Cd speciation

Figure 3.2. Changes of Cd speciation in soil solutions of a typical Israeli calcareous soil with pH 4-9 (after Hirsh and Banin, 1990, with permission from Soil Sci. Soc. Am)...

Xue, H.-B. and Sigg, L. (1998). Cd speciation and complexation by natural organic ligands in freshwater, Anal. Chim. Acta, 363, 249-259. 

Potentially toxic metals such as Al, Cd, and Pb have been reported in breast milk. As they may also contaminate drinking water it is not surprising that they are found in infant formulas at levels even higher than in breast milk. Cow s milk-based formulas exhibit lower toxic metals levels [13]. For example, the exposure to Cd for an infant fed a soy formula turned out to be about 20 times higher than for a breast-fed infant. Interestingly, gastrointestinal Cd uptake is believed to be higher in newborns than in adults. So far studies on Cd speciation in formula milk are very scarce, but it has been reported that Cd in formulas is distributed between caseins and LMW compounds [12]. 

B. Michalke, P. Schramel, Protein fraction and Cd-speciation in human breast milk by HPLC and voltammetry, J. Trace Elem. Electrol. Health Dis., 4 (1990), 163-167. 

Dissolved Organic Matter-Solution Speciation Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on measured total dissolved Cd concentration and computation of free Cd2+ using chemical equilibrium models. The accuracy of the... 

In order to quantify these tractions and to assess the mobility and bioavailability of different forms, scientists have attempted to extract the different fractions using a range of chemical extractants. Soil Cd speciation is fundamental to understanding the mobility and bioavailability of different chemical forms of soil Cd. 

Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on the measured total dissolved Cd concentration and computation of free Cd using chemical equilibrium models. The accuracy of the speciation depends on the correctness of Cd-complex stability constants (Turner, 1995). For example, the proportion of free Cd (as Cd ) in solution varies from 16 to 82% depending on the log K values for Cd-(DOM) dissolved organic matter complexes (Table 4). The stability constants of Cd-humics are particularly uncertain. The reported high values of pCd + ranged... 

Although much effort has been spent testing different soil extraction techniques for characterizing metal phytoavailability, Sharma and Shupe (1977) found a surprisingly good relation (r = 0.883 for Cd) between total metal concentrations in the soil and total metal concentrations in plants. The statistical relationship between total soil and plant metal concentration may be fortuitous, since the total amount of Cd in soil is seldom indicative of its effect on Cd accumulation in plants (Cottenie et al., 1983). More attention should be paid to Cd speciation in relation to Cd bioavailabihty, especially in the rhizosphere. 

It is advantageous to combine theroretical (thermodynamic) calculations, sequential extraction schemes, and spectroscopies (e.g., synchrotron-based methods, e.g. EXAFS and XANES) to advance our understanding of Cd speciation in the rhizosphere which is bathed in root exudates and microbial metabohtes. This information is fundamental in order to advance the frontiers of knowledge on the biogeochemistry of soil Cd at the molecular level. 

The preconcentration step is in particular useful in a saline matrix (e.g., seawater Cd speciation). For instance, ICP-MS is susceptible to polyatomic ion interferences from seawater and its performance is much better after such a matrix removal step. 

The development of routine and easy handling procedures for continuous and real-time speciation of trace metals in waters has led, in the last years, to the development of microsensors coupled to voltammetric techniques. Microelectrodes offer several advantages for speciation measurements in real-world samples, including their application in low ionic strength media (e.g., freshwaters), reproducibility, and sensitivity. Some Cd speciation studies carried out in river waters, heavily loaded with suspended material, using microelectrodes demonstrated that most of Cd was associated with colloidal material. In addition, this technique also enables the determination of the corresponding complexation stability constants for Cd and protons. 

MTs in invertebrates have also been named MT-like proteins, MLPs, because they differ significantly from those of vertebrates. In spite of these differences, both vertebrate and invertebrate MTs have been isolated for Cd speciation analysis using similar analytical procedures. A typical preparative procedure to extract MTs from animal tissues is given in Scheme 1. 

On the other hand, it is known that Cd accumulation in soil and water now poses a major environmental problem due to various industrial and agricultural activities (as reported in previous sections). In fact, the first papers on Cd speciation were mainly focused on such environmental issues. 

Although few studies have been carried out for Cd speciation in biological fluids (e.g., human serum, human milk, cow milk, and formula milk), in cow and formula milk Cd seems to be bound to caseins while in human serum Cd appears to be complexed by ceruloplasmin, a well-known glycoprotein for metal storage. 

Due to nanoliter per minute flows of CE the concentration-based detection limits are seriously degraded in comparison with those obtained by LC, limiting the application of CE in Cd speciation analysis of real samples. In any case, there are CE strategies (e.g., large volume sample stacking) that are able to preconcentrate the analytes and have been successfully applied to real samples in order to improve the detectability of Cd species in MTs by CE-ICP-MS. Figure 2 shows typical electrophoretic results by a hybrid technique for speciation of Cd-MTs in fish samples of environmental monitoring interest. 

Cu, and Zn bound to MTs (such metal-MTs concentrations are interrelated). In this sense, LC-ID-ICP-MS has been applied to the speciation of Cd, Cu, and Zn in liver and kidney of fish under Cd stress. In Figure 4, the Cd speciation profile obtained by ID-ICP-MS in fish liver after online separation by anionic-exchange chromatography is illustrated. The ID quantification of Cd in each chromatographic peak provides the metal amount (nanogram) bound to different proteins. As can be seen, the relative amount of Cd (peak area) bound to each protein is considerable, pointing out the importance of Cd speciation studies in investigating bioindicators of environmental contamination. 

On the other hand, combination of CE with ID-ICP-MS has also been successfully applied to Cd speciation, but using a double focusing ICP-MS detector. The determination of Cd, Cu, and Zn molar ratios in MTs, using such instruments, which enables the determination of and without interferences, has already been described. 

Crews et al. have applied their size-exclusion separations to a study of Cd speciation in pig kidney. Cadmium in different proteins could be distinguished in cooked, uncooked, and digested pig kidney. The bulk of the soluble cadmium in retail pig kidney was associated with a metallothionein-like protein, some of which survived cooking and digestion. The detection limits of ICP-MS were sufficient to permit studies of Cd speciation at normal levels of Cd previous work on this subject by other methods was generally restricted to abnormally high levels of Cd [45]. 

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