Thermo Orbitrap Fusion Series - Figure 30. Visual Representation of the LIT Mass Analyzer Operation

4
Ion Transmission and Mass Analysis
Mass Analyzers
42 Orbitrap Fusion Series Hardware Manual Thermo Scientific
Summary of Linear Ion Trap Mass Analyzer Operation
The processes that occur in the LIT mass analyzer can be broken down into four steps:
1. Ion storage
2. Ion isolation (SIM or MS
n
[n >1])
3. CID (MS
n
[n >1])
4. Ion scan-out (the ion detection step)
For MS/MS applications, the quadrupole typically performs the ion isolation. For MS
n
(n > 1) applications, the LIT mass analyzer performs the ion isolation and CID steps n–1
times.
For SIM or MS
n
(n > 1) analyses, the LIT mass analyzer applies the ion isolation waveform
voltage to the X-rods, in combination with a ramp of the main AC voltage to a new storage
voltage, to eject all ions except those of the selected mass-to-charge ratio.
For MS
n
(n > 1) analyses, the LIT mass analyzer applies the resonance excitation AC voltage
to the X-rods to cause CID. Product ions with a mass-to-charge ratio of less than the
minimum storage mass-to-charge ratio are not stored.
Finally, the sample ions or product ions are either scanned out or sent to the Orbitrap mass
analyzers. To scan out the ions, the main RF voltage ramps up from low voltage to high
voltage, and simultaneously the mass spectrometer applies the resonance ejection AC voltage
to the X-rods to facilitate ejection. As the main RF voltage increases, ions of increasing
mass-to-charge ratios 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’s low pressure trap, while initial trapping, isolation, and dissociation
of the ions occur in the high pressure trap.
Figure 30 illustrates the ion scan-out process.
Figure 30. Visual representation of the LIT mass analyzer operation (low pressure cell,
Orbitrap Fusion MS)
Dynode in the ion
detection system