PNNL

Abstract

Ion transfer and storage using inhomogeneous radio frequency (RF) electric fields in combination with gas-assisted ion cooling and focusing constitutes one of the basic techniques in mass spectrometry today. The RF motion of ions in the bath gas environment involves a large number of ion-neutral collisions that leads to the internal activation of ions. The degree of ion activation required in various applications may range from a minimum possible activation to an intense ion heating that results in ion fragmentation. Several research groups proposed using the effective temperature as a measure of ion activation under conditions of multiple ion-neutral collisions. We have developed a formalism for the effective ion temperature that accurately predicts the degree of ion activation for a given operation mode of a specific RF ion trap or ion guide. We show that RF ion activation results in near-thermal energies for ions occupying an equilibrium position at the center of an RF trap, whereas increased ion activation can be produced by shifting ions off-center, e.g., by means of an external DC electric field. The ion dissociation in the linear quadrupole ion trap using the dipolar DC ion activation has been observed experimentally and interpreted in terms of the effective ion temperature.