Thermo Scientific TSQ Series Hardware Manual page 50

2
Functional Description
Mass Spectrometer
With mid-range collision gas pressure, fragmentation efficiency might vary from 15 percent to
65 percent for various compounds. As the collision gas pressure increases, the fragmentation
efficiency for all compounds approaches 100 percent due to multiple collisions. The collection
efficiency decreases, however, due to scattering.
Overall CID efficiency: The product of the collection efficiency and the fragmentation efficiency.
The overall CID efficiency exhibits a maximum with intermediate pressure. As the pressure
increases beyond the optimum value, more and more collisions take place, the probability of
scattering increases, and fewer and fewer ions pass through the collision cell. Scattering results in
the collection efficiency decreasing. The fragmentation efficiency also decreases as the pressure is
decreased from its optimum value, because fewer and fewer collisions take place.
Quadrupole Offset Voltage
The quadrupole offset voltage is a dc potential applied to the quadrupole rods in addition to the
ramping dc voltage. The offset voltage applied to the two rod pairs of the assemblies is equal in
amplitude and equal in sign. The quadrupole offset voltage accelerates or decelerates ions and,
therefore, sets theTKE of the ions as they enter the quadrupole rod assembly.
In general, for a given experiment, the TSQ has fixed offset voltages for Q1 and Q2. However, in
MS/MS experiments, the quadrupole offset voltage applied to Q3 usually varies as a scan proceeds.
The TSQ automatically computes the Q3 quadrupole offset voltage necessary and then varies the
voltage, as appropriate, as each scan proceeds.
The offset voltage applied to Q2 (which contains the collision cell) is responsible for the collision
energy. The collision energy is the difference in potential between the ion source (where parent
ions are formed) and Q2 (where they collide with collision gas). As the offset voltage on Q2
increases, the TKE of the parent ions also increases. As a result, increases in the Q2 offset voltage
increase the energy of ion/Ar collisions. The collision energy is generally set to one value for an
entire scan and can be set from 0 to ±200 V.
Before obtaining any mass spectra, the TSQ tunes Q1 in the Q1MS scan mode (Q2 and Q3 rf
voltage only), and tunes Q3 in the Q3MS scan mode (Q1 and Q2 rf voltage only). During tuning,
the TSQ determines the optimum quadrupole offset voltage for Q1 and for Q3.
Mass Analyzer Lenses
The TSQ system mass analyzer has three lens sets. See
and Q2 are designated L21, L22, L23; those between Q2 and Q3 are designated L31, L32, L33;
and the lens between Q3 and the ion detection system is designated as L4 (or the exit lens). All of
the lenses have circular holes in their centers through which the ion beam passes.
The lens assemblies also retain the three rod assemblies to ensure accurate and automatic axial
alignment of the rod assemblies.
The L2x lens set (between Q1 and Q2) and the L3x lens set (between Q2 and Q3) serve these
functions:
• To minimize the amount of collision gas that enters the mass analyzers (Q1 and Q3) from the
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collision cell (Q2). (For high-mass transmission, it is important to maintain a low pressure in
the mass analyzers.)
Figure 26 on page 29. Those between Q1
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