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

Ions stored in an electrodynamic ion trap can be forced from the center of the ion trap to regions of higher radio frequency (RF) electric fields by exposing them to a dipolar DC (DDC) potential applied across opposing electrodes. Such ions absorb power from the trapping RF field, resulting in increased ripple motion at the frequency of the trapping RF. When a bath gas is present, ions undergo energetic collisions that result in "RF-heating" sufficient to induce fragmentation. DDC is therefore a broad-band (i.e., mass-to-charge-independent) means for collisional activation in ion traps with added bath gas. Under appropriate conditions, the internal energy distribution of an ion population undergoing dissociation can be approximated with an effective temperature, T_eff. In such cases, it is possible to determine thermal activation parameters, such as Arrhenius activation energies and A-factors, by measuring dissociation kinetics. In this work, the well-studied thermometer ion, protonated leucine enkephalin, was subjected to DDC activation under rapid energy exchange conditions and in two separate bath gases, N₂ 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 experimental conditions to T_eff. It was also possible to quantitatively evaluate a model described by Tolmachev et al. that can be used to predict T_eff. It was found that the model, which was derived under the assumption of an atomic bath gas, accurately predicts T_eff when Ar was used as the bath gas but overestimates T_eff when N₂ was the bath gas. Adjustment of the Tolmachev et al. model for a diatomic gas resulted in an underestimate of T_eff. Thus, use of an atomic gas can provide accurate activation parameters, while an empirical correction factor should be used to generate activation parameters using N₂.