Preconcentration means to increase the concentration of a sample prior to analysis or detection. For example you can do preconcentration for an non-volatile organic sample with evaporation its solvent.

What is meant by sample pre-concentration?

Pre-concentration just refers to the process of concentrating a sample before analysis, so that trace components won't be overlooked.

How do you pre concentrate a sample?

Solvent removal is the most simple method for preconcentrating species in solution. In some cases, though not always, the sample is heated to dryness. For a detection limit of 0.1-0.01 g, the water volume to be evaporated in order to ensure precise analyses at the parts-per-billion level is 10-100 mL.