Electrochemical detection potentiometric detectors

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. 

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. 

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. 

Recently, electrochemical detection methods, namely, conductimetry, amperometry, and potentio-metry, have also become accessible. All three variants of electrochemical detection are intrinsically simpler than the optical methods, and their success depends highly on the electrode materials and designs used. Conductivity detection relies on measurement of the differences between the conductivities of the analyte and the separation electrolyte this provides a direct relationship between migration times and response factor, and makes this detector universal. On the contrary, amperometric detection is restricted to electroactive species and potentiometric detection is not possible for certain small ions with multiple charges. Conductimetric detection works better for inorganic compounds since the higher mobility of... 

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. 

Potentiometric detectors and formation of new t3 pes of sensors including detectors without internal solution (all-solid-state). The composition of ion-selective layers was considerably and aptly modified. New solutions and additives in the membrane electrodes were used and tested looking for analogy with biological membranes. Special applications of such electrodes in medicine were studied. Development of methods of the flow analysis, which was introduced to analytical chemistry in the last quarter of the twentieth century. A number of original results in the methodology of that method were obtained at the University of Warsaw, especially in the use in detection of electrochemical techniques. Number of interesting and new results were obtained in electrophoretic methods. 

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. 

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... 

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. 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

Since its introduction in 1975 ion chromatography (IC) has been used in most areas of analytical and environmental chemistry. Although the conductivity detector is still the most popular, other types of detection can be applied for different analytes. These include the following methods electrochemical (e.g., amperometric, potentiometric), photometric (UV/Vis, chemiluminescence), and spectrometric detectors used mainly in hyphenated techniques. 

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. 

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. 

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