A device for mass selective determination of at least one ion or of a plurality of ions is used, for example, in a measuring apparatus having an ion trap. The ion trap has a ring electrode having a first opening. A first electrode is arranged at the first opening. Furthermore, an amplifier for providing a radio-frequency storage signal for the ion trap and a first transformer are provided, said first transformer being connected to the amplifier and the first electrode in such a way that the radio-frequency storage signal is coupled into the first electrode via the first transformer.