speciation

in chemistry
https://doi.org/10.1351/goldbook.ST06861
Distribution of an element amongst defined chemical species in a system.
Source:
PAC, 2004, 76, 1033. 'Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)' on page 1070 (https://doi.org/10.1351/pac200476051033)
This definition replaces an earlier definition of speciation.