2
Functional Description
Mass Spectrometer
Thermo Scientific
TSQ Quantum XLS Series User Guide
35
Then, at a later time, both rf and dc voltages change, and ions of the next mass-to-charge ratio
(for example,
m/z
181) are allowed to pass, while all other ions (including
m/z
180) become
unstable and undergo unbounded oscillations. This process continues, with ions of one
mass-to-charge ratio after another being transmitted, as the rf and dc voltages change in value.
The potentials on the quadrupole rods can be changed rapidly and precisely. At the end of the
scan, the mass spectrometer discharges the rf and dc voltages to zero and repeats the process.
The precision quadrupole rods of the mass spectrometer provide excellent sensitivity, peak
shape, resolution, and high mass transmission.
Collision Cell and CID Efficiency
In the MS/MS scan modes, the mass spectrometer applies a large voltage of opposite polarity
to the Q2 rod pairs between scans, which empties the collision cell. This process ensures that
no ions remain in the collision cell from scan to scan.
The collision cell quadrupole rod assembly (Q2), which always acts as an ion transmission
device, is a quadrupole array of square-profile rods. A variable rf voltage charges the rods,
which creates an electrostatic field that gives stable oscillations to ions in a wide window of
mass-to-charge ratios.
The collision cell surrounds Q2 and is usually pressurized from about 1 × 10
–
3
to
4 × 10
–
3
Torr with argon collision gas. The collision cell is where collision-induced
dissociation (CID) takes place.
CID is a process in which an ion collides with a neutral atom or molecule and then, because
of the collision, dissociates into smaller fragments. The mechanism of dissociation involves
converting some of the translational kinetic energy (TKE) of the ion into internal energy. This
collision places the ion in an excited state. If the internal energy is sufficient, the ion
fragments.
Three expressions convey the efficiency of the CID process:
• Collection efficiency
• Fragmentation efficiency
• Overall CID efficiency
Collection efficiency:
The ion flux ratio measured at the exit of the collision cell and at its
entrance. With no collision gas present, the mass spectrometer obtains virtually 100 percent
collection efficiency. Collection efficiency is a mass-dependent parameter. For example, with
mid-range collision gas pressure, the collection efficiency might vary from about 50 percent
for comparatively less massive ions (which are more prone to scatter) up to 75 percent for
comparatively more massive ions (which are less prone to scatter).
