Sensitivity anodic stripping voltammetry

The methods of investigation of metal species in natural waters must possess by well dividing ability and high sensitivity and selectivity to determination of several metal forms. The catalytic including chemiluminescent (CL) techniques and anodic stripping voltammetry (ASV) are the most useful to determination of trace metals and their forms. The methods considered ai e characterized by a low detection limits. Moreover, they allow detection of the most toxic form of metals, that is, metal free ions and labile complexes. 

Analysis of total zinc by anodic stripping voltammetry is problematic because of interference by the hydrogen wave in acidified samples, and due to the inability to detect organically complexed zinc at natural pH values near 8 [ 185]. An improved understanding of zinc in marine systems now requires rapid, sensitive analytical methods that are less prone to contamination, and that can be performed at sea [624],... 

Because Nafion film can preconcentrate organic cations at a substrate electrode surface, Nafion-coated electrodes can be used as sensitive and selective sensors for electroactive organic cations. The film-entrapped ion is then either oxidized or reduced and the resulting current is recorded and related to the concentration of ions in the aqueous solution (qualitatively similar to anodic stripping voltammetry). It was shown that ion-exchange voltammetric determination of the dication methyl viologen at a Nafion-film-coated electrode results in an improvement of three orders of magnitude in detection limit relative to an uncoated electrode.46... 

Explain what is done in anodic stripping voltammetry. Why is stripping the most sensitive voltammetric technique What are its limitations ... 

The voltammetric sensitivity can be improved further by analyte preconcentration in conjunction with stripping analyses (cf. Chapter 5). Anodic stripping voltammetry (ASV) (Section 6.5) is the best known of the stripping techniques, and is capable of detecting concentrations as low as 10 " mol dm . Differential pulse voltammetry, when applied to stripping, can further improve the accuracy of electroanalytical measurement and, in principle, further improve the sensitivity of the technique. 

Most of our understanding of the marine chemistry of trace metals rests on research done since 1970. Prior to this, the accuracy of concentration measurements was limited by lack of instrumental sensitivity and contamination problems. The latter is a consequence of the ubiquitous presence of metal in the hulls of research vessels, paint, hydrowires, sampling bottles, and laboratories. To surmount these problems, ultra-clean sampling and analysis techniques have been developed. New methods such as anodic stripping voltammetry are providing a means by which concentration measurements can be made directly in seawater and pore waters. Most other methods require the laborious isolation of the trace metals from the sample prior to analysis to eliminate interferences caused by the highly concentrated major ions. 

Analysis. Colorimetry with proper reagents (such as nitrophenylfluo-ronone) permits analysis down to about 100 ppb. ETAAS detects Sn down to 1 ppb, and ICPMS is effective down to 0.1 ppb, as is anodic stripping voltammetry. Spot testing involves the use of cacotheline or diazine green (a dye made by reacting diazotized safranine with dimethylaniline). Sensitivity of these is about 50 ppm. 

The concentration levels of most trace metals and metalloids lie below 1000 pg P . Therefore, the classical methods of analysis do not have the required sensitivity. Among the instrumental techniques that have been extensively used for the analysis of biological materials include, atomic absorption spectrometry, plasma emission spectrometry, anodic stripping voltammetry and neutron activation analysis. 

Anodic stripping voltammetry (ASV) is a very sensitive instrumental technique for the measurement of metals in solution. Of particular importance are determinations of reactive or ASV-labile metal concentrations. ASV-labile metal is defined as the fraction of the total metal concentration that is measured under a defined set of ASV and solution conditions. Labilities of metal species in natural water have been related to toxicities. Thus, one objective of speciation analysis carried out by ASV is to find conditions where the ASV-labile fraction is a close approximation to the toxic fraction of a metal. In experimental terms, the ASV-labile metal should be equal to the electroactive fraction of the metal. The latter... 

Figure 14.7 shows the technique of anodic stripping voltammetry with collection at a double hydrodynamic electrode30. The fact that it is possible to control the potentials of generator and detector electrodes independently is used to increase sensitivity. This procedure has been used with success at rotating and wall-jet electrodes4,27... 

Olsen et al. (48, 20) have described an interesting method for the determination of lead in polluted seawater using FIA and flame atomic absorption spectroscopy. The system incorporates a Chelex-100 column for on-line preconcentration of the sample. The preconcentration and elution step improves the detection limit for lead by a factor of four (50 nM). Further increases in sensitivity are easily possible. The combination of this preconcentration step with a more sensitive detector, such as anodic stripping voltammetry, may make possible the determination of trace metals in seawater on a routine basis. 

Improvements on a computer-controlled instrument for performing trace-metal analysis by anodic stripping voltammetry are presented and discussed. The ease of operation of the instrument has been improved by the use of carbon-disc electrodes and spool-type Teflon valves. The device has been used to measure Zn, Cd, Pb, and Cu in estuarine waters recently an attempt was made to measure Cu in surface oceanic waters. Although the sensitivity and accuracy of the instrument appear insufficient for the measurement of Cu in oceanic surface waters, the approach appears promising for future work. 

Analytical methods have been developed which are sensitive enough to measure the low concentration levels of trace metals in seawater. Well defined methods, like emission spectroscopy, neutron activation analysis, anodic stripping voltammetry, atomic absorption spectroscopy, and mass spectroscopy, can be used individually or collectively to obtain the necessary data on trace metal concentrations. So why, even with these well developed methods, are we not getting reliable results from the analysis of trace metals in natural water ... 

Anodic stripping voltammetry (ASV) with the tubular mercury graphite electrode (TMGE) possesses adequate sensitivity and precision under repeated use to characterize zinc in San Diego Bay water. The TMGE, made by electrolysis of a mercuric nitrate solution to form a thin mercury film inside a graphite tube, is described elsewhere (I). 

For chemical monitoring, a list of priority substances has been established that includes metals such as cadmium, lead, and nickel. As far as metals are concerned, voltammetric techniques and more precisely electrochemical stripping analysis has long been recognized as a powerful technique in environmental samples. In particular, anodic stripping voltammetry (ASV) coupled with screen-printed electrodes (SPEs) is a great simplification in the design and operation of on site heavy metal determination in water, for reasons of cost, simplicity, speed, sensitivity, portability and simultaneous multi-analyte capabilities. The wide applications in the field for heavy metal detection were extensively reviewed (Honeychurch and Hart, 2003 Palchetti et al., 2005). 

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