Technical Description: System
Cracking Patterns
B-18
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
These ions result from fragmentation of the molecule under electron
impact in the ion source. In hydrocarbon species these ions are
normally the most abundant since longer chain molecules are
statistically more likely to be broken. The relative intensities of
fragment peaks are dependent on the flux of electrons within the ion
source (trap current) and the energy of the electrons from the
filament (electron energy).
An example of a fragmentation ion would be the m/e 27 amu peak in
ethylene. The molecular ion for ethylene has a mass of 28 amu. The
27 amu ion arises from the loss of a hydrogen atom from the
molecule.
These ions result from the sample gas molecule losing more than one
electron during the ionization process which gives rise to multiple
positive charges. The relative intensities of multiple-charged ions
depend upon the energy of the electrons from the filament. It should
be noted that multiple-charged isotope and fragment peaks can also
occur further complicating the spectrum. An example of a multiple-
charged ion peak is the
40
Ar
++
ion at m/e 20 amu.
In some species it is possible to produce ion peaks that are a
combination of multiple-charged ions and fragment ions. An
example of this would be the m/e 14 amu peak for nitrogen, where
the peak is produced from a combination of
14
N
2
++
ions and
14
N
+
ions. Since the multiple-charged ions have a different energy from
the singly charged ions, the observed peak shape resulting from
combinations of the two types may be slightly degraded.
These peaks arise from a variety of causes, such as ion-molecule
interactions within the ion source. The product ions can appear to
have strange (and perhaps seemingly impossible) chemical
structures. The relative intensities of these ions are usually very low
since they normally result from collisions within the ion source
between molecules in the gas stream. Relative intensities are very
strongly dependent on the partial pressure of the different gas species
present, and on the total gas pressure within the ion source. The
intensities of these peaks are usually unstable in time and should be
avoided for quantitative analytical work.
Examples of these types of ions are the m/e 30 amu peak observed in
air due to the interaction of nitrogen and oxygen within the ion
source to produce NO and the m/e 29 peak observed in methane
produced by the interaction of a methane ion with another methane
molecule to give a C
2
H
5
+
ion at m/e 29 amu.
Fragment Ions
Multiple-Charged Ions
Rearrangement and
Recombination Peaks
