An apparatus for separating ions spatially and in sequential order of mass-to-charge (m/z) ratio, the apparatus comprising: an electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) comprising a linear quadrupole ion guide having a length defining a longitudinal axis which extends in an axial direction between a first end thereof and a second end thereof, the second end opposite the first end, and the first end being configured to introduce a beam of ions into an ion transmission space of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), the beam of ions comprising ions having m/z ratios within a first range of m/z ratios; and an electronic controller in electrical communication with the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) and configured to apply an RF potential and a DC potential to at least an electrode of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) for generating a ponderomotive RF electric field that urges ions radially toward the longitudinal axis of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), and a DC electric field that urges ions out of the ion transmission space of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) in a direction orthogonal to the said longitudinal axis thereof, such that a ratio of the strength of the ponderomotive RF electric field to the strength of the DC electric field varies monotonically along the length of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), wherein the generated electric field supports the extraction of ions having different m/z values at respective different positions along the length of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), in one of increasing and decreasing sequential order of m/z ratio with increasing distance from the first end. an electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) comprising a linear quadrupole ion guide having a length defining a longitudinal axis which extends in an axial direction between a first end thereof and a second end thereof, the second end opposite the first end, and the first end being configured to introduce a beam of ions into an ion transmission space of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), the beam of ions comprising ions having m/z ratios within a first range of m/z ratios; and an electronic controller in electrical communication with the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) and configured to apply an RF potential and a DC potential to at least an electrode of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) for generating a ponderomotive RF electric field that urges ions radially toward the longitudinal axis of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), and a DC electric field that urges ions out of the ion transmission space of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400) in a direction orthogonal to the said longitudinal axis thereof, such that a ratio of the strength of the ponderomotive RF electric field to the strength of the DC electric field varies monotonically along the length of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), wherein the generated electric field supports the extraction of ions having different m/z values at respective different positions along the length of the electrode arrangement (400;500, 502, 504, 506; 700; 800; 400A, 400B; 1100; 1200; 1300; 1400), in one of increasing and decreasing sequential order of m/z ratio with increasing distance from the first end.