Electrochemical methods detector

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

Crompton [21] has reviewed the use of electrochemical methods in the determination of phenolic and amine antioxidants, organic peroxides, organotin heat stabilisers, metallic stearates and some inorganic anions (such as bromide, iodide and thiocyanate) in the 1950s/1960s (Table 8.75). The electrochemical detector is generally operated in tandem with a universal, nonselective detector, so that a more general sample analysis can be obtained than is possible with the electrochemical detector alone. 

Electrochemical (EC) detectors have been used for detection in CE. EC methods offer an advantage over the spectroscopic detection methods because electrochemistry that occurs directly at an electrode surface is not limited by the small dimensions inherent in The... 

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. 

Two developmental difficulties existed in the application of electrochemical methods to CE detection. The first involved the cross-talk or interference that resulted from the high voltages used in CE separations. These dc voltages may be as high as 50 kV, and the resulting electroosmotic currents can be up to six orders of magnitude greater than the faradaic currents measured at an amperometric detector poised... 

Cyanide in aqueous matrices is usually measured by colorimetric, titrimetric, electrochemical methods or by headspace gas chromatography with a nitrogen-specific detector, after pretreatment to produce hydrogen... 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

Photo-acoustic spectroscopy has been used for ultratrace levels of Hg in air and snow (de Mora etal. 1993). X-ray fluorescence is nondestructive, rapid, requires minimal sample preparation, and was, for example, used successfully to determine the maximal level of mercury in maternal hair to assess fetal exposure (Toribora et al. 1982). However, the procedure is less sensitive compared to AAS and INAA if no pre-concentration is used. Electrochemical methods have been replaced as detectors in chromatography by other instrumental techniques because of poorer detection limits. High-performance liquid chromatography (HPLC) with reductive amperometric electrochemical reduction, however, was shown to be capable of speciating Hg(II), methyl- ethyl- and phenylmercury, with detection limits <2pgL (Evans and McKee 1987). 

Electrochemical HPLC detectors, 15 Electromotive force (EMF) method, pXa values and, 9,12 Ephedrine, 5... 

The first is a UV-vis-absorption or absorbance spectrophotometric (the phenomenon responsible for the signal is absorption, however, what is actually measured is absorbance, UV) detector. The second is a molecular fluorescence luminescence spectrophotometric (commonly called fluorescence, FL) detector. Other non-mass-spectrometric detectors designed for HPLC include refractive index (RI), electrochemical (EC), and of a more recent vintage, the light-scattering evaporative detector (LSED). RI and LSED HPLC detectors are not sensitive enough to meet the needs of TEQA. Electrochemical HPLC detectors have the required sensitivity, but due to frequent fowling of electrode surfaces they have not really found a place in TEQA. This author knows of no EPA methods as yet that incorporate EC HPLC detectors. For this reason, EC HPLC detectors will be not considered. 

The detection of the separated zones in the tube can be carried out in different modes detection of underivatized carbohydrates through direct UV detection, indirect UV detection, or by electrochemical methods. Carbohydrates with labeled chromoph-ore groups can be detected by UV or fluorescence detectors. 

The hydrodynamically well-defined conditions of flow systems are an ideal environment for electrochemical detectors, resulting in enhanced performance characteristics. The surface sensing properties of most electrochemical methods require particular attention in the construction of suitable flow-through cells. Efficient and repeatable mass transport toward the electrode surface is necessary, and dead volumes should be small. Various flow-through cells have been designed for electrochemical detection, all of which can be derived from the basic configurations depicted in Figure 4. 

Ascorbic acid (vitamin C) in a 50.0-mL sample of orange juice was analyzed by an electrochemical method that gave a detector current of 1.78 ulA. A standard addition of 0.400 mL of 0.279 M ascorbic acid increased the current to 3.35 julA. Find the concentration of ascorbic acid in the orange juice. 

Standard addition. Vitamin C was measured by an electrochemical method in a 50.0-mL sample of lemon juice. A detector signal of 2.02 p,A was observed. A standard addition of 1.00 mL of 29.4 mM vitamin C increased the signal to 3.79 p.A. Find the concentration of vitamin C in the juice. 

In addition to being a green house gas, CO2 is an important component for metabolism process of plant and many living creatures [34]. Thus, reliable and selective CO2 detectors are needed for a variety of applications including environmental and health monitoring [35, 36], fire detection [37], and controlling of fermentation [38]. There are several types of commercially available CO2 gas sensors and most of them are based on nondispersed infrared (NDIR) and electrochemical methods. 

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... 

Hyphenated instruments consist of an inlet that separates and preconcentrates analytes prior to their introduction into an integral detector. A hyphen can be used to separate the two components when the name is written in abbreviated form—for example, GC-MS. The separation module is the GC and the detection module is the MS. As hyphenated instruments became more common, the hyphen often was dropped, and that is the convention used in this book (i.e., GCMS rather than GC-MS). Separations alone, such as in thin layer chromatography, are not instrumental methods in the traditional sense, but mating a separation technique to a detector creates an instrument that is capable of what is called a hyphenated technique. The basis of most separation modules is selective partitioning, a topic discussed in the previous chapter. However, there is a group of hyphenated instruments in which separation is achieved with electrochemical methods, an increasingly important topic in forensic chemistry. 

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