In a mass spectrometer, a method for trapping ions includes providing at least first and second multipole rod sets positioned in tandem, introducing a plurality of ions into the first rod set, applying an RF potential to at least one of said rod sets to generate a radial trapping potential within said rod sets, applying a radial DC potential to said first rod set so as to modulate said radial trapping potential set as a function of m/z of said ions, and applying a DC potential between said two rod sets to provide an axial bias potential between said two rod sets. The method can further comprise selecting an axial barrier potential to selectively extract ions having an m/z ratio less than a threshold from said first rod set into said second rod set.