The invention pertains to a method and apparatus to separate and quantify particles using time-variable force fields. The force fields can be for dielectrophoresis (positive or negative), electrophoresis or electrohydrodinamic. In a first aspect of the method, the fields are translated and/or modified in space at a speed substantially comparable to the speed of translation of the fastest particles in the sample so that only these follow by changing position, while the slowest particles are not affected. According to the invention the translation and/or modification of the force field can also occur with varying speed, which is especially useful when this happens with periodic law on a field with spatial periodicity. In one aspect of the method, a force field with spatial periodicity is translated and/or modified in a first direction at high speed, for such a period of time as to cause a movement equal to the spatial period of the field, and at low speed in a second direction, opposite to the first one, for s
