Thermo Orbitrap Fusion Series - Page 59

4
Ion Transmission and Mass Analysis
Mass Analyzers
Thermo Scientific Orbitrap Fusion Series Hardware Manual 41
AC Voltages Applied to the X-Rods
The Orbitrap Fusion Series MS applies ion isolation waveform voltage, resonance excitation
RF voltage, and resonance ejection RF voltage to the X-rods to stimulate motion of the ions in
the direction of the ion detection system. When the AC frequency applied to the rods equals
the frequency of the motion of the trapped ion (which depends on its mass), the ion gains
kinetic energy. If the magnitude of the applied voltage is large enough or the ion is given
sufficient time, the mass analyzer ejects the ion in the direction (that is, X direction) of the ion
detection system.
The ion isolation waveform voltage is a multifrequency resonance ejection waveform that acts
during the ion isolation step of SIM or MS
n
(n > 1) applications. The ion isolation waveform
voltage, combined with the main RF voltage, ejects all ions except those of a selected
mass-to-charge ratio or narrow ranges of mass-to-charge ratios.
During the collision-induced dissociation (CID) step of MS
n
(n > 1) applications, the
Orbitrap Fusion Series MS applies a resonance excitation AC voltage to the X-rods to
fragment precursor ions into product ions. Ion motion is enhanced and the ion gains kinetic
energy. After many energetic collisions with the helium damping gas, which is present in the
mass analyzer, the ion gains enough internal energy to cause it to dissociate into product ions.
The mass analyzer then analyzes the product ions.
During ion scan-out, the resonance ejection AC voltage allows for the ejection of ions from
the mass analyzer, which improves mass resolution and sensitivity. The mass spectrometer
applies resonance ejection AC voltage during the ramp of the main RF voltage. Ions
consecutively move into resonance with the resonance ejection RF voltage. When an ion
approaches resonance, it moves farther away from the center of the mass analyzer and is
subsequently ejected.
Helium Damping Gas
The LIT mass analyzer cavity contains helium that is used as a damping gas and a collision
activation partner. The collisions of the ions entering the LIT with the helium slow the ions so
that the RF field in the LIT can more efficiently trap them.
The presence of helium in the LIT cavity significantly enhances sensitivity and mass spectral
resolution. Before their ejection from the mass analyzer cavity, sample ions collide with
helium atoms. These collisions reduce the kinetic energy of the ions, which results in damping
the amplitude of their oscillations. The mass analyzer then focuses the ions to the axis of the
cavity rather than allow them to spread throughout the cavity, which would spread their
ejection times and degrade the mass spectral resolution.
Helium in the LIT mass analyzer cavity also serves as a collision activation partner. During the
CID step of an MS
n
(n > 1) analysis, the resonance excitation AC voltage that is applied to the
X-rods drives precursor ions energetically into the helium atoms. After gaining sufficient
internal energy from the resulting collisions, the precursor ion dissociates into one or more
product ions.