The International Union for Pure and Applied Chemistry (IUPAC) has published guidelines or recommendations for the definition of speciation analysis:
Speciation analysis is the analytical activity of identifying and/or measuring the quantities of one or more individual chemical species in a sample.
The chemical species are specific forms of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure.
The speciation of an element is the distribution of an element amongst defined chemical species in a system.
In case that it is not possible to determine the concentration of the different individual chemical species that sum up the total concentration of an element in a given matrix, that means it is impossible to determine the speciation, it is a useful practice to do fractionation instead.
Fractionation is the process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g. size, solubility) or chemical (e.g. bonding, reactivity) properties.
D.M. Templeton, F. Ariese, R. Cornelis, L.-G. Danielsson, H. Muntau, H.P. van Leeuwen, R. Lobinski, IUPAC Guidelines for Terms Related to Speciation of Trace Elements, Pure Appl. Chem., 72/8 (2000) 1453-1470.
Many authors talk about trace element speciation when they actually only present methods and results for fractionation such as sequential leaching. On the other hand, some authors present data on trace element speciation, but never use the term "speciation". This is for example frequently happening for publication s in the field of isotopes.