Flow injection analysis , electrochemical detection

In another approach the electrocatalytic activity of lead oxide was enhanced with fluoride doping. The F-doped lead oxide-modified electrode leads to the fabricatirai of an electrochemical detection system for flow injection analysis to detect the chemical oxygen demand (COD) in water samples. The combination of flow injection analysis with electrochemical detection of COD results in the development of a low-cost, rapid, and easily automated detection system with minimum reagent consumption. The basic principle of the F-doped lead oxide electrode is the generation of hydroxyl radicals which are subsequently utilized for the oxidation of COD pollutants in order to determine the COD value. It is a multistep process at first, hydroxyl radicals will be produced at the surface of the F-PbOa electrode by the anodic discharge of water ... 

In a quantitative flow injection analysis a calibration curve is determined by injecting standard samples containing known concentrations of analyte. The format of the caK-bration curve, such as absorbance versus concentration, is determined by the method of detection. CaKbration curves for standard spectroscopic and electrochemical methods were discussed in Chapters 10 and 11 and are not considered further in this chapter. 

Progress has been made in developing electrochemical methods for detection of amino acids without derivatization.74 75 Evaporative light scattering (ELSD) is also a promising detection method.76 Flow-injection analysis... 

Flow injection analysis (F1A). In this technique, introduced by Ruzicka and Hansen, a small amount of sample is injected into a liquid flow (see Fig. 5.16), which apart from being automated is normally continuous, but can include the use of stopped-flow, merging zones extraction techniques in addition to FIA scanning and methods based on intermittent pumping89. The principles of FIA and the versions just mentioned will now be briefly discussed on the basis of the excellent review of Ruzicka and Hansen89 in order to understand the appli-cational possibilities of electrochemical detection in this technique. 

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

Electrochemical detection under convective conditions has been applied widely in freshwater measurements. In addition, seawater measurements have been combined with flow injection analysis (FIA) and high pressure liquid chromatography (HPLC) (D.C. Johnson et al., 1986). Well-developed commercial product lines exist, and detection limits are typically in the range of femtomoles. For in situ, shipboard, and land-based measure-... 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

A few other types of anion sensors have been mentioned recently in the literature. Tetrathiofulvalene microcrystals immobilised at a platinum electrode displayed electrochemical properties that were affected by the presence of anions in solution, with some selectivity for anions such as bromide [ 145]. A flow-injection analysis system using anion-exchange columns for separation and polyaniline electrodes as detectors could detect dichromate down to 0.004 ppb and could be used for seawater samples [146]. 

Fig. 31. The determination of ethylamine by flow-injection analysis with electrochemical detection. Chronoampero-gram with repeated 20 pi injections of 0.25 mM ethylamine in 0.1 M carbonate buffer, pH 10, to a flowing (0.5 mL min-1) carbonate buffer solution. Reprinted with permission from [146], Copyright (1997) American Chemical Society.

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. 

Voltammetric studies have demonstrated that thiocholine is oxidized at +0.15-0.20 V on CNT-modified GC electrodes as compared to its higher oxidation potential on unmodified GC (+0.7 V), carbon paste (+0.6 V), and gold electrodes (+0.9 V). A lower detection limit of 5 x 10 M thiocholine was obtained under the optimal batch conditions, while the detection limit was further improved down to 3 X 10 M by the electrochemical detection in flow injection analysis. 

Electrochemical Detection for High-Performance Liquid Chromatography (ELCD) and Flow-Injection Analysis (ED-FIA)... 

Tougas, T.P. Recent developments in electrochemical detection for flow injection analysis. Int. Lab. 1988, 18, 17-28. 

Neumayr, M. Friedrich, O. Sontag, G. Pittner, F. Flow-injection analysis with electrochemical detection for determination of salicychc acid in pharmaceutical preparations. Anal. Chim. Acta 1993, 273, 469-475. 

Amperometry is when voltammetry is used at a fixed potential and the current alone is followed. This current is proportional to the concentration of a species in a stirred or flowing solution. It is, therefore, a very suitable detection method for use in flow injection analysis systems and in chromatographic separations its use as a detector for separation techniques is discussed in that section. The current is the result of the electrochemical oxidation or reduction of the analyte after application of a potential pulse across the working and auxiliary electrodes. 

Phenol can also be determined by flow injection analysis with electrochemical detection (FI AEC). In FI AEC the sample is injected directly into... 

T. R. Lindstrom, The Determination of Mercury at Trace Levels by Flow Injection Analysis with Electrochemical Detection. Diss. Abstr. Int. B, 41 (1981) 3021. 

T. Toyoda, S. S. Kuan, and G. G. Guilbault, Determination of Lactate Dehydrogenase Isoenzyme (LD-1) Using Flow Injection Analysis with Electrochemical Detection after Immunochemical Separation. Anal. Lett., 18 (1985) 345. 

R. E. A. Escott and A. F. Taylor, Determination of Water by Flow Injection Analysis Using Karl Fischer Reagent with Electrochemical Detection. An-alysty 110 (1985) 847. 

H. A. Mottola, C.-M. Wolff, A. lob, and R. Gnanasekaran, Potentiometric and Amperometric Detection in Flow Injection Analysis. In Modern Trends in Analytical Chemistry (Proc. of Two Scientific Symps., Mdtrafured, Hungary, 1982. I. Electrochemical Detection in Flow Analysis ) (E. Pungor, I. Bizas and G. E. Veress, Eds.). Anal. Chem. Symp. Series, 18 (1984) 49. 

M. E. Herrera, Analytical and Neurochemical Studies with Flow Injection Analysis and Electrochemical Detection. Part I. Pulsed Flow Electrolysis with the Reticulated Vitreous Carbon Electrode. Part II. On-Line Determination of Secreted Catecholamines from Cultured Adrenal Cells. Diss. Abstr. Int. B, 46 (1986) 1894. 

Hybrid techniques such as HPLC-EC or electrochemical detection in combination with flow injection analysis (FIA) will foreseeably play a much more important role in the environmental analysis. 

Analysis in flowing solutions, as performed in particular with high performance liquid chromatography (HPLC) and flow injection analysis, (FIA) has developed rapidly over the last decade and now plays an important function in most analytical laboratories throughout the world. There is little doubt, however, that even HPLC lacks the resolving power required to solve analytical problems in complex matrices with minimal sample preparation. Often, the resolving power of the detection method is called upon to assist in the solution of these problems. This is particularly true with electrochemical detection (ED) systems which offer a certain degree of selectivity based on differences in oxidation or reduction potentials of the species to be determined. In recent years, the advent of chemically modified electrodes (CMEs) has provided a stimulus to further improve both the sensitivity and selectivity of ED systems used in HPLC and FIA. 

Electrochemical detection (ED) in high performance liquid chromatography (HPLC) and flow injection analysis (FIA) continues to be of increasing importance for the determination of trace quantities of compounds which are difficult to detect spectrophotometrically. 

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