Dynamic collision-induced dissociation (DCID) in a quadrupole ion trap using a two-frequency excitation waveform: I. Effects of excitation frequency and phase angle

This study describes the application of a two-frequency excitation waveform to the end-cap electrodes of a quadrupole ion trap (QIT) during the mass acquisition period to deliberately fragment selected precursor ions. This approach obviates the need for a discrete excitation period and guarantees on-resonant excitation conditions without any requirement for resonant tuning; it is therefore faster than the conventional approach to collision-induced dissociation (CID) in QITs. The molecular ion of n-butylbenzene is used as thermometer molecule to determine the energetics of the new excitation procedure. The excitation waveform, consisting of two closely spaced sinusoidal frequencies, has an interference pattern that displays nodes and crests in the time domain. The energetics (determined by the product ion ratios of 91/92 Th) and CID efficiencies are highly dependent on the excitation amplitude, the relative position of the excitation frequencies in the Mathieu stability diagram, and whether the ions come into resonance during a node or crest of the excitation waveform. Under highly energetic conditions, ratios of 91/92 as large as 15 can be obtained at concomitant CID efficiencies of 10%, indicating internal energies in excess of 10 eV at the time of fragmentation. These extremely high internal energies far exceed the energetics achievable using conventional on-resonance excitation in QITs, indicating that the collisional heating rate is very fast in the new approach. Under less energetic conditions CID efficiencies as high as 70% are possible, which compares favorably with results obtained by conventional on-resonance excitation. Correlation analyses are used to determine the conditions that simultaneously optimize energetic and efficient fragmentation conditions.