Separation of ions in a quadrupole mass filter is based on achieving a stable
trajectory for ions of specificm/zvalues. Only ions that undergo stable motion
in bothx-andy-directions will remain within the device and be detected by the
detector as they emerge from the quadrupole. WhenU=0 (au= 0, rf only
mode), a wide band of ions with m/z values below a certain cut-off
corresponding toqu= 0.908 are transmitted. As the value ofU/Vis increased,
the resolution increases so that only ions within a narrowm/zwindow are
transmitted. A mass spectrum may be generated by scanning the values ofU
and V at a constant U/V ratio, while keeping the rf frequency fixed
(Chernushevich et al., 2001).
11.2.3.4 Quadrupole Ion Traps (QIT) The fundamental working principle of
quadrupole ion traps is the same as described for quadrupole mass filters, but
in three dimensions rather than two (March, 1997). A QIT is composed of
three electrodes: two end caps and one ring. One end cap electrode has a single
small central aperture through which ions can be gated periodically and the
other has several small apertures arranged centrally through which ions pass to
a detector. The mass analysis equation for a QIT operated in the mass-selective
instability mode is
m=z¼8 V
qzðr 02 þ 2 z^20 Þv^2whereVis the rf potential,r 0 is the radius of the ring electrode,z 0 is the distance
from the center to the end cap, andvis its angular frequency. In contrast to
quadrupole mass filters, mass spectra with ion traps are generated by making
the ion trajectories unstable in a mass-selective manner in which ions are
moved along theqzaxis by raising the rf voltageVuntil they become unstable
at the boundary, whereqz= 0.908. Ions of progressivem/zvalues are ejected
and detected by an electron multiplier; hence called mass-selective instability.
Due to the small footprint, low cost and ability to perform sequential
fragmentation experiments (MSn), this mass spectrometry technique gained a
lot of popularity in the past decade. The main issue that limits the performance
of 3D ion traps is that only a limited number of ions can be trapped in the
center of the device before reaching the space-charge limit of stability. This
limitation was overcome in the linear ion trap (LIT) MS, which is a geometrical
variation of the 3D ion trap (Douglas et al., 2005). LIT consists of four parallel
hyperbolic rods with two end caps, in which ions are confined radially by a
two-dimensional radio frequency (rf) field, and axially by stopping potentials
applied to end electrodes. LIT has two major advantages over 3D IT: a larger
ion storage capacity and a higher trapping efficiency. LIT can hold almost
10–100 times more ions than 3D traps because ions are more radially focused
in the LIT while in a 3D ion trap they are focused at the center of the trap. In
addition, ions are injected into the linear trap through an end cap and then
ejected from it. Doing so allows the use of two detectors on each side of the
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