The invention concerns a microfluidic system (1) comprising: at least one channel (2) for the flow of fluid having an inlet (4), an outlet (5) and a longitudinal axis (7) extending between the inlet (4) and the outlet (5), said channel (2) comprising a capture zone (3), and the cross section of the channel (2) orthogonal to the longitudinal axis (7) of the channel (2) increasing in size in the capture zone (3), from the inlet (4) towards the outlet (5) of the channel (2); and means (6) for applying a magnetic field having a decreasing intensity in the capture zone (3) of the channel (2), from the inlet (4) towards the outlet (5) of the channel. The invention also concerns a method for treating a sample that can be implemented with this system.