Extraction wherein extractants of increasing strength or different chemical character are used in order to extract analytes of interest.
Note: The most commonly used approaches in elemental analysis are the BCR (now termed Standards, Measurement and Testing Program) and Tessier procedures.
Source:
PAC, 2016, 88, 517. 'Glossary of terms used in extraction (IUPAC Recommendations 2016)' on page 526 (https://doi.org/10.1515/pac-2015-0903)