Preconcentration falls under the general heading of sample preparation techniques. Any analytical technique that requires a dissolved sample, and there are many, often requires a lot of sample preparation. If after dissolving a sample by microwave digestion for example you find that the analyte concentration is too low for analysis, you need to bring the analyte to a higher concentration. There are many ways to do this, a variety of extraction techniques are used to isolate desired analytes from a sample.
For our purposes the preconcentration step can be effectively achieved in a much more low tech. fashion. Once the liquids from the microwave extraction vessels are quantitatively transferred to another container, the liquid can simply be boiled off on a hot plate (in a hood!) such that the solution volume is decreased to achieve the desired concentration. The solution is then cooled, transferred to a volumetric flask and diluted to the appropriate volume for analysis.