preconcentration

in trace analysis
https://doi.org/10.1351/goldbook.P04803
An operation (process) as a result of which the ratio of the concentration or the amount of microcomponents (trace constituents) and macrocomponents (matrix) increases. The term enrichment is not recommended.
Source:
PAC, 1979, 51, 1195. (Separation and Preconcentration of Trace Substances. I-Preconcentration for Inorganic Trace Analysis) on page 1197 [Terms] [Paper]