The processes that occur in the LIT mass analyzer can be broken down into four steps:
- Ion storage
- Ion isolation
- Resonance excitation collision-induced dissociation (CID)
- Ion scan-out (the ion detection step)
For MS/MS applications, the quadrupole typically performs the ion isolation. For MSn (n ≥ 3) applications, the LIT mass analyzer automatically performs the ion isolation.
When ion-trap based waveform isolation is used, the LIT mass analyzer applies the ion isolation waveform voltage to the X-rods in combination with a ramp of the main supplementary AC voltage to a new storage voltage to eject all ions, except those with the selected m/z values.
The MS can apply a resonance excitation AC voltage to the X-rods to cause resonance CID. Product ions with m/z values of less than the minimum storage m/z are not stored.
Finally, the sample ions or product ions are scanned out. To scan out the ions, the main RF voltage ramps up from low voltage to high voltage, while the MS simultaneously applies the resonance ejection AC voltage to the X-rods to facilitate ejection. As the main RF voltage increases, ions of increasing m/z values are brought into resonance with the supplementary AC voltage and become unstable and eject through the slots in the X-rods. Most of these ions are focused toward the ion detection system where they are detected. The scan-out step can only occur in the LIT mass analyzer low-pressure trap, while initial trapping, isolation, and dissociation of the ions occur in the high-pressure trap.