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 q_z = 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 q_z = 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.