Ion trapping scheme with improved mass range

A method of trapping ions in an ion trapping assembly comprising an ion trap (100, Fig. 2) and an ion cooling device, e.g. a collision cell (120), comprises: applying an RF trapping voltage having a first amplitude V1 to the ion trapping assembly to capture introduced ions within a first range of m/z ratios; cooling said trapped ions; and then reducing the amplitude of the RF voltage to a second, lower amplitude V2 to reduce the low mass cut-off (LMCO) of the ion trapping assembly; and trapping, at said second RF amplitude, introduced ions within a second range of m/z ratios, the lower mass limit of the second mass range being below the LMCO when the first RF amplitude is applied. Further intermediate RF trapping amplitudes (V3-V5, Fig. 6) may be applied to the trapping assembly to successively reduce the LMCO and the lower mass limit of introduced ions that are trapped therein. The same effect may also be achieved by increasing the frequency of the RF trapping voltage.