Speciation species transformation

The most frequent, reliable approach for element speciation in real samples today uses combined - or hyphenated - techniques, where species are selectively separated (e.g., by a chromatographic technique) and then the elements in the various chemical forms are selectively detected. To enhance the quality of measurements, molecule-selective detection is also coupled to separation devices. These combinations provide extended flexibility and broad applicability. Disadvantages are that, as complexity increases, the risk increases of system failure. Besides, species equilibrium can be drastically altered during separation due to dilution, some components can be removed, and so on. Under such conditions, species transformation and destruction is a likely consequence. Therefore, the total separation time should be shorter than the transformation rate of species. Figure 17.1 gives an overview of the strategy and various speciation methods mostly used for element speciation in human milk. 

Elemental As is not considered poisonous, but many arsenic compounds are extremely toxic. Arsenic differs in toxicity in different oxidation states. Trivalent arsenic (As(III)) is considerably more toxic than pentavalent arsenic (As(V)). The toxicity of arsenic is not only dependent on its chemical form but also on its solubility and mobility in soils. Understanding the speciation and species transformation of arsenic is essential to determining its toxic effect on plants and animals. The chemical form determines the availability to animals and plants. 

Generally, the sample should be measured after sample collection as quickly as possible with as limited a sample preparation as possible. If sample storage is necessary for some time, subsamples should be stored under different conditions to allow the identification of species transformation during storage (e.g., oxidation, methylation, or demethylat-ion). Lower temperatures, — 20°C to 4°C, are generally more useful than the addition of preservatives and acid, which may have an influence on the speciation. 

Chapters on sample introduction and hyphenated sample treatment and ICP systems have also been further updated since the last edition. No doubt that chromatographic, electrophoresis, flow injection and field flow fraction separations have extended ICP-MS (and AES) measurements as the mainstay of elanental specia-tion measurements in biological and environmental fields. Without the combination of these separation techniques and ICP measurements, elemental speciation applications would be severely hampered... if not impossible (Chapter 18). The ability to measure P and S with high sensitivity has opened up new opportunities in proteomics, for example. Species-specific and unspecific isotopic dilution (ID-MS) has been critical in quantifying speciation analysis and revealing recovery errors (Chapter 13). Species-specific techniques have been applied to identify species transformations, resulting in the development of multi-species methods whereas, hyphenated species-unspecific ICP-ID-MS determinations of heteroatoms such as sulfur have become a common quantification technique in proteomics. 

The core techniques developed for mercury speciation take advantage of gaseous species separation after an initial derivatisation step. However GC may lead to some problems and there is increasing motivation to use LC separation methods, for which the derivatisation step and all implications connected with it, like species transformation, etc., is avoided. With LC, inorganic mercury and methylmercury compounds can be determined, but it is also possible to analyse less volatile or non-volatile species such as mersalic acid or aromatic mercury compounds, which are not accessible for GC separation. 

Wang, X-M and Waite, T D (2010), Iron speciation and iron species transformation in activated sludge membrane bioreactors , Water Research, 44 3511-3521. 

It must be noted that sometimes calcination is beneficial to create active species. Notable examples are the Sn-beta speciation [176] and generation of extra-framework Al-Lewis sites in beta zeolite for organic transformations... 

The list of elements and their species listed above is not exhaustive. It is limited to the relatively simple compounds that have been determined by an important number of laboratories specializing in speciation analysis. Considering the economic importance of the results, time has come to invest in adequate CRMs. There is a steadily increasing interest in trace element species in food and in the gastrointestinal tract where the chemical form is the determinant factor for their bioavailability (Crews 1998). In clinical chemistry the relevance of trace elements will only be fully elucidated when the species and transformation of species in the living system have been measured (ComeUs 1996 Cornelis et al. 1998). Ultimately there will be a need for adequate RMs certified for the trace element species bound to large molecules, such as proteins. 

In some cases, resins have been used to try to determine only the plant or more generally the biological availability of an ionic species. Resins placed in soil have also been used to study ion speciation, soil microbiology, various phosphorus measurements, soil nutrient supply rate, nutrient transformations and movement, and micronutrient and metal toxicity [22-25],... 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

Temperature-time transformation The temperature-time transformation, or T-t-T method (e.g., Seifert and Virgo, 1975), is the oldest method in geospeedometry. In this method, a reasonably high initial temperature is given, and equilibrium species concentrations are calculated. This speciation is assumed to be the initial speciation. The final species concentrations after cooling down (i.e., at present day) are measured and hence known. To reach the present-day species... 

In 2006, the speciation of metals and metalloids (As, Bi, Hg, Pb, Sb, Se and Sn) associated with alkyl groups and biomacromolecules in the environment was critically reviewed by Hirner.85 More than 60 species of alkylated metals and metalloids have been found in different ecosystems and terrestrial locations all over the world.85-87 These alkylated metals or metalloids are of interest due to their toxicological properties (e.g. monomethyl mercury, MMHg, which gained worldwide attention during the Minamata tragedy, and are not only known to be produced by microbial methylation within most anaerobic compartments of the environment, but also in the course of enzymatic transformation during human metabolism.85... 

Another intensively studied element in speciation analysis is arsenic. The biological and environmental effects of arsenic species and their transformation pathways have been studied in numerous papers.40- 42 Both arsenite and arsenate accumulate in living tissues because of their affinity for proteins, lipids and other cellular compounds.43 Arsenic species can undergo transformation via... 

The transformation of contaminants by sulfur species in anaerobic environments can involve both reduction and nucleophilic substitution pathways. These processes have been studied extensively (67-74), but the complex speciation of sulfur makes routine predictions regarding these reactions difficult. 

Figure 2.2 According to Darwin s theory of natural selection, species evolved by a mechanism of small continuous changes that today is known as phyletic gradualism. In this framework, evolutionary change can be realized by two distinct processes (A) a gradual transformation of a species without any increase in the total number of species (phyletic transformation), and (B) a gradual separation of an ancestral species in two or more descendant species (phyletic speciation).

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