Dissociation Kinetics in Quadrupole Ion Traps: Effective Temperatures under Dipolar DC Collisional Activation Conditions

Ions stored in an electrodynamic ion trap can be forcedfrom thecenter of the ion trap to regions of higher radio frequency (RF) electricfields by exposing them to a dipolar DC (DDC) potential applied acrossopposing electrodes. Such ions absorb power from the trapping RF field,resulting in increased ripple motion at the frequency of the trappingRF. When a bath gas is present, ions undergo energetic collisionsthat result in RF-heating sufficient to induce fragmentation.DDC is therefore a broad-band (i.e., mass-to-charge-independent) meansfor collisional activation in ion traps with added bath gas. Underappropriate conditions, the internal energy distribution of an ionpopulation undergoing dissociation can be approximated with an effectivetemperature, T-eff. In such cases, it is possible to determinethermal activation parameters, such as Arrhenius activation energiesand A-factors, by measuring dissociation kinetics. In this work, thewell-studied thermometer ion, protonated leucine enkephalin, was subjectedto DDC activation under rapid energy exchange conditions and in twoseparate bath gases, N-2 and Ar, to measure T-eff as a function of the ratio of DDC and RF voltages. As a result,an empirically derived calibration was generated to link experimentalconditions to T-eff. It was also possible to quantitativelyevaluate a model described by Tolmachev et al. that can be used topredict T-eff. It was found that the model, which was derivedunder the assumption of an atomic bath gas, accurately predicts T-eff when Ar was used as the bath gas but overestimates T-eff when N-2 was the bath gas. Adjustment of theTolmachev et al. model for a diatomic gas resulted in an underestimateof T-eff. Thus, use of an atomic gas can provide accurateactivation parameters, while an empirical correction factor shouldbe used to generate activation parameters using N-2.

Automated summary

In an electrodynamic ion trap, ions can be forced to higher frequency regions by applying a dipolar DC potential across opposing electrodes. This results in increased ripple motion and collisional activation, especially when a bath gas is present. The internal energy distribution of ions can be approximated by an effective temperature, and thermal activation parameters can be determined through dissociation kinetics. By subjecting the thermometer ion, protonated leucine enkephalin, to dipolar DC activation in different bath gases, and measuring T-eff as a function of DDC and RF voltages, a calibration and prediction model for T-eff can be established.