Therefore, this approach remains as a very seldom application of isotope-based mass
spectra interpretation.
A second interesting element is sulphur with a A+2 type isotope pattern of
32 S
and
34
S. The relative abundance with around 4% is higher as compared to carbon.
Hence, the A+2 signal is better visible and is used to identify not only a contribution
of sulphur to the molecular structure but, especially, to point to the number of
sulphur atoms by interpreting the intensity of the A+2 signal (see Fig. 5.10).
However, the by far most interesting application of isotope interpretation is
related to the halogens chlorine and bromine, both very important elements in
many organic pollutants such as PCBs, dioxins or brominated flame retardants.
Both elements exhibit significant A+2 pattern with high relative abundances, the
relevant stable isotopes are
35 Cl/
37 Cl (ratio ca. 3:1) and
79 Br/
81
Br (ratio ca. 1:1).
Based on the isotope pattern, it is easy to deduce the total number of chlorine and
bromine atoms in fragment or molecular ions. There is a systematic development of
isotope pattern that are clearly visible in mass spectra. For fragments containing one
chlorine atom, the signals have a A+2 pattern with the natural relative abundance of
ca. 3:1. If two chlorine atoms appear, the superimposition of the two atoms and their
isotopes forms a A+2+2 pattern with rel. abundances of 9:6:1 as illustrated in
Fig. 5.11. The same approach applied to bromine reveals a A+2 pattern with 1:1
intensities for one bromine atom containing signals, but a A+2+2 pattern with rel.
abundances of 1:2:1 for two bromine atom containing signals (Fig. 5.9). Consequently, it is obvious that these unique patterns can be used for calculating the
number of chlorine and bromine atoms in individual fragment or molecule ions.
These interpretations can be supported by the differences of the corresponding
signals with m/z 35 or 79 as well as m/z 70 or 158 representing the loss of individual
halogen atoms or the loss of Cl 2 or Br 2 molecules (see Fig. 5.11).
Noteworthy, hydrogen, nitrogen or oxygen isotopes are not used for spectra
interpretation in common organic mass spectrometry. But sometimes more exotic
elements receive interest due to their isotope pattern. In Organic Geochemistry one
example is related to the element tin. Peralkylated tin organic molecules can be
analyzed by GC/MS due to their volatility and lipophilicity. Tin exhibit a very
unique pattern of stable isotopes with elevated relative abundances. Based on this
pattern, tin organic compounds can be easily detected in mass spectrometry. This is
illustrated in Fig. 5.12 for biomethylated tributyl tin, a formerly common ingredient
in antifouling paints.
In summary, there are various aspects that can be used for an ab initio interpretation of mass spectra. These approaches cover the examination of the overall pattern
for differentiating aliphatic and aromatic moieties, the influence of branching on
fragment intensities, the interpretation of the molecular ion intensity, the consideration of preferred fragmentation at hetero atoms, the observation of even to odd
fragment distribution and, finally, the identification of isotope pattern.
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5 GC/MS Data Evaluation
spectra interpretation.
A second interesting element is sulphur with a A+2 type isotope pattern of
32 S
and
34
S. The relative abundance with around 4% is higher as compared to carbon.
Hence, the A+2 signal is better visible and is used to identify not only a contribution
of sulphur to the molecular structure but, especially, to point to the number of
sulphur atoms by interpreting the intensity of the A+2 signal (see Fig. 5.10).
However, the by far most interesting application of isotope interpretation is
related to the halogens chlorine and bromine, both very important elements in
many organic pollutants such as PCBs, dioxins or brominated flame retardants.
Both elements exhibit significant A+2 pattern with high relative abundances, the
relevant stable isotopes are
35 Cl/
37 Cl (ratio ca. 3:1) and
79 Br/
81
Br (ratio ca. 1:1).
Based on the isotope pattern, it is easy to deduce the total number of chlorine and
bromine atoms in fragment or molecular ions. There is a systematic development of
isotope pattern that are clearly visible in mass spectra. For fragments containing one
chlorine atom, the signals have a A+2 pattern with the natural relative abundance of
ca. 3:1. If two chlorine atoms appear, the superimposition of the two atoms and their
isotopes forms a A+2+2 pattern with rel. abundances of 9:6:1 as illustrated in
Fig. 5.11. The same approach applied to bromine reveals a A+2 pattern with 1:1
intensities for one bromine atom containing signals, but a A+2+2 pattern with rel.
abundances of 1:2:1 for two bromine atom containing signals (Fig. 5.9). Consequently, it is obvious that these unique patterns can be used for calculating the
number of chlorine and bromine atoms in individual fragment or molecule ions.
These interpretations can be supported by the differences of the corresponding
signals with m/z 35 or 79 as well as m/z 70 or 158 representing the loss of individual
halogen atoms or the loss of Cl 2 or Br 2 molecules (see Fig. 5.11).
Noteworthy, hydrogen, nitrogen or oxygen isotopes are not used for spectra
interpretation in common organic mass spectrometry. But sometimes more exotic
elements receive interest due to their isotope pattern. In Organic Geochemistry one
example is related to the element tin. Peralkylated tin organic molecules can be
analyzed by GC/MS due to their volatility and lipophilicity. Tin exhibit a very
unique pattern of stable isotopes with elevated relative abundances. Based on this
pattern, tin organic compounds can be easily detected in mass spectrometry. This is
illustrated in Fig. 5.12 for biomethylated tributyl tin, a formerly common ingredient
in antifouling paints.
In summary, there are various aspects that can be used for an ab initio interpretation of mass spectra. These approaches cover the examination of the overall pattern
for differentiating aliphatic and aromatic moieties, the influence of branching on
fragment intensities, the interpretation of the molecular ion intensity, the consideration of preferred fragmentation at hetero atoms, the observation of even to odd
fragment distribution and, finally, the identification of isotope pattern.
110
5 GC/MS Data Evaluation