Functional Description
MS Detector
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______________________ Finnigan LTQ Hardware Manual _______________________
the rod pairs produces a two-dimensional quadrupole field within the mass
analyzer cavity. This time-varying field drives ionic motion in the radial
(X,Y) direction. Ionic motion must be stable in the radial direction for an ion
to remain trapped. (A stable trajectory is an oscillatory trajectory that is
confined within the mass analyzer.) During ion scan out, the system produces
a mass-dependent instability to eject ions from the mass analyzer in the radial
direction.
When the amplitude of the main RF voltage is low, all ions above a minimum
mass-to-charge ratio are trapped. This RF voltage is referred to as the storage
voltage, and the minimum mass-to-charge ratio is usually chosen to be greater
than the mass-to-charge ratios associated with air, water, and solvent ions.
During ion scan out, the main RF voltage is ramped at a constant rate
corresponding to approximately 11,000 u/s (for unit resolution). As the main
RF voltage increases, ions of increasing mass-to-charge ratio become
successively unstable in the radial direction and are ejected from the mass
analyzer. The voltage at which an ion is ejected from the mass analyzer is
defined as its resonance voltage. The ejection of ions of each mass-to-charge
ratio occurs over a very short time. Many of these ions are detected by the ion
detection system.
Ion Isolation Waveform Voltage, Resonance
Excitation RF Voltage, and Resonance Ejection RF
Voltage Applied to the Exit Rods
The ion isolation waveform voltage, resonance excitation RF voltage, and
resonance ejection RF voltage are ac voltages that are applied to the exit rods
to stimulate motion of the ions in the direction of the ion detection system.
The voltages applied to the exit rods are equal in amplitude but are 180
o
out of
phase to one another. When the RF frequency applied to the rods equals the
resonance frequency of a 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 ion is ejected from the mass analyzer in the
direction of the ion detection system (X direction).
The ion isolation waveform voltage consists of a distribution of frequencies
between 5 and 500 kHz containing all resonance frequencies except for those
corresponding to the ions to be trapped. The ion isolation waveform voltage
acts during the ion isolation step of SIM, SRM, CRM, or MS
n
(n > 1) full scan
applications. The ion isolation waveform voltage, in combination 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. The ion isolation waveform
voltage is calculated by the LTQ and automatically applied at the correct time.
During the collision induced dissociation step of SRM, CRM, or MS
n
(n > 1)
full scan applications, the resonance excitation RF voltage is applied to the
exit rods to fragment parent ions into product ions. The resonance excitation
RF voltage is not strong enough to eject an ion from the mass analyzer.
