One of the advantages for field-flow fractionation is that the sample fractions obtained from one or several runs can be concentrated and reinjected in-line. Procedure of this method is simple and the trouble caused by impurities contained in carrier solvent can be avoided. This technique can be used to study the band broadening effect caused by the system or to recycle a particular fraction of polymers/particles with broad molecular weight/particle size distribution. The approach for carrying out this concentration and reinjection procedure is shown in detail. Experiment for thermal field-flow fractionation is described by using polystyrene with molecular weight 20.6×106

but the principle is unversal for other kind of FFF subtechniques.