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Mass Spectrometry in Forensic Science 165

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FIGURE 5 Separation of mass fragments with quadrupole instrument. Ions (M ,M ,M , ... ) are accelerated out
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of the ion source into the space between four parallel rods with an oscillating electric field. At a certain ac and dc
+
potential, ions with a specific m/z (M ) value will become stable and oscillate in a fixed path through the quadrupole
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+ +
to reach the detector to be recorded, whereas the unstable ones (M ,M , ... ) will stick to the rods.
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1 2
and 50% of these are then ejected from the cell through differs from the beam-type scanning MS by detecting the
a hole in its bottom to be recorded by a detector. Unlike unstable ions, whereas the magnetic sector or quadruple
the quadrupole or magnetic sector instrument, in which instrument scans the stable ones, conditions that may offer
the separation and detection of an ion occur in a contin- rather different analytical possibilities.
ues process, the ITD operates in two steps separated in
time: ion accumulation and mass analysis. The ITD also 4. Tandem Mass Spectrometer
In tandem mass spectrometry (MS/MS), two mass spec-
trometers and a collision chamber are hooked up in se-
ries. Early devices utilized two magnetic sector instru-
ments, but because these instruments became bulky and
high priced, modern tandem devices are nearly all based
on the use of quadrupole or ITD apparatus. The operating
scheme of MS/MS is shown in Fig. 7. An advantage of
MS/MS is that it provides high selectivity and extra infor-
mation about the nature of an analyte, as well as reduced
chemical background noise, which results in an increase in
the signal-to-noise ratio for a detected peak. Even though
thismethod,asjudgedfromthenumberofscientificpapers
reported, does not seem to be in common use for foren-
sics today, MS/MS will probably become the method of
choice in the years to come.

D. Mass Focusing
FIGURE 6 Separation of mass fragments with ion trap detector.
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Ions (M ,M ,M , ... ) are introduced into a closed cavity and, The MS test can be run in the full scan mode or with se-
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under the control of the applied electric field, the ions are forced lected ion monitoring (SIM). In the full scan mode, the
to move in an orbit within the space of the cell. When the ac or
dc potential is changed, the motion of ions with given m/z values mass analysis covers a range of m/z values, whereas one
+ or a limited number of m/z values are selected for the
becomes unstable (M ), and they are then ejected from the cell
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through a hole in its bottom to be recorded by a detector. exam with SIM. When picking between the two options,