Metal speciation measurements

Stolzberg [143] has discussed potential inaccuracies in trace metal speciation measurement in the determination of copper and cadmium by differential pulse polarography and ASV. 

Tomaszewski, L., Buffle, J. and Galceran, J. (2003). Theoretical and analytical characterisation of a flow-through permeation liquid membrane with controlled flux for metal speciation measurements, Anal. Chem., 75, 893-900. 

Stolzberg, R.3., 1977. Potential inaccuracy in trace metal speciation measurements by DPP. Anal. Chim. Acta, 92 193-196. 

Standardisation of speciation schemes - Despite the significant advances that have been made in metal speciation measurement techniques over the past 30 years, much remains to be done. The methods that have been developed do not provide an absolute breakdown of metal species, but rather operationally defined classifications. Because of this operational nature, the standardisation of the procedures becomes essential if different results are to be compared. So far, this has rarely been achieved. 

Metal speciation procedures, which have been verified under controlled laboratory conditions and evaluated by means of bioassays, will require further verification in order to determine their ecological effects. For example, how does the response of the bioassay test species to a toxic metal fraction relate to the toxicity to larger organisms such as fish in the natural environment Bioaccumulation of metals in populations has been very difficult to relate to metal speciation measurements. There is a challenge for analytical chemists to develop metal speciation procedures that are relevant to ecotoxicology (Morrison and Wei, 1991). 

Li, W., H. Zhao, P.R. Teasdale, and F. Wang. 2005. Trace metal speciation measurements in waters by the liquid binding phase DGT device. Talanta 67 571-578. 

Because ground waters, like other natural waters, are dilute solutions of many compounds, metal speciation measurements are difficult. Therefore, the metal-complexing properties of natural waters are operationally defined by many factors, including the analytic method used for speciation, the conceptual and mathematical models used to analyze the data, the range of titrant metal concentrations used, and conditions such as pH, ionic strength, and temperature. The analytical methods used to determine metal speciation all have inherent assumptions and limitations. Most published studies of metal complexation in natural waters have used one analytical method. However, confirmation of results (e.g. stability constants, ligand concentrations) by independent methods would add confidence to such results. In the present work, three independent methods were used. 

N. Parthasarathy, M. Pelletier, M.L. Tercier-Waeber, J. Buffle, On-line coupling of flow through voltammetric microcell to hollow fiber permeation liquid membrane device for subnanomolar trace metal speciation measurements. Electroanalysis 13 (2001) 1305—1314. 

Principles and Characteristics The fastest growing area in elemental analysis is in the use of hyphenated techniques for speciation measurement. Elemental spe-ciation analysis, defined as the qualitative identification and quantitative determination of the individual chemical forms that comprise the total concentration of an element in a sample, has become an important field of research in analytical chemistry. Speciation or the process yielding evidence of the molecular form of an analyte, has relevance in the fields of food, the environment, and occupational health analysis, and involves analytical chemists as well as legislators. The environmental and toxicological effects of a metal often depend on its forms. The determination of the total metal content... 

The interpretation of previous attempts at measuring the impact of metals on microbially mediated processes has been hindered by the use of a wide range of experimental conditions and measurements. Already, a shift from studies based on total metal concentration to those based on bioavailable metal concentrations has occurred. The next step will entail accurately predicting and measuring metal speciation patterns in order to identify microbial responses to metal speciation. Only then will it be possible to develop more effective methods to quantify and mitigate deleterious effects of metals on the myriad processes that microbes mediate in the environment. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

The choice of techniques for a specific problem depends not only on the nature of the material in question but also on the types of answer required. Methods for the determination of metal speciation range from very simple separations based on bulk physical properties to detailed structural analyses, which can measure interatomic distances and orientations. The complete chemical identification of a total unknown is a different problem from one in which a distinction must be made between, perhaps as few as two, possibilities. The foregoing chapter concentrates on techniques for chemical identification, which, together with the separation techniques which are discussed in depth elsewhere, make the determination of metal speciation possible. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

Buffle, J. (1981b) Calculation of the surface concentration of the oxidized metal during the stripping step in the anodic stripping techniques and its influence on speciation measurements in natural waters. J. Electroanal. Chem. Interfacial Electrochem., 125, 273-294. 

Zhang, H. and W. Davison. 2001. In situ speciation measurements. Using diffusive gradients in thin films (DGT) to determine inorganically and organically complexed metals. Pure Appl. Chem. 73 9-15. 

In soil research, the term speciation is often applied to operationally defined fractionation of heavy metals into five or more components.25 Typically, water soluble, exchangeable, organically bound (which includes what is in biomass), amorphous oxide bound, crystalline oxide bound, and residual fractions are measured.26 Sometimes residual fractions are further subdivided according to particle size distributions to give amounts in sand, silt, and clay fractions. Similar fractionation procedures are often applied to aquatic sediments.27 In arid regions, often the calcium carbonate bound fractions of heavy metals are also measured.28 Because of the constraints of detection limits, generally only cadmium, copper, iron, manganese, and zinc are usually monitored by flame spectrometry in such heavy metal speciation studies.28... 

Recently, attempts have been made to develop biomimetic methods, simulating plant uptake of metals. An example of such a method is DGT (diffusive gradients in thin films), developed by Zhang et al. (2001), for measuring metal availability to plants. In this case, metal accumulation in a chelex layer is measured. By taking into account thickness of the diffusive layer covering the chelex layer and contact time with the soil sample, it is possible to estimate the available metal concentration in the soil solution. The DGT method may also be used to estimate metal speciation in surface water (Zhang 2004). 

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