Collision-induced dissociation in quadrupole ion traps: Application of a thermal model to diatomic ions

Dissociation of the tantalum oxide cation, a strongly bound diatomic, is simulated for the multiple-collision environment of a quadrupole ion trap mass spectrometer using a model based on thermal unimolecular reaction theory. The intact diatomic ion is assigned a specific internal temperature at which it undergoes collisional activation and deactivation during a random walk in energy space. Collisional energy transfer is assumed to proceed via independent vibrational and rotational processes, as described by the refined impulse approximation and exponential transition probability, with dissociation occurring when the vibrational energy exceeds the rotational-energy-dependent barrier for dissociation. Processing the data from many such random walks yields the simulated dissociation kinetics and time-dependent internal energy distribution of an ion population at the specified internal temperature. Comparison of experimental dissociation rates with those obtained via simulations performed over a series of temperatures enables prediction of internal temperatures and corresponding internal energy distributions for tantalum oxide ion populations undergoing resonance excitation. Although the simulations indicate that rotational-energy transfer can lead to significantly higher dissociation rates than those associated with purely vibrational-energy transfer, the results obtained in this study suggest that ion internal temperatures in the tens of thousands of degrees are required nonetheless to dissociate a strongly bound diatomic ion such as tantalum oxide, and the experimental data demonstrate that resonance excitation in a quadrupole ion trap can achieve the necessary temperatures.