Instrumentation electrochemical detectors

Photometric detectors are the most popular in CE instruments including diode array detectors. Laser-induced fluorescence (LIE) detection and electric conductivity detectors are also popular. LIE is particularly sensitive and powerful for detecting low concentration analytes. However, most analytes are not natively fluorescent and some derivatizations are necessary. Conductivity detector is useful for the detection of non-ultraviolet (non-UV) absorbing analytes such as inorganic ions or fatty acids. Both LIE detection and conductivity detectors are commercially available and easy to interface with conventional CE instruments. Electrochemical detectors are also useful for selective high-sensitivity detection. Several techniques have been developed to circumvent the problem of strong effects of electrophoretic field on electrochemical detection, but despite this, commercial electrochemical detectors are not used extensively. 

Many applications for ion analysis use a UV detector with indirect detection, though other electrochemical, laser-induced fluorescence (LIE), or mass spectrometry detectors have been described. The main advantage of UV detection is its availability on commercial instruments and that both UV-absorbing and non-UV-absorbing analytes may be detected. Nowadays, electrochemical detectors are also available specific background electrolytes (BGEs) must be used and the detector has to be adapted to existing CE instruments. 

It is important for the reader to recognize that there is no single optimal electrochemical detector, but, rather, a whole family of devices that can be adapted to solve a variety of problems. To make the best choice, there is no substitute for a little knowledge of the principles of electrochemical instrumentation. Even more important is an understanding of the chemistry involved when a redox-active molecule ventures into the high-field region near an electrodesolution interface. 

The instrument is composed of an interface to plug the disposable cartridge, a temperature controller, a multichannel pumping device and valves as well as a multiplexer electrochemical detector for sequential or parallel detection of the amperometric events occurring in each of the eight channels for some protocols, it can also serve to deliver different reagents such as the secondary antibody or the substrate and thus provide fully automatic assays. 

Dionex 2010i ion chromatograph, Dionex electrochemical detector, Houston Instrument Omniscribe chart recorder. 

Refs. [i] Ryan TH (ed) (1984) Electrochemical detectors. Fundamental aspects and analytical applications. Plenum Press New York [ii] Vdha f (1982) Gas and liquid analyzers. In Svehla G (ed) Wilson and Wilsons comprehensive analytical chemistry, vol. XVII. Elsevier, Amsterdam [iii] Mount AR (2003) Hydrodynamic electrodes. In Bard A, Strat-mann M, Unwin P (eds) Instrumentation and electroanalytical chemistry. Encyclopedia of electrochemistry, vol 3. Wiley-VCH, Weinheim, pp 134... 

Poppe, H. Electrochemical detectors. In Instrumentation for High-Performance Liquid Chromatography, Huber, J.F.K., Ed. Elsevier Amsterdam, 1978, pp. 131-149. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Because electrochemical experiments involve a direct conversion of chemical information to electricity, the instrumentation can be relatively simple. There is, for example, no need for high-quality power supplies to drive a light source or operate a photomultiplier tube. On the other hand, because the process often measures nanoamperes (or less) of current, electrochemical detectors are particulariy subject to dectrical interferences, and proper grounding can be crucial to successful experiments at high current-to-voltage gains. 

Electrochemical detector an instrument with dual glassy carbon working electrodes (W1 and W2), positioned in parallel (Bioanalytical Systems). Set the operating potential ofWl at 600 mV (the maximal oxidation potential of 5HT) and of... 

As in the previous work, the flow injection system consisted of a Cole Parmer Mas-terflex peristaltic pump, Rheodyne 7125 injector, Bioanalytical System (BAS) TL-5A flow through electrochemical cell and either an IBM EC/230 or a BAS CV 37 potentiostat. These instruments respectively are designed for use with an electrochemical detector for liquid chromatography and for low current measurements such as with ultramicroelectrodes. A pulse dampener was also included in the flow system. 

The instrument, Figure 1, involves a two-arm Mach-Zehnder interferometer constructed from mono-mode fiber optic waveguides. One arm of the interferometer is coated with an immobilized enzyme, while the other is used as a reference. Both arms of the interferometer are firmly held in the middle of a conduit that is incorporated into a flow injection analysis (FIA) system, serving as a substitute for the normal absorbance, fluorescence, pH, or electrochemical detectors. A common source of phase coherent light is launched down both arms of the interferometer. 

Sensitive and selective detection techniques are of crucial importance for capillary electrophoresis, microfluidic chips, and other microfluidic analysis systems. Electrochemical detector has attracted considerable interest in these fields owing to its high sensitivity, inherent miniaturization of both the detection and control instrumentation, low cost and power demands, and high compatibility with microfabrication technology. The commonly used electrochemical detection approaches can be classified into three general modes COTiductimetry, potentiometry, and amperometry. 

Figure 2. Simple basic post-chromatographic derivatization manifold. The detector may be a photometer, fluorimeter, electrochemical detector or any other appropriate type of instrument.

In the first chapter we recount some of the historical milestones and briefly cover the most basic principles of ion chromatography, or IC as it is often called. The various components and hardware of IC instruments are described in Chapter 2, but it is not our intention to discuss specific commercial instruments. Chapter 3 has been updated to include advances in column technology and promising new columns, such as monolithic columns. Chapter 4 on detectors has been expanded to include new material on the contactless conductivity detector (CCD) and pulsed electrochemical detectors. 

Electrochemical detectors of several types are currently available from instrument manufacturers. These devices are based on amperometry, voltammetry, coulometry, and conductometry. The first three of these methods are discussed in Chapters 24 and 25. 

Applications of this approach include measurement of neurotransmitters in dialysates and single cell analysis. Microfabricated instrumentation with recycling electrochemical detector may further improve the detection limit for species that can be oxidized and reduced multiple times. Microfabrication may also make this approach more widely available. The main limitation to greater use of CLC with microelectrodes for EC detection is lack of commercial instrumentation that would allow simple implementation of the principles outlined above. 

The association of a spectrometer with the liquid chromatograph is usually for the purpose of structure elucidation of the eluted solute, a procedure that will be discussed in a later chapter. The association of tui atomic spectrometer with the liquid chromatograph, in contrast, is almost exclusively for the specific detection of the metalic and semi-metalic elements. The atomic spectrometer is a highly specific detector, and for element detection perhaps more so than the electrochemical detector. However, in general, a flame atomic absorption spectrometric (AAS) system is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed then multi-element detection is possible. The inductively coupled plasma spectrometer can also, under some circumstances, provide multi-element detection but all three instruments are extremely expensive particularly in terms of an LC detector. It follows that most LC/AAS combinations employ a flame atomic absorption spectrometer or occasionally an atomic spectrometer fitted with a graphite furnace. Furthermore the spectrometer is usually set to monitor one element only, throughout the development of any given separation. 

A Cecil CE212 variable wavelength UV detector (Cecil Instruments Ltd., Cambridge, Great Britain) was used in conjunction with the electrochemical detector in some of the investigations. 

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