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Functional Description
Mass Spectrometer
Thermo Scientific
TSQ Quantum XLS and TSQ Quantum GC User Guide
37
Three expression 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).
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.
Typically, low mass ions require a higher collision energy to dissociate than high mass ions. To
compensate for this, the data system allows you to specify a mass-dependent collision energy
ramp.
With a 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 is increased 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. This 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 magnitude 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.