Thermal dissociation in the quadrupole ion trap: ions derived from leucine enkephalin

Abstract

Rates of dissociation of protonated leucine enkephalin and the b 4 + fragment ion derived from protonated leucine enkephalin have been measured as a function of helium bath gas temperature in a quadrupole ion trap. Dissociation rates were observed to be insensitive to the amplitude of the trapping voltage over the range of values studied. This observation, along with theoretical arguments based on predicted levels of “rf heating,” indicates that any internal excitation of the ions due to ion trap storage is minimal. The bath gas temperature can therefore be used to characterize the internal temperatures of the ions. This approximation is expected to be most valid for high mass ions and low mass bath gases, such as helium. Activation parameters were obtained from Arrhenius plots of the rate data, and master equation modeling of the activation, deactivation, and dissociation processes was performed to provide an indication as to how closely these ions approached high-pressure limit behavior. Protonated leucine enkephalin more closely approached the high-pressure limit than the b 4 + ion due to its larger size and the fact that the activation parameters were derived from somewhat lower dissociation rates. These studies suggest that the quadrupole ion trap operated in the presence of a light, heated bath gas can be used to obtain Arrhenius activation parameters from the dissociation kinetics of relatively high mass ions.