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Ion Transmission and Mass Analysis
Mass Analyzers
36 Orbitrap Fusion Series Hardware Manual Thermo Scientific
Applied RF and DC Fields
In a quadrupole rod assembly, because rods opposite each other in the array connect
electrically, the four rods are considered two pairs of two rods each. The mass spectrometer
applies RF and DC voltages to the rods. As shown in Figure 24, although the RF voltages
applied to the four rods are the same, the two pairs are 180 degrees out of phase (that is, one
pair has a positive voltage and the other is negative).
Figure 24. Polarity of the RF and DC voltages applied to the Q1 rods
The quadrupole coil module provides the voltages for operating the quadrupole. The RF
voltage applied to the quadrupole rods is of constant frequency (approximately 1.1 MHz) and
varies from 0 to 10 000 Vac peak-to-peak amplitude. The DC voltage varies from 0 to
±800 V.
When the mass spectrometer applies both RF and DC voltages, quadrupole Q1 acts as a mass
filter. When it applies only the RF voltage, the quadrupole acts as ion transmission devices. In
the ion transmission mode, the quadrupole allows ions in a wide window of mass-to-charge
ratios to pass.
Mass Filtering
The variable amplitude of RF and DC voltages applied to the quadrupole rods generates an
electric field that gives stable oscillations to ions with a specific mass-to-charge ratio and
unstable oscillations to all others. When the mass spectrometer applies one particular set of
RF and DC voltages to the quadrupole, only ions of one mass-to-charge ratio (for example,
m/z 609) are maintained within bounded oscillations as their velocity carries them through
the mass filter. At the same time, all other ions undergo unbounded oscillations. These ions
strike one of the rod surfaces, become neutralized, and are pumped away, or they are ejected
from the rod assembly.
The more closely the electrostatic field generated by a set of quadrupole rods approximates a
hyperbolic geometry, the better their operating characteristics are. As a result, the precision
quadrupole rods of the Orbitrap Fusion Series MS provide excellent sensitivity, peak shape,
resolution, and high mass transmission.
RF voltage + DC voltage
RF voltage 180° out of phase – DC voltage
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