A mass spectrometry method in which a trapping field signal (such as a three-dimensional quadrupole trapping field signal or other multipole trapping field signal) set to store ions of interest is superimposed with a notch-filtered broadband ("filtered noise") signal, and ions are formed or injected in the resulting combined field. The filtered noise signal resonates all ions (except selected ones of the ions) from the combined field, so that only selected ones of the ions remain trapped in the combined field. The combined filtered noise and trapping field signal (the "combined signal") is then changed to excite the trapped ions sequentially, so that the excited ions can be detected sequentially. The invention can be applied to perform an (MS).sup.n or CI, or combined CI/(MS).sup.n, mass spectrometry operation.