Additional set of four short, cylindrical electrodes, mounted co-linearly with a transmission quadrupole mass spectrometer that is excited with some fraction of the AC potential but not the DC potential applied to the main quadrupole. The Brubaker lens thus acts as a high-pass filter for ion m/z, but the resulting delay in application of the DC potential component also results in a reduction in defocusing by fringe fields and thus significantly improves overall ion transmission efficiency at a given mass resolution.