Effects of target gas in collision-induced dissociation using a double quadrupole mass spectrometer and radiofrequency

Threshold collision-induced dissociation of diatomic molecules: A case study of the energetics and dynamics of O2− collisions with Ar and Xe

The energetics and dynamics of collision-induced dissociation of O2- with Ar and Xe targets are studied experimentally using guided ion-beam tandem mass spectrometry. The cross sections and the collision dynamics are modeled theoretically by classical trajectory calculations. Experimental apparent threshold energies are 2.1 and 1.1 eV in excess of the thermochemical O2- bond dissociation energy for argon and xenon, respectively. Classical trajectory calculations confirm the observed threshold behavior and the dependence of cross sections on the relative kinetic energy. Representative trajectories reveal that the bond dissociation takes place on a short time scale of about 50 fs in strong direct collisions. Collision-induced dissociation is found to be remarkably restricted to the perpendicular approach of ArXe to the molecular axis of O2-, while collinear collisions do not result in dissociation. The higher collisional energy-transfer efficiency of xenon compared with argon is attributed to both mass and polarizability effects.

Measurement of collision-induced dissociation rates for tantalum oxide ions in a quadrupole ion trap

A study of factors influencing the collision-induced dissociation (CID) rate of strongly bound diatomic ions effected via resonance excitation in a quadrupole ion trap is presented. From these studies, an approach to measuring the CID rates is described wherein product ion recovery is optimized and the effect of competitive processes (e.g., parent ion ejection and product ion reactions) on rate measurements are prevented from influencing rate measurements. Tantalum oxide ions (dissociation energy = 8.2 eV), used as a model system, were formed via reactions of glow discharge generated Ta+ ions with residual gases in the ion trap. Neon (0.5 mtorr) was found to be a more favorable target gas for the dissociation of TaO+ than He and Ar, but collisional activation of TaO+ ions in neon during ion isolation by mass selective instability necessitated ion cooling prior to dissociation. A 25 ms delay time at qz = 0.2 allowed for kinetic cooling of stored TaO+ ions and enabled precise dissociation rate measurements to be made. CID of TaO+ was determined to be most efficient at qz = 0.67 (226 kHz for m/z 197). Suitable resonance excitation voltages and times ranged from 0.56 to 1.2 V(p-p) and 1 to 68 ms, respectively. Under these conditions, measurement of rates approaching 80 s(-1) for the dissociation of TaO+ could be made without significant complications associated with competing processes, such as ion ejection.