Thermo Scientific TSQ Series Hardware Manual page 49

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.
At the end of the scan, the rf and dc voltages are discharged to zero, and the process repeats.
The TSQ system can rapidly and precisely change the potentials on the quadrupole rods. The rf
and dc voltages in the TSQ mass spectrometer can be scanned over the full mass range of the
system, m/z 10 to 3000, in 0.85 seconds.
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 TSQ mass spectrometer provide excellent sensitivity, peak shape,
resolution, and high mass transmission.
Collision Cell and CID Efficiency
In the MS/MS scan modes, the TSQ applies a large voltage of opposite polarity to the 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
with argon collision gas. The collision cell is where collision-induced dissociation 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 TSQ 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).
Fragmentation efficiency: The fraction of the ion flux at the exit of the collision cell that results
from fragmented ions. Fragmentation efficiency depends directly on the stability of the ion and
indirectly on the mass of the ion. The more stable the ion, the less likely a given collision will
fragment the ion. The more massive the ion, the greater its ability to distribute the vibrational
energy imparted by a collision. As a result, ion fragmentation might decrease.
Thermo Scientific
2
Functional Description
Mass Spectrometer
to 4 × 10
-3 -3
Torr
TSQ Series Hardware Manual 31