relative preconcentration

in trace analysis
An operation (process) as a result of which the ratio of concentration or amounts of microcomponents and main macrocomponents increases. This is a particular example of a separation, but at greatly differing concentrations of components. The ratio of the masses of the initial and final samples is not of great importance. Frequently, the main purpose of relative preconcentration is the replacement of a matrix unsuitable for analysis by a suitable medium. In some cases, it is difficult to trace a boundary between the absolute and the relative preconcentrations as, for example, in preconcentration by zone melting.
PAC, 1979, 51, 1195 (Separation and preconcentration of trace substances. I - Preconcentration for inorganic trace analysis) on page 1197