A time-of-flight mass spectrometer, comprising: an ion source for supplying sample ions, a segmented linear ion storage device having a longitudinal axis for receiving sample ions supplied by the ion source, at least one segment (15, 17) of the ion storage device defining an extraction region of the device; voltage supply means for supplying, in use, to the ion storage device trapping voltage effective to trap sample ions, or ions derived from sample ions in said extraction region to form an ion cloud and for supplying to the ion storage device extraction voltage for causing ejection of the ion cloud from said extraction region in an extraction direction orthogonal to said longitudinal axis, and a time-of-flight mass analyser for performing mass analysis of ions ejected from the extraction region; characterised in that the segment (15, 17) includes overlapping first electrode means (71, 72) comprising linear electrodes (73 - 79) and second electrode means comprising electrodes (81 - 83) formed in a 2D arrangement to produce ion clouds of 2D shape; said trapping voltage includes first RF trapping voltage supplied, in use, to said first electrode means and, with the assistance of cooling gas, being effective to form a substantially one-dimensional, axially-extending ion cloud, and second RF trapping voltage supplied, in use, to said second electrode means and being effective to transform said substantially one-dimensional, axially-extending ion cloud into a substantially two-dimensional ion cloud having a central plane orthogonal to said extraction direction, in response to the linear electrodes (73 - 79) being switched off and the 2D electrodes (81 - 83) being switched on. an ion source for supplying sample ions, a segmented linear ion storage device having a longitudinal axis for receiving sample ions supplied by the ion source, at least one segment (15, 17) of the ion storage device defining an extraction region of the device; voltage supply means for supplying, in use, to the ion storage device trapping voltage effective to trap sample ions, or ions derived from sample ions in said extraction region to form an ion cloud and for supplying to the ion storage device extraction voltage for causing ejection of the ion cloud from said extraction region in an extraction direction orthogonal to said longitudinal axis, and for supplying, in use, to the ion storage device trapping voltage effective to trap sample ions, or ions derived from sample ions in said extraction region to form an ion cloud and for supplying to the ion storage device extraction voltage for causing ejection of the ion cloud from said extraction region in an extraction direction orthogonal to said longitudinal axis, and a time-of-flight mass analyser for performing mass analysis of ions ejected from the extraction region; characterised in that the segment (15, 17) includes overlapping first electrode means (71, 72) comprising linear electrodes (73 - 79) and second electrode means comprising electrodes (81 - 83) formed in a 2D arrangement to produce ion clouds of 2D shape; said trapping voltage includes first RF trapping voltage supplied, in use, to said first electrode means and, with the assistance of cooling gas, being effective to form a substantially one-dimensional, axially-extending ion cloud, and second RF trapping voltage supplied, in use, to said second electrode means and being effective to transform said substantially one-dimensional, axially-extending ion cloud into a substantially two-dimensional ion cloud having a central plane orthogonal to said extraction direction, in response to the linear electrodes (73 - 79) being switched off and the 2D electrodes (81 - 83) being switched on.