In one aspect, a mass spectrometer is disclosed, which comprises a Fourier Transform (FT) mass analyzer having an input port for receiving ions and an exit port through which the ions exit the FT mass analyzer, a detector disposed downstream of said FT analyzer for detecting ions exiting the FT analyzer, and a multi-segment ion guide having a plurality of segments, said multi-segment ion guide being disposed upstream of said FT mass analyzer and having an input port for receiving ions and an output port through which ions exit the FT mass analyzer. The segments of the ion guide are configured to be independently activated via application of a DC offset voltage thereto so as to adjust a length through which ions passing through the ion guide experience collisional cooling.