Electrochemical detection flowing stream

Electrochemical oxidation-reduction of eluting mixture components is the basis for amperometric electrochemical detectors. The three electrodes needed for the detection, the working (indicator) electrode, reference electrode, and auxiliary electrode, are either inserted into the flow stream or imbedded in the wall of the flow stream. See Figure 13.13. The indicator electrode is typically glassy carbon, platinum, or gold, the reference electrode a silver-silver chloride electrode, and the auxiliary a stainless steel electrode. Most often, the indicator electrode is polarized to cause oxidation of the mixture components... 

Research has been done showing that rapid pressnre-driven LC analysis can be done with little solvent consumption, demonstrating this as a viable process analytical tool. Using electrokinetic nanoflow pumps LC can be miniaturized to the point of being a sensor system. Developments in terms of sampling to enable sampling directly from a process stream, to the separation channel on a chip are critical for the application of miniaturized process LC. The components (valves and pumps) required for hydrodynamic flow systems appear to be a current limitation to the fnll miniatnrization of LC separations. Detection systems have also evolved with electrochemical detection and refractive index detection systems providing increased sensitivity in miniaturized systems when compared to standard UV-vis detection or fluorescence, which may require precolumn derivatization. 

Electrochemical measurements can be readily adapted for on-line monitoring. An electrochemical detector uses the electrochemical properties of target analytes for their determination in a flowing stream. An electrochemical flow system, based on an SWV operation at a carbon-fiber-based detector, for use in the on-line continuous monitoring of trace TNT in marine environments was developed [16]. Such flow detector offers selective measurements of sub-part-per-million concentrations of TNT in untreated natural water samples with a detection limit of 25ppb. It responds rapidly to sudden changes in the TNT concentration with no apparent carryover. About 600 runs can be made every hour with high reproducibility and stability (e.g., relative standard deviation (RSD) = 2.3%, n = 40). The system lends itself to full automation and to possible deployment onto various stationary mobile platforms (e.g., buoys and underwater vehicles). 

In flow injection analysis [32] with electrochemical detection a sample is injected into an electrolyte carrier stream dispersion of the sample plug into the carrier stream occurs so that electrolyte is effectively added to the sample—with consequent sample dilution—before reaching the electrode. Even so, by using a capillary flow injection system nanolitre sample volumes can be investigated [33]. In continuous flow systems, electrolyte often has to be added to the sample beforehand, also leading to sample dilution. 

Atsushi, A., Matsue T., and Uchida, I. Multichannel electrochemical detection with microelectrode array in flowing streams. Anal. Chem. 1992, 64, 44—49. 

Ultraviolet (UV) spectroscopy, mass spectrometry (MS), refractive index (RI) detection, and electrochemical detection (ECD) are common online monitoring techniques for analytical chromatography. UV and RI are regularly used for monitoring preparative operations as well. To employ MS or ECD in a high-flow scheme, usually a side stream must be diverted from the main eluate line via a flow splitter so that what passes through the detector has a flow rate that is acceptable for an analytical-scale system. 

Electrochemical detectors were reported used by 21% of the respondents to the detector survey (47). Electron transfer processes offer highly sensitive and selective methods for detection of solutes. Various techniques have been devised for this measurement process, with the most popular being based on the application of a fixed potential to a solid electrode. Potential pulse techniques, scanning techniques, and multiple electrode techniques have all been employed and can offer certain advantages. Two excellent reviews of electrochemical detection in flowing streams have appeared (59,60), as well as a comprehensive chapter in a series on liquid chromatography (61). 

There were situations where the analyte needed to be converted into a more detectable species, and thus reagent addition was often required. These steps were efficiently accomplished in a flow system, as demonstrated in the determination of chlorine in waters [8]. A potassium iodide solution merged with the sample flowing stream, and the mixture passed through a coil, so that enough time for the stoichiometric release of iodine was achieved, and the liberated species was electrochemically determined. 

The primary drawback to air-segmented systems is the bubble. These air bubbles are compressible, thereby creating pulsations in the flowing stream. In addition, for most detectors, the air bubble must be removed before the sample passes through the detection cell. In addition, the precision of bubble size has been difficult to control. This variation in bubble size adds to the irreproducibility of the system. If electrochemical or other static sensitive detectors are used, the segmented system, which builds up static charge, will present severe problems... 

T. N. Morrison, K. G. Schick, and C. O. Huber, Determination of Ethanol by Air-Stream Separation with Flow Injection and Electrochemical Detection at a Nickel Oxide Electrode. Anal. Chim. Acta, 120 (1980) 75. 

Wang, J., T. Golden, and R. Li (1988). Cobalt phthalocyanine/cellulose acetate chemically modified electrodes for electrochemical detection in flowing streams Multifunctional operation based upon the coupling of electrocatalysts and permselectivity. AnoZ. Chem. 60(15), 1642-1645. 

Electrochemical detectors are very popular in liquid chromatography. Electron transfer processes offer highly sensitive and selective methods for detection of solutes in flowing streams. Various techniques have been devised for these measurements, with the most popular being based on the... 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

Graphite electrodes used as liquid chromatography detectors are relatively free from filming effects. The amount of material normally electrolyzed is very small and the continuous washing of the surface by the flow stream appears to keep film formation at a minimum. Nevertheless, if electrochemical detectors are used in liquid chromatography, the quantities injected (especially of serotonin compounds) should not exceed several hundred nanograms. Indeed, if higher amounts are to be detected, one can use less sensitive, nonelectrochemical detector systems. 

One of the most active areas of electrochemical research and development in the last decade has been the use of electrochemical detectors for flowing stream analysis. Of particular interest is the analysis of complex solutions containing several chemical species. The use of high performance chromatography coupled to electrochemical detection (LCEC) is an important development in this regard. In most applications of conventional LCEC, such as in clinical, biomedical or organic analysis, discrete samples are analysed. 

When working with complicated matrices (biological samples or food products), the selectivity of electrochemical methods (ED) can usually be increased if they are combined with effective separation techniques, e.g. capillary electrophoresis and liquid chromatography (Rychlik 2011 Trojanowicz 2011). In separation science, electrochemical detection is used to detect and measure response analytes in flowing streams after separation by HPLC or capillary electrophoresis (Trojanowicz 2011). The former was introduced in the mid-20th century and is still an actively developing analytical technique. Employment of a new generation of columns, new detector types, new software and the... 

A three-dimensional modular setup for a miniaturized analysis system for flowing streams is presented The system uses silicon micromachined pumps and flow manifolds in combination with electrochemical sensors or optical detection Applications range from simple ion concentration measurements with ISFETs to a multi-step chemical analysis of phosphate Miniaturization of the flow systems leads to a substantial reduction in reagent consumption... 

The FI determination of acesulfame-K, cyclamate, and saccharin in wines, yogurts, diet soft drinks, and sweetener tablets was reported by Nikolelis and Pantoulias (2001). The detection was performed with filter-supported bilayer membranes from lyophilized egg phosphatidylcholine, using two Ag/AgCl reference electrodes biased by an external power supply at 25 mV. Transient electrochemical signals were recorded with a different time of appearance for each artificial sweetener after the injection of the sample into the flow stream, thus allowing their selective detection in mixtures. 

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