Voltammetric detectors

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

To overcome these problems, most voltammetric detectors have used pulsed waveforms such as staircase , squarewaveand differential pulseThe current is sampled at the end of the pulse after the charging current has decayed. In addition, because the charging current is typically the major current source, iR problems are not as severe. Last has described a coulostatic detector based on charge pulses instead of potential pulses which eliminates iR and charging current... 

Lunte et al. have recently developed a unique voltammetric detector using... 

Electrochemical detection is sensitive and selective, and it gives useful information about polyphenolic compounds in addition to spectra obtained by photodiode array detectors. Differences in electrochemically active substituents on analogous structures can lead to characteristic differences in their voltammetric behavior. Because the response profile across several cell potentials is representative of the voltammetric properties of a compound, useful qualitative information can be obtained using electrochemical detection (Aaby and others 2004). 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

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.

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

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 need to measure concentrations in very small volumes is not restricted to biological systems. For example, open tubular columns for liquid chromatographic separations offer the advantage of increased resolution, but because their internal diameters may be as small as 15 pm, the amount of material in the eluted peaks is very small. Thus, the use of these columns requires detectors that can be used with low concentrations in small volumes. Jorgenson and co-workers showed that this could be accomplished by the insertion of a 10-pm-diameter, cylindrical electrode made from a carbon fiber into the end of the column [4]. The close fit between the column wall and the fiber ensured that a large fraction of the eluting molecules were electrolyzed. When the electrochemical data were collected in a voltammetric mode, the resolved compounds could be classi-... 

Electrochemical detectors can be broadly classified as either amperometric or voltammetric. An amperometric detector is one in which the potential applied to the detecting electrode is held constant and the resulting current is measured as a function of time. A voltammetric detector is one in which the applied potential is varied with time and the current response is measured as a function... 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

Many published articles on HPLC-ECD refer to the use of one of three voltammetric detectors (amperometric, coulometric, or polarographic). More detailed information on principles and techniques of various electrochemical detection modes can be obtained from the recent book, Coulometric Electrode Array Detectors for HPLC (34). There are also two electrode array detectors, the coulometric electrode array system and the CoulArray detector, currently available. Both detectors offer the qualitative data of PDA and the extreme sensitivity of ECD (34). The... 

Abbreviations AOD, Acousto-optical deflection BCB, bisbenzyocyclobutadiene CCD, indirect contact conductivity detection CL, chemiluminescence ECD, electron capture detector FCS, fluorescence correlation spectroscopy FRET, fluorescence resonance energy transfer ICCD, integrated contact conductivity detection GMR, giant magnetoresistive LED-CFD, light emitting diode confocal fluorescence detector LIF, laser-induced fluorescence LOD, limit of detection MALDI, matrix-assisted laser desorption ionization PDMS, poly(dimethylsiloxane) PMMA, poly(methylmetha-crylate) SPR, surface plasmon resonance SVD, sinusoidal voltammetric detection TLS, thermal lens spectroscopy. 

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