What does 'preconcentration techniques' mean?
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.

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.(IUPAC)



There are lots of methods for preconcentration: solvent evaporation, using a different phase (ex: a solid or liquid that could absorbs sample from the solvent) and ...