Individual speciation

Individual speciation—the most difficult type encountered in speciation analytics, involving the broadest range of analytical work. Its purpose is to separate, detect, determine, and identify all species of an element in a sample. 

In this chapter we have encountered the most important analytical aspects of acids and bases (a) their individual speciation, as described by the mass action law, and as reflected in the distribution and logarithmic concentration diagrams, (b) their buffer action, and (c) their neutralization, as exploited in acid-base titrations. 

Figure 52.5 shows that the solubility of amorphous silica is independent of pH between 4 and 9 above pH 9 the solubility increases because of the formation of monosilicate, disilicate, and multimer ions. DS coatings are deposited at 90°C — equilibrium constants are not available at this temperature. Silica solubility data are available [24], and hence it is possible to represent the insolubility-solubility domains of silica at this temperature (if not the details of individual speciation), as in Figure 52.6. The predominant reason behind Her proposing that coatings be deposited at ca. 90°C would be the increase by about a factor of 2.5 available in solubility compared to 25°C, as well as possible dehydration and porosity aspects of resultant films [25]. Included in Figure 52.6 are dashed lines that describe the trajectory of successive aliquots of aqueous silicate solution (pH ca. 13.5, point S) added to the titania slurry (at pH ca. 10). The steadily increasing silica concentration in the slurry is depicted by the solid vertical arrow in Figure 52.6.

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

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... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

The number of potential hydrocarbon isomers in the naphtha boiling range (Tables 10.1 and 10.2) renders complete speciation of individual hydrocarbons... 

Among the above-defined thermodynamic entities, the individual ionic activity coefficients are particularly useful, because they allow practical calculation of the speciation state of an aqueous phase, linking individual ionic molalities to the energy balance. We will see in the following section how these coefficients may be derived. 

Methods for Determining Biomarkers of Exposure and Effect. Nickel concentrations in hair, nails, blood, or urine are elevated in exposed individuals. A correlation has been established between nickel levels in urine, plasma, and feces in occupationally exposed workers and nickel levels in air (Angerer and Lehnert 1990 Bemacki et al. 1978 Hassler et al. 1983). If the identity of the nickel compounds to which workers are exposed is known, nickel levels in urine and plasma can be used as a biomarker for nickel exposure (Sunderman 1993). Available analytical methods can determine the nickel levels in these media in both unexposed and occupationally exposed persons. Methods to determine nickel speciation in biological media require further development. 

Biodiversity can also be considered within the species or even within the population. This is called genetic diversity, signifying that each individual has its own genetic make-up. This allows the species to adapt to environmental changes and furnishes the seed for speciation, by which biological evolution occurs. 

Individual VOC. The term VOC is commonly used to describe speciated measurements of individual organics. The almost universal approach to the identification and measurement of individual VOC is GC with either FID or mass spectrometry (MS). GC-MS is used to establish the identity of a particular compound through the combination of retention times and mass spectra and, of course, can also be used for quantification. However, for a given type of air mass, GC-FID is commonly used for more extensive quantitative measurements after the individual peaks have been identified. For reviews of various aspects of sampling and measurement of VOC in air, see Westberg and Zimmerman (1993), Apel et al. (1994), Klemp et al. (1994), Sacks and Akard (1994), and Dewulf and Van Langen-hove (1997). 

In the last few years, we have seen the application of isotope dilution methodologies to some new analytical fields. One of these is elemental speciation , where the aim is to determine individual chemical species in which an element is distributed in a given sample. IDMS has also proved its usefulness in element speciation, in which either species-specific or species-unspecific spikes can be used. For example, species-specific IDMS is nowadays used in several laboratories as an effective tool to validate analytical procedures for speciation and to investigate and document eventual interconversion between species. In addition, the study of induced variations in the isotopic composition of a target element can also provide insight into various (bio)chemical and physical processes isotopic analysis is, therefore, also of increasing importance in biological studies. 

Speciation. The method used to resolve a complex air matrix into individual species is dependent on the collection procedure that was used. Gaseous samples are separated into the individual components with gas chromatography, whereas samples in liquid media (derivatized and absorbed) are usually resolved on a liquid or ion chromatograph. 

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