Stability studies speciation analysis

Many problems occur in Se-speciation analysis, owing e.g. to risks of adsorption on container walls, instability of species or contamination, insufficient separation efficiency of the chromatographic techniques, problems of conversion yield of selenite to selenate etc. Prior to conducting an interlaboratory project on this topic, it was hence decided to assess the stability of selenite and selenate according to various factors (effects of container materials, additives, temperature and light). The study focused on tests of effects of physicochemical parameters on solutions stored in polyethylene and PTFE containers. Container volumes were 100 and 500 mL for polyethylene and 500 and 1000 mL for PTFE. Stock and initial working solutions were prepared in 1 and 5 L polyethylene containers previously cleaned with nitric acid (at pH 2) and rinsed with Milli-Q water. The stock solutions were prepared with sodium selenite and sodium selenate (purity >98%). 

Specific chapters then focus on different projects on speciation analysis. Chapter 4 deals with interlaboratory studies on methylmercury in fish and sediment Chapter 5 describes the collaborative projects to certify organotins in sediment RMs and mussel tissues Chapter 6 gives an overview of the certification project on trimethyllead in simulated rainwater and urban dust Chapter 7 describes the certification project on arsenic species in fish tissues Chapter 8 focuses on the intercomparison and tentative certification of Se(IV) and Se(VI) in simulated freshwater Chapter 9 deals with a feasibility study to stabilize Cr species in solution followed by the certification of Cr(III) and Cr(VI) in lyophilized solutions and welding dust Chapter 10 gives a review of methods used for A1 speciation Chapter 11 develops the overall collaborative project to standardize single and sequential extraction procedures for soil and sediment analysis, followed by interlaboratory studies and certification of soil and sediment reference materials. 

The project to improve the quality control of lead speciation analysis was started in 1990 by a feasibility study on the stability of alkyllead species in solution [118], and was concluded in 1991. The first interlaboratory study was conducted in 1992 [119] and was followed by a second exercise carried out in 1993 [120]. The certification campaign of trimethyllead in artificial rainwater and urban dust was conducted in 1995-96. 

Martin 89) has provided an extensive survey of the bioinorganic chemistry of AF that is of central importance to considerations of AF speciation. Much of his analysis concerns the tabulation and interpretation of stability constants and we shall not duplicate this. In the following sections we describe spectroscopic studies of AF speciation that complement calculations based on stability constants. 

The stability constant is probably the most important quantitative parameter for the characterization of a metal-ligand complex in that it provides a numerical index of the affinity of the metal cation for the ligand and allows the development of quantitative models able to predict the speciation of metal ions in the system studied. Several different theoretical and experimental approaches have been attempted for the determination of stability constants of metal—HS complexes and modeling metal-HS complexation reactions. Data analysis and interpretation is, however, still controversial, due to the intrinsically complex and ill-defined nature of HSs. The multiligand, polyelectrolitic nature of HS macromolecules results in the inability to describe quantitatively the types, concentrations, and strengths of the several nonidentical binding sites in HSs and in the impossibihty to ascertain and measure the stoichiometry of metal-HS complexation (MacCarthy and Perdue, 1991). 

A recent review has highlighted the stability of chemical species (in speciation studies) with respect to environmental matrices [1], These results are summarized in Table 4.2. It should be noted that samples must be analysed as quickly as possible after collection. The times given in this table are the maximum times that samples should be stored before analysis. It should also be noted that the guidelines given are general in nature. For example, many organophosphorus pesticides (not shown in Table 4.2) can be preserved by the addition of hydrochloric acid. However, as an exception to this, the (organophosphorus) pesticide diazinon breaks down when acidified. 

Abstract This chapter reports an analysis of literature dedicated to the speciation of cadmium in various environmental compartments, i.e., atmosphere, natural waters, soils and sediments. The difficulty of the cadmium speciation studies, due to the variability of composition of different natural systems and to the low cadmium concentration in the environment, is highlighted. As an alternative approach, cadmium behavior is assessed by modelling its reactivity towards the main classes of ligands usually present in natural systems. The stability of cadmium complexes with... 

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