True speciation

The most widely studied interactions between biologically active ligands and organotin(lV) cations relate to the amino acids and their derivatives (N- or S-protected amino acids and peptides), though new data on several of the most commonly occurring amino acids are still being published. This is specially true for aqueous speciation studies. Nice and very detailed reviews were published in this area by Molloy and Nath. ... 

Presently, the precise determination of the true dissolved Th fraction in water samples remains a challenge. Results from ultrafiltration experiments on organic-rich water samples from the Mengong river tend to demonstrate that Th concentration is less than 15 ng/L in absence of DOC (Table 2 and Viers et al. 1997), and that Th is still controlled by organic carbon in the final filtrate of the ultrafiltration experiments. The latter conclusion is also supported by the results obtained for the Kalix river (Porcelli et al. 2001). These results therefore not only raised the question of the determination of the amount of dissolved Th in water but also of the nature of Th chemical speciation. 

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

This section is based upon the papers of Johnson and Pytkowicz (7j and of Sipos et al. (8). The concepts used are the formation of coulombic ion-pairs between the major ions of seawater (NaCio, NaHC03°, NaC03", NaS04, and similar pairs for Ca2+ and Mg2+) and the formation of true complexes such as PbCl+, PbCl2°. CdCl+, PbOH+, PbC03o, etc. The coexistence of these two types of entities implies competition, e.g., for Cl ions, one has NaCl° and PbCl+, and yields trace metal speciation quite different from those obtained in the absence of ion pairs. In Table IV are shown the fractions of the major ions which are free and ion-paired. 

Uncorrected and corrected results for the trace metal speciation of lead are presented in Table V. It can be seen that the competition between true complexes and coulombic ion-pairs modifies considerably the speciation of lead. The fractions do not add up to 100% because species such as PbCl2° Pbc ", etc., were not entered into the table. 

The discipline of analytical chemistry is wide and catholic. It is often difficult for a food chemist to understand the purist concerns of a process control chemist in a pharmaceutical company. The former deals with a complex and variable matrix with many standard analytical methods prescribed by Codex Alimentarius, for which comparability is achieved by strict adherence to the method, and the concept of a true result is of passing interest. Pharmaceuticals, in contrast, have a well-defined matrix, the excipients, and a well-defined analyte (the active) at a concentration that is, in theory, already known. A 100-mg tablet of aspirin, for example, is likely to contain close to 100 mg aspirin, and the analytical methods can be set up on that premise. Some analytical methods are more stable than others, and thus the need to check calibrations is less pressing. Recovery is an issue for many analyses of environmental samples, as is speciation. Any analysis that must... 

So what is the gist of all these explanations They serve to make room for a new interpretation of the fossil record. According to the new model speciation was a fait accompli when the first animals began to produce fossils. It happened even earlier, the true edge-condition for speciation must be placed at the Hadean-Precambrian transition and that means that it was a chemical and not a biological phenomenon. 

The term species generally refers to the molecular forms of an element or a cluster of atoms of different elements in a given (in this case solid) matrix (Bernhard et al., 1986a). The term form is also used to indicate uncertainty or lack of knowledge about the exact nature of the species one expects to find in an environmental sample. Conditional speciation assessment techniques like sequential extractions or leaching tests are not able to yield information at the true molecular level in solid phases thus the more general term form is used rather than species when referring to the results of such techniques commonly applied to date to soils and sediments. 

Just choosing the most widely applied procedure (namely that of Tessier et al., 1979) could yield data of doubtful reliability for a particular matrix or objective, but may nevertheless allow comparison with results of many other studies. In practice, there is always an optimisation necessary between compatibility and reliability. The limitations reported here and elsewhere lead to the conclusion that results given by sequential sediment extraction experiments can be used for an assessment of specific release scenarios particularly related to changing pH, complexing ligand availability and redox environments rather than for true metal speciation in sediments. The latter can be achieved only by using intrumental speciation techniques, either alone or in combination with sequential extraction. It is in this area of research that new developments have appeared since the first edition of this volume. Particularly... 

In a review of available data relating to the physico-chemical speciation of plutonium in the Irish Sea and western Mediterranean, Mitchell et ai. (1995) concluded that a high percentage of the plutonium is present as Puv and not retained by a 1 kD filter, thus demonstrating that plutonium in the oxidised state is in true solution. The data also indicate that a significant proportion of plutonium in the reduced state is associated with colloids and that the size of the colloidal particles or aggregates involved (<10kD) is considerably smaller than those observed in non-saline waters. 

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