Study of the Enhancement of Dipolar Resonant Excitation by Linear Ion Trap Simulations

Resolution improvements in dipolar resonant excitation have been examined in a round-rod quadrupolar collision cell for values of the Mathieu characteristic exponent β equal to n/p, where n and m are small integers (prime β values) versus other β values where n and p are not small (ordinaryβ values). The trajectories of ions moving in the time-varying electric fields of a quadrupole with and without buffer-gas molecules were calculated to determine the relationship of prime and ordinary β values to frequency resolution for resonant ion excitation and ejection. For prime β values, the ion trajectory in the hyperbolic quadrupole field will be exactly periodic with a period of at most 4πp/Ω, where Ω is the angular frequency of the main drive radio-frequency (RF) potential. Ion trajectory simulations with prime β versus ordinary β values show that the motion of ions with prime β values have simpler trajectories of shorter periods. Frequency response profiles (FRPs) for round-rod quadrupoles at zero pressure show that dipolar resonant excitations with primeβ values exhibit significantly narrower bandwidths than those with ordinary β values. Simulations show that at 0.05 to 0.8 mTorr of nitrogen, it is possible to reduce the FRP bandwidth by 20% (measured at 50% depth).