A mass spectrometer is disclosed comprising a mass selective ion trap or mass analyzer arranged upstream of an ion guide. Ions are scanned out of the mass selective ion trap or mass analyzer and are received in one or more axial potential wells created or formed within the ion guide. One or more transient DC voltages or potentials are preferably applied to the ion guide in order to create a plurality of axial potential wells which are translated along the length of the ion guide. Ions are released in packets from the exit of the ion guide and are orthogonally acceleration into a drift or flight region of an orthogonal acceleration Time of Flight mass analyzer with a relatively high duty cycle.