suggestions can be used for obtaining partial structure information on the measured
molecule.
The comparison with mass spectral libraries has certainly its limitations. High
similarity does not imply clear evidence for identification. There are various examples pointing to be cautious with such simple interpretations. One example is related
to positional isomers. Mass spectra of positional isomers (e.g. substitution at phenyl
moieties) are often highly similar or even identical. An example is given in
Fig. 5.16a. Here, the 4-methyl and 6-methyl isomers cannot be distinguished solely
based on their mass spectra. Also the mass spectra of n-alkenes and corresponding nalkanols are highly similar (see Fig. 5.16b) due to elimination of H 2 O from alcohols
in the ion source forming n-alkenes. Therefore, both substances exhibit nearly the
same mass spectra and cannot be differentiated based on their mass spectral
properties.
These examples point to more comprehensive requirements for an unambiguous
identification of substances based on GC/MS. Noteworthy, for both examples a
differentiation of the potential target compounds can be achieved by their chromatographic behavior. Positional isomers have slightly but different gas chromatographic
retention times. Alkenes and alcohols exhibit not only different GC retention times
but can be also easily separated by fractionation.
Therefore, an unambiguous identification of a compound by GC/MS can only be
achieved by comparison of both mass spectral as well as gas chromatographic
properties with those of a reference substance.
General Note
An organic substance is unambiguously identified by GC/MS if the mass
spectrum and the gas chromatographic retention time match those of a reference substance under identical analytical conditions.
5.1 Identification
117
molecule.
The comparison with mass spectral libraries has certainly its limitations. High
similarity does not imply clear evidence for identification. There are various examples pointing to be cautious with such simple interpretations. One example is related
to positional isomers. Mass spectra of positional isomers (e.g. substitution at phenyl
moieties) are often highly similar or even identical. An example is given in
Fig. 5.16a. Here, the 4-methyl and 6-methyl isomers cannot be distinguished solely
based on their mass spectra. Also the mass spectra of n-alkenes and corresponding nalkanols are highly similar (see Fig. 5.16b) due to elimination of H 2 O from alcohols
in the ion source forming n-alkenes. Therefore, both substances exhibit nearly the
same mass spectra and cannot be differentiated based on their mass spectral
properties.
These examples point to more comprehensive requirements for an unambiguous
identification of substances based on GC/MS. Noteworthy, for both examples a
differentiation of the potential target compounds can be achieved by their chromatographic behavior. Positional isomers have slightly but different gas chromatographic
retention times. Alkenes and alcohols exhibit not only different GC retention times
but can be also easily separated by fractionation.
Therefore, an unambiguous identification of a compound by GC/MS can only be
achieved by comparison of both mass spectral as well as gas chromatographic
properties with those of a reference substance.
General Note
An organic substance is unambiguously identified by GC/MS if the mass
spectrum and the gas chromatographic retention time match those of a reference substance under identical analytical conditions.
5.1 Identification
117