scan rate. This approach is named single ion monitoring SIM or single ion recording
SIR and is dominantly used for sensitive quantitative analyses of preselected target
analytes. Noteworthy, the full information on the compounds gets lost, since only
very few ions or fragments are detected.
General Note
Fragmentation is a main clue to obtain structural information based on mass
spectrometry. Gathering information on the smaller fragments allows insights
into their molecular structure. Reassembling of all these moieties (like a
puzzle) allows to characterize or even predict the structure of the whole
molecule.
A significant improvement of mass accuracy and sensitivity has been developed
early in the 1940s by adding an electrostatic sector between ion source and magnetic
sector for focusing the ion beam prior to the mass separation. This double field mass
spectrometers are still in use in many laboratories but are relatively expensive, since
they can exhibit a high mass resolution (for mass resolution see Sect. 4.2.2).
Therefore, an alternative method has been developed. Instead of a simple magnetic sector field, a quadrupole field can be used (see Fig. 4.27b). The basic idea is
the same, for ions with different m/z ratios stable and instable trajectory exist also in
quadrupole fields allowing a scanning similar to the sector field mass spectrometer.
Core of the separator are four metal tubes fed by both DC and AC voltage resulting
in a quadrupole field that is tunable by the individual voltages. The quadrupole field
forces the ions on a sort of coil pathway. Under stable conditions the radius remain
constant, but for lower masses the radius increase with time and pathway, whereas
for higher masses the radius decreases. Both the sensitivity as well as the scan rate
are similar as compared to sector field mass spectrometer, but the mass resolution
is low.
A further technical approach in mass spectrometry is based on a linear acceleration of the ions. Once all ions get the same impulse at a defined time point their
velocity is different according to their m/z ratio and, consequently, their time of
flight for a certain distance. This phenomenon is used in time-of-flight mass spectrometry (see Fig. 4.27c). The time the ions need for a defined pathway is measured
and the time is directly correlated with the m/z values. Advantage of this technique is
a contemporary detection of all generated ions and a resulting higher sensitivity.
Further on, the mass resolution can be very high depending inter alia on the flight
distance. This points to one challenge of this technique to build devices with
sufficient stretches of way. In modern devices the pathway is optimized by reflecting
the ion beam to increase the distance.
Finally, a fourth approach is used in mass separation that also detects all ions
generated. The name of this technique exactly describes the method, the ion-trap
mass spectrometry (see Fig. 4.27d). The ions get trapped in a special trap consisting
of different electrodes producing dynamic electrostatic fields. The resulting field
directs the ions on complex but stable orbits within the ion source. Under these
4.2 Mass Spectrometry MS
67
SIR and is dominantly used for sensitive quantitative analyses of preselected target
analytes. Noteworthy, the full information on the compounds gets lost, since only
very few ions or fragments are detected.
General Note
Fragmentation is a main clue to obtain structural information based on mass
spectrometry. Gathering information on the smaller fragments allows insights
into their molecular structure. Reassembling of all these moieties (like a
puzzle) allows to characterize or even predict the structure of the whole
molecule.
A significant improvement of mass accuracy and sensitivity has been developed
early in the 1940s by adding an electrostatic sector between ion source and magnetic
sector for focusing the ion beam prior to the mass separation. This double field mass
spectrometers are still in use in many laboratories but are relatively expensive, since
they can exhibit a high mass resolution (for mass resolution see Sect. 4.2.2).
Therefore, an alternative method has been developed. Instead of a simple magnetic sector field, a quadrupole field can be used (see Fig. 4.27b). The basic idea is
the same, for ions with different m/z ratios stable and instable trajectory exist also in
quadrupole fields allowing a scanning similar to the sector field mass spectrometer.
Core of the separator are four metal tubes fed by both DC and AC voltage resulting
in a quadrupole field that is tunable by the individual voltages. The quadrupole field
forces the ions on a sort of coil pathway. Under stable conditions the radius remain
constant, but for lower masses the radius increase with time and pathway, whereas
for higher masses the radius decreases. Both the sensitivity as well as the scan rate
are similar as compared to sector field mass spectrometer, but the mass resolution
is low.
A further technical approach in mass spectrometry is based on a linear acceleration of the ions. Once all ions get the same impulse at a defined time point their
velocity is different according to their m/z ratio and, consequently, their time of
flight for a certain distance. This phenomenon is used in time-of-flight mass spectrometry (see Fig. 4.27c). The time the ions need for a defined pathway is measured
and the time is directly correlated with the m/z values. Advantage of this technique is
a contemporary detection of all generated ions and a resulting higher sensitivity.
Further on, the mass resolution can be very high depending inter alia on the flight
distance. This points to one challenge of this technique to build devices with
sufficient stretches of way. In modern devices the pathway is optimized by reflecting
the ion beam to increase the distance.
Finally, a fourth approach is used in mass separation that also detects all ions
generated. The name of this technique exactly describes the method, the ion-trap
mass spectrometry (see Fig. 4.27d). The ions get trapped in a special trap consisting
of different electrodes producing dynamic electrostatic fields. The resulting field
directs the ions on complex but stable orbits within the ion source. Under these
4.2 Mass Spectrometry MS
67