types of flow-through detection cells, each type being characterized by parameters such as the length, diameter, and shape of its detection channel, which determine the laminar character of the liquid flow under the given experimental conditions and the predominant mode of the mass transport within the cell. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

The three most common modes of operation of electrochemical detection are amperometric, coulometric, and potentiometric. An amperometric detector is an electrochemical cell that produces a signal proportional to the analyte concentration usually the percentage of the analyte that undergoes the redox reaction is very low, about 5%. 

With respect to chromatography, electrochemical detection means amperometric detection. Amper-ometry is the measurement of electrolysis current versus time at a controlled electrode potential. It has a relationship to voltammetry similar to the relationship of an ultraviolet (UV) detector to spectroscopy. Whereas conductometric detection is used in ion chromatography, potentiometric detection is never used in routine practice. Electrochemical detection has even been used in gas chromatography in a few unusual circumstances. It has even been attempted with thin-layer chromatography (TLC). Its practical success has only been with liquid chromatography (LC) and that will be the focus here. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

Conductometric detectors respond to all ions, but the other detectors respond only to certain electroactive ions. In this book, electrochemical detection will refer to amperometric, or potentiometric detectors, but will not refer to conductometric detectors. 

Microscale fluidic systems use small volumes so sensitivity of detection can be a challenge. Any detector for chip-based LC needs to be small and ideally have low power consumption. It is generally a problem of interfacing. Flow cell geometry is also a big factor, e.g. a U cell instead of linear flow cell can give a ten-fold increase in sensitivity for absorbance measurements. Electrochemical detection is very common, mainly ampero-metric and potentiometric, and very amenable to detection on chip. Fluorescence is more sensitive than UV-Vis absorbance and chemiluminescence is sensitive down to a single molecule, similar to LIF. 

Ganthier M. and Chamberland A., Solid-state detectors for the potentiometric determination of gaseons oxides, J. Electrochem. Soc., 124, 1579-1583, 1977. 

Unlike the commonly made potentiometric pH measurement, electrochemical detection is an amperometric (current) measurement, at controlled potential. Electrochemical detection involves a chemical redox reaction, in contrast to ultraviolet or fluorescence detection where a passive, physical absorption of radiation occurs. The reaction occurs at an electrode suiface, placed in or alongside the flow of effluent from the column. Either an oxidation or reduction may be forced to occur by judicious selection of a potential applied to the cell by the controlling potentiostat. The potential is a source of electrochemical selectivity, in the same manner as the wavelength selected with a variable wavelength UV detector. In essence the electrode acts as an oxidizing or reducing agent of variable power. 

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