A method and apparatus for chemical ionization of analyte gas particles in a carrier gas by introducing primary ions, characterized in that the primary and product ions are accelerated by a rotating electric field orthogonal to that direction (5) in which the ions are transported towards the exit (3) of the reaction volume (1). This can, for example, reduce unwanted cluster formation without increasing the transport speed of the ions through the reaction chamber, which improves, for example, the product ion yield. The apparatus of the invention achieves this by means of N≥3 rod electrodes (6) to which N AC voltages U1(t), . . . , UN(t) with N different phase positions ascending in one sense of rotation φ1, . . . , φN are applied.