Voltammetric detector, chromatographic

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.

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-... 

When analyzing complex samples by HPLC, the selection of detection system is important. If chromatographic separation is incomplete or analytic concentration very low, then the more universal and insensitive ultraviolet absorbance (UV) detectors are not satisfactory, and other detection systems must be used. Fluorescense (FL) detection is a sensitive and selective alternative for those compounds that fluoresce. Electrochemical (EC) detection differs from UV and FL in that it is based on a chemical reaction rather than a physical phenomenon, and is the best choice for the many electroactive compounds. In this system analytes are either oxidized or reduced on the electrode surface. More precisely, this technique may be called ampero-metric or voltammetric detection, though in practice it is commonly referred to as EC. 

A hydrodynamic voltammogram is a current-potential curve which shows the dependence of the chromatographic peak height on the detection potential. The technique used to obtain the necessary information is voltammetric flow injection analysis. in which an aliquot of the analyte is injected into the flowing eluent prior to the detector and the peak current recorded. This is repeated many times, the detector potential being changed after each injection, until the peak current - potential plot reaches a plateau or a maximum, as shown... 

Voltammetric sensors are constructed in such a way that very little depletion of the species to be determined occurs and is nearly equal to C2. That condition is maintained in most chromatographic detectors, where A and V are small, and v high. The 6 is inversely proportional to the convection and must be constant, as well as to the activity of the working electrode. 

Stulik, K., Pacakova, V., Weingart, M. and Podolak, M. (1986) Operation parameters of voltammetric high-performance liquid chromatographic detectors with copper electrodes and application to a determination of some fodder biofactors. J. Chro-matogr, 367, 311-321. 

In specific circumstances where there happen to be no interferences, the simple voltammetric response of an electrode can be a Very convenient monitor in process streams and effluents. Such applications, however, have little generality. An application of electrochemical celts of increasing importance is as a detector for high-pressure liquid chromatographs. Here, the chromatographic column should have already separated the components of the mixture and the ability to record complete i-E responses quite rapidly can be used to identify components and to confirm complete separation (does the curve change through a peak ),... 

Chromatopolarography appeared to be an important method, because on a well-chosen chromatographic column, substances which had almost identical voltam-metric characteristics (half-wave potentials) could be separated, if their affinities to the column-filling material (stationary phase) were sufficiently different. After separation, they could be polarographically detected and their content determined. This method may be considered a precursor of many combined voltammetric techniques developed in forthcoming years and specifically electrochemical detectors in high performance liquid chromatography (HPLC) and microfluidics. 

As summarised in Table 3.4, some electroanalytical methods have been certified by standardisation bodies for the chemical characterisation of ambient water samples, mostly in the class of inorganic substances. Conductometric detection is used in direct method for ionic constituents and also as chromatographic detector for individual cations and anions. Total, inorganic and organic carbon in water can be also assayed by conductometric detection. Amperometric detection has been certified for dissolved oxygen and cyanide. ISE potentiometry is used for standardised measurements of a set of ions and also for the evaluation of water oxidation-reduction potential. Voltammetric detection is the base for diverse methodologies oriented to the determination of trace elements including the most relevant elemental pollutants. 

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