Abstract: An apparatus for a selective fractionation of ultrafine particles includes at least three separating columns fluidically connected in series by connecting lines. An infeed is arranged to feed into a connecting line which is arranged upstream of each separating column. Each connecting line comprises an inlet for a suspension of ultrafine particles to be separated and an inlet for at least one additional mobile phase. The inlets are alternately operated. A discharge branches off from a connecting line which is arranged downstream of each separating column. Each connecting line comprises an outlet for a first and a second discharge suspension of the ultrafine particles. The outlets are alternately operated. A control means provides a simultaneous switching of the through-flow switching position of the shutoff valves at the inlets and outlets. At least one magnetic field source for a magnetic field is arranged in each separating column.