Types of Chemical Species

The International Union for Pure and Applied Chemistry (IUPAC) has published guidelines or recommendations for the definition of speciation analysis. According to these definitions, chemical species are specific forms of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure.

In the literature the following types of elemental species are often distinguished:

Type: Oxidation species
Examples: Fe(II)/Fe(III); As(III)/As(V); Sb(III)/Sb(V); Se(IV)/Se(VI); Cr(III)/Cr(VI)
Interest: Study of redox processes; Different toxicity

Type: Complexes
Examples: Cis-Platin; Humic acid complexes, Gd-based contrast agents
Interest: Study of complex stability, study of the influence of ligands on complex characteristics such as mobility, bioavailability and toxicity

Type: Organometallic compounds
Examples: Methylmercury; Tetraethyllead; Tributyltin; Arsenobetaine
Interest: Different toxicity; bioavailability and bioaccumulation in organisms; contamination of food; fate in the environment; degradation reactions

Type: Biomolecules
Examples: Metalloproteins; Arsenosugar; Selenoamino acids
Interest: Study of biological function; Study of metabolism;

The different types of species have different stabilities and therefore require different attention with respect to sampling, sample preparation and analysis. Oxidation species and complexes do have limited thermodynamic and kinetic stability and therefore can easily be transformed during analysis. Biomolecules do have limited stability mostly under biological conditions with respect to temperature, pH and redox state and can easily be degraded during analysis under harsh conditions. Organometallic compounds are the most stable chemical species and therefore can often be separated even under more harsh conditions as used for gas chromatographic separation.

Related EVISA Resources: Brief summaries