the ion cloud with the intention to mitigate the effects of coulomb repulsion in typical operation the trapped ions are cooled by collisions with a buffer gas such as helium towards the centre of the ion trap 6 as the trapped ions get closer together their individual charges repel other trapped ions keeping them apart by coulomb repulsion this is the so called space charge effect eventually the trapped ions will cool through collisions with buffer gas towards the centre of the ion trap 6 and approach the limits imposed on the size of the ion cloud by the space charge effect coulomb repulsion is a prime factor in determining the size of the ion cloud in the ion trap and the size of the ion cloud can give rise to deleterious effects in respect of mass linearity and resolution in a mass scan or ion isolation reducing the size of the ion cloud by mitigating the effects of coulomb repulsion by means of charge compensation reduces the resulting energy spread of the ejected ions and produces either a a corresponding improvement in mass resolution for the same ion density or b an improvement in signal intensity for the same mass resolution depending on the number of compensating charges introduced to the trap in a preferred embodiment the ion trap 6 is coupled to a time of flight tof analyser not shown such as described by kawatoh in u s pat no 6 380 666 april 2002 a known limitation in achieving the highest mass resolution combined with high signal intensity in this type of configuration is the spatial distribution and velocity of the ions at the time of fast ejection from the ion trap 6 into.