Ions and charged droplets move from the nozzle (6) towards the orifice (22) of a charged-particle transport device or the desolvation pipe (7). This particle motion is governed by the distribution of the pseudo-potential along particle trajectories. There are RF-voltages applied to neighboring electrodes (241-246) of the electrode array (24) cause the charged particles to substantially hover above the electrode array (24). Right before the ions come to the electrode array (24) they thus experience a repelling force “F” perpendicular to the surface of the electrode array (24). This force “F” causes an effective barrier (B) right before the electrode array (24) and consequently a pseudo-potential well (A) where the charged particles stop their motion parallel to the plume axis (D). Thus they accumulate around the center line (C) of this well (A). Applying additionally to the RF-potentials also DC-potentials to neighboring electrodes within the electrode array (24) small DC-fields can be formed within the well area (23). These additional DC-fields drive the charged particles towards the axis of symmetry (C) and thus towards the orifice (22) of a charged-particle transport device or the desolvation pipe (7). Thus, many of the charged particles which would normally impinge on the wall (21) around the orifice (22) can now be analyzed.