A mass spectrograph in which vaporised ions are confined by an AC electric field. As the radio-frequency voltage that confines the ion is changed, the ion will escape the trap at a voltage characteristic of its mass-to-charge ratio, and be detected. Sequences of polypeptides and polynucleotides may be determined by expulsion from the trap of all but one kind of large mass ion, fragmentation of this ion by collision with helium atoms and analysis of the mass spectrum of the resulting fragments. A variant of this electrical-type ion trap uses a magnetic field to confine ions.