Mercury , amperometric detection

Electrochemical sensors based on amperometric detection are popular small instruments that can be used to directly probe samples. The electrodes in these sensors do not always have to be made of metal, e.g. platinum or mercury, and do not always have to be bare. A commonly employed working electrode is the rotating disk electrode, which is preferred over the DME for easily reduced species and anodic reactions. It works by convection mixing of the solution so that fresh sample is constantly passed over the surface. 

M.R. Hackman and M.A. Brooks, Differential pulse amperometric detection of drugs in plasma using a dropping mercury electrode as a high-performance liquid chromatographic detector, J. Chromatogr., 1981, 222, 179-190. 

W.R. Jin and Y. Wang, Determination of cysteine by capillary zone electrophoresis with end-colunm amperometric detection at a gold/mercury amalgam microelectrode without deoxygenation, J. Chromatogr. A, 1997, 769, 307-314. 

Weber, P.L. Lunte, S.M. Capillary Electrophoresis with Pulsed Amperometric Detection of Carbohydrates and Glycopeptides. Electrophoresis 1996 17, 302-309. O Shea, T.J. Lunte, S.M. Selective Detection of Free Thiols by Capillary Electrophoresis-Electrochemistry Using a Gold/Mercury Amalgam Microelectrode. Anal. Chem. 1993 65, 247-250. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

In method (c), the NOC after HPLC separation was photolyzed by a UV lamp (254 10 nm), and the charged nitrite species was determined amperometrically (79). The denitrosation reaction was found to be dependent on the wavelength of the UV light, lamp intensity, exposure time, and pH of the solution. The effluent from the HPLC column was passed through a capillary PTFE tubing coiled around a 40-W mercury lamp. The electrochemical detector used permitted either single- or dual-mode detection corresponding, respectively, to detection limits of 60 pg and 20 pg for NDMA. The method was applied to the determination of NDMA in beer and of... 

Lui et al. [109] have described an automated system for determination of total and labile cyanide in water samples. The stable metal-cyanide complexes such as Fe(CN)63 are photo-dissociated in an acidic medium with an on-line Pyrex glass reaction coil irradiated by an intense mercury lamp. The released cyanide is separated from most interferences in the sample matrix and is collected in a dilute sodium hydroxide solution by gas diffusion using a hydrophobic porous membrane separator. The cyanide ion is then separated from remaining interferences such as sulphide by ion exchange chromatography and is detected by an amperometric detector. The characteristics of the automated system were studied with solutions of free cyanide and metal-cyanide complexes. The results of cyanide determination for a number of wastewater samples obtained with this method were compared with those obtained with the standard method. The sample throughput of the system is eight samples per hour and the detection limit for total cyanide is 0.1 pg L 1. 

Electrochemical Detectors. The only electrochemical detector in current use is amperometric. However, some workers have used the term cou-lometric for detectors that operate at a high current efficiency and others have used the term polarographic when the electrode is mercury. The acronym LCEC is in common use to represent LC with electrochemical detection. 

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). 

The reaction continues and current passes until all the iodide is used up. At this point some means of endpoint detection is needed. Two methods are commonly adopted. The first uses an amperometric circuit with a small imposed voltage that is insufficient to electrolyze any of the solutes. When the mercury ion concentration suddenly increases, the current will rise because of the increase in the concentration of the conducting species. The second method involves using a suitable indicator electrode. An indicator electrode may be a metal electrode in contact with its own ions or an inert electrode in contact with a redox couple in solution. The signal recorded is potentiometric (a cell voltage vs. a stable reference electrode). For mercury or silver we may use the elemental electrodes, because they are at positive standard reduction potentials to the hydrogen/hydrogen ion couple. 

A square-wave amperometric titration has been used for the determination of the total available heavy metals in water samples. This method involves the direct anodic oxidation of mercury in the presence of excess EDTA. The resulting mercury wave is used to detect the endpoint of the amperometric titration by running a polarogram after each successive addition of an aliquot of EDTA. The successful utilization of this method lies in the ability to discriminate between Ca(n) and heavy metals, such as Cu(II) and Zn(II). Thus, in practice, it involves 1 1 dilution of samples with 0.2 mol 1 acetate buffer (pH 4.8), prior to the amperometric titration. At this pH, heavy metals, such as Fe(III), Hg(II), Ni(II), Cu(II), Pb(n), Zn(II), Cd(II), Co(II), and Al(III), are completely ( 99%) converted to EDTA complexes. Furthermore, the presence of Ca(II) does not interfere with the determination of the available heavy metals under these conditions. As little as 1 pmol 1 of available heavy metals has been successfully determined in water samples by this method. 

Bioanalytical Systems were the first to manufacture metal electrodes that would interchange with their standard GCE blocks in their amperometric detector cell. Thiols could be detected by forming an amalgam on a gold electrode. The static electrode had increased sensitivity relative to the mercury-pool electrode requiring similar low potentials but importantly was easier to operate. Nobel metal electrodes work at potentials intermediate between those of mercury-based electrodes and those required by a GCE (Figure 5.1). With the development of these metal electrodes the availability and the applicability of the methods described above were extended. 

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