Terminology of elemental speciation - An IUPAC perspective

The International Union of Pure and Applied Chemistry (IUPAC) provides definitions of chemical or elemental species, and speciation or speciation analysis, that are based on different levels of chemical structure. Thus, species of an element are described at the level of isotopic composition, oxidation state, inorganic compounds, organic complexes and organometallic structures, and complexes with macromolecules. A number of examples to illustrate these various levels of speciation are presented for individual elements, with a focus on their importance to biological, medicinal and environmental chemistry. Fractionation is distinguished from speciation, a species requiring a defined structure while a fraction is determined operationally. A number of issues not fully addressed by the current definitions, such as the degree of kinetic or thermodynamic stability, analytical measurability, how to address macromolecular heterogeneity and polydispersity, and the question of carbon speciation are discussed.