Properties of a multipole ion trap studied by evaporative ion losses

We have studied a scheme for analysing the effective trapping potential for ions in a 22-pole ion trap, based on the strong dependence of the lifetime of the ions on the buffer gas temperature. We distinguish two regimes for ion losses, either over one of the two end cap electrodes (axial direction) or over the effective radiofrequency potential (radial direction), and examine these schemes under different trap settings. We have found that evaporation over one of the end caps yields an effective temperature of the ions that is much larger than the buffer gas temperature. This may be a hint for an enhancement of the high-energy tail of the ions' velocity distribution. We also observe and investigate a breakdown of thermalisation for trap temperatures lower than 25 K. Measurements for OH- and D- anions have shown that this feature is independent of both the buffer gas to ion mass ratio and the trapping parameters. (C) 2014 Elsevier B.V. All rights reserved.