An operation (process) as the result of which microcomponents are transferred from the sample of larger mass into the sample of smaller mass, so that the concentration of the microcomponents is increased. Examples include the decrease in solvent volume during distillation or evaporation, and the transfer of microcomponents from an aqueous solution into a smaller volume of organic solvent by extraction.
PAC, 1979, 51, 1195. 'Separation and Preconcentration of Trace Substances. I-Preconcentration for Inorganic Trace Analysis' on page 1197 (https://doi.org/10.1351/pac197951051195)