Page 276 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009G-958 July 18, 2001 0:57

164 Mass Spectrometry in Forensic Science

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FIGURE 4 Separation of mass fragments with magnetic sector instrument. Ions (M ,M ,M , ... ) are accelerated
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out of the ion source into the flight tube of the magnetic sector instrument, whose magnetic field, perpendicular to
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the trajectories of the ions, forces the charged particles to move in circular orbit. Only those ions (M ) with a given
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m/z value will, at a given acceleration and magnetic strength, become stable and exit the magnetic mass separator to
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be detected, whereas the unstable ones (M ,M , ... ) will stick to the wall of the flight tube.
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when ions travel in a beam of an oscillating electric field, 3. Ion Trap Detector
their trajectories become bent. The quadrupole mass filter
is made up by four parallel rods, about 10 cm long and The working principle of a third type of mass filter, the ion
1 cm in diameter, which in a cross section are arranged trap detector (ITD), is illustrated in Fig. 6. It operates like
to form a square box with a gap in between the four bars. the quadrupole based on the concept that the trajectories of
The fragments are allowed to travel in this space along ions, traveling in an oscillating electric field, become influ-
the rods, where they are accelerated in the oscillating enced by the wave frequency. Dissimilar to the quadrupole
electric field set up by dc and ac currents applied to the filter, however, the ion separation of the ITD occurs in a
rods. At a certain ac and dc potential, ions with a specific closed cavity, where the dc and ac currents applied to a
m/z value will become stable and oscillate in a fixed ring electrode and an end cap of the cell set up the elec-
path through the quadrupole to reach the detector to be tric field. The molecules enter the cell in which they are
recorded. The fragments with m/z values that do not suit ionized by an electron beam, and under the control of the
the applied potentials will become unstable and, as in the given electric field the ions are forced to move in an orbit
magnetic sector instrument, will not reach the detector within the space of the cell. When the ac or dc potential
but will stick to the rods. is changed, the motion of some ions becomes unstable,