2
Functional Description
Mass Spectrometer
36
TSQ Quantum XLS and TSQ Quantum GC User Guide
Thermo Scientific
Mass Analysis
The Q1 and Q3 rods are charged with a variable ratio of rf voltage and dc voltage (
These potentials give rise to an electrostatic field that gives stable oscillations to ions with a
specific mass-to-charge ratio and unstable oscillations to all others.
At any given instant, one particular set of rf and dc voltage values is being applied to the mass
analyzer rods. Under these conditions, only ions of one mass-to-charge ratio (for example,
m/z
180) are maintained within bounded oscillations as their velocity carries them through
the mass analyzer. During this 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.
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 is
repeated.
The potentials on the quadrupole rods can be changed rapidly and precisely. The rf and dc
voltages in the mass spectrometer can be scanned over the full mass range of the system,
m/z
10 to 3000, in 0.6 s.
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.
