Electrochemical methods with speciation

I) Faradaic electrochemical methods. From a general analytical point of view, electrochemical techniques are very sensitive methods for identifying and determining the electroactive species present in the sample and, in addition, they also are able to carry out speciation studies, providing a complete description of the states of oxidation in which the ionic species are present in the object. Other applications and improvements obtained by their hyphenation with other instrumental techniques, such as atomic force microscopy (AFM), will be described in the following chapters. 

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 advantages of SIA methodology indicated above and the commercial availability of such instrumentation generated wide interest in it and led to the development of numerous different applications in various fields of chemical analysis. They are most commonly used with spectrophotometric detection but numerous electrochemical detections were also employed (Barnett et al., 1999 Perez-Olmos et al., 2005). Sequential injection analysis methods with various detections were developed for environmental analysis (Cerda et ah, 2001 Miro and Hansen, 2006 Mesquite and Rangel, 2009), and also for speciation analysis (Van Staden and Stefan, 2004). These are considered a useful tool for clinical and biochemical analysis (Economou et ah, 2007), and even as an alternative approach to process analytical chemistry (Barnett et ah, 1999). 

Brugmann [784] discussed different approaches to trace metal speciation (bioassays, computer modelling, analytical methods). The electrochemical techniques include conventional polarography, ASV, and potentiometry. ASV diagnosis of seawater was useful for investigating the properties of metal complexes in seawater. Differences in the lead and copper values yielded for Baltic seawater by methods based on differential pulse ASV or AAS are discussed with respect to speciation. 

A number of organometallic compounds show promise for LCEC study, and a few have already been examined in detail (especially mercury alkyls) [9]. More highly conjugated organic compounds such as a,a-unsaturated ketones and imines are occasionally good candidates, but at this time UV detectors frequently outperform electrochemical detectors for such systems. At this writing there have been only a few reported LCEC studies of metal ions or metal complexes. Perhaps the major reason for this is that very little modern LC has been carried out on them using any detector. It is difficult to compete with atomic spectroscopy techniques for the determination of most elements, but as speciation becomes more important, it seems likely that more LCEC methods will be developed for metal complexes. 

The literature reports many analytical techniques used for the speciation analysis of As in multifarious matrices, including food. The usual methods are GC or LC coupled with spectroscopic or electrochemical detection [185, 248] the standard detectors are AAS, ICP AES and ICP MS [192, 246, 249]. One of the more favoured techniques for the speciation analysis of As is HG AAS coupled with... 

Chromatographic methods are especially useful for iodine speciation when coupled with ICP-MS, electrochemical detection, or chemiluminescence detection. The total iodine can be determined following various ashing procedures with a moderately low detection limit. 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

Operative conditions can be adjusted so that only metals with rate of dissociation of their complexes, within a desired range, are included in the electroactive fraction. Conditions that can be adjusted to achieve selectivity are deposition potential, electrode rotation rate, solution stirring, pulse frequency, potential scan rate, temperature, pH, etc. As electrochemical techniques require much less sample handling than other speciation methods, such as solvent extraction, dialysis or ultrafiltration, the potential sources of contamination are highly reduced. An in depth discussion of the pro and cons of electrochemical speciation is far beyond this article. Theoretical aspects and applications have been covered in great detail by Niirnberg, Florence et al., cf. ° and references therein. 

To cite a few examples, HPLC was coupled with SIA for the simultaneous determination of several heavy metals by means of nitro-PAPS (polyfiuoroalkyl phosphate esters) complexes [43]. An SIA—HPLC—atomic fluorescence spectrometry (AFS) system was proposed for As speciation in seafood extracts, implementing standard addition method for simultaneous quantification of four As species [44]. An SIA-HPLC with electrochemical detection was proposed using a homemade microcolumn SPE coupled to SIA in order to automate the sample cleanup, extraction and detection of sulfonamides [45]. 

After basic studies on the speciation of arsenic by polarography with the DME (see, e.g., references (52, 59)), recent methods are based on more efficient electrochemical stripping analysis with gold electrodes," " either in the disc configura-... 

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