All principal statements presented so far are relatively simple. Normally, in more
complex mass spectra superimposition of different aspects are observable. This
makes interpretation more complex but allows some deeper insights. This will be
exemplified in the following by the interpretation of fatty acid methyl esters and its
iso- and anteiso-isomers. In Fig. 5.8 indicative sections of mass spectra of
unbranched pentadecanoic acid methyl esters and iso-methyl (methyl branched at
the next-to-last position) and anteiso-methyl (methyl branched at the third-to-last
position) substituted homologues are illustrated. All methyl esters have one common
fragmentation by losing the Me-O moiety leading to fragments with M-31 m/z, here
resulting in m/z 225. Accompanied there is the loss of ethyl units forming peaks with
M-29 m/z, here as m/z 227. The relative abundance of these two peaks allow to
distinguish the isomers, because the first fragmentation is not influenced by the
isomerism. However, the fragmentation leading to m/z 227 is preferred in the
anteiso-isomer due to branching at the corresponding position but nearly impossible
for the iso-isomer. These circumstances induce an elevated 227/225 ratio for the
anteiso-isomer and a strong depletion of this value for the iso-isomer as compared to
the 227/225 ratio for the unbranched isomer.
A second example illustrating more complex mass spectrometric interpretation is
related to a specific rearrangement, the so-called McLafferty rearrangement. It is
also related to the mass spectrometric behavior of fatty acid methyl esters but
focusses solely on the functional group (see Fig. 5.9). At the methyl ester group
the aliphatic chain can be folded to a quasi-six ring and as a result a hydrogen atom
shifts from the aliphatic side chain to the carbonyl oxygen. This unstable transition
gets stabilized by elimination of an olefinic moiety and the formation of an unsaturated ester unit forming a very characteristic m/z 74 peak. This peak represents the
key fragment for identification of methyl esters. A comparable rearrangement of
10
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160 170 180 190 m/z
0
44
N
100
114
185
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
114
100
Fig. 5.7 Mass spectra of dihexylamine illustrating the nitrogen rule
106
5 GC/MS Data Evaluation
complex mass spectra superimposition of different aspects are observable. This
makes interpretation more complex but allows some deeper insights. This will be
exemplified in the following by the interpretation of fatty acid methyl esters and its
iso- and anteiso-isomers. In Fig. 5.8 indicative sections of mass spectra of
unbranched pentadecanoic acid methyl esters and iso-methyl (methyl branched at
the next-to-last position) and anteiso-methyl (methyl branched at the third-to-last
position) substituted homologues are illustrated. All methyl esters have one common
fragmentation by losing the Me-O moiety leading to fragments with M-31 m/z, here
resulting in m/z 225. Accompanied there is the loss of ethyl units forming peaks with
M-29 m/z, here as m/z 227. The relative abundance of these two peaks allow to
distinguish the isomers, because the first fragmentation is not influenced by the
isomerism. However, the fragmentation leading to m/z 227 is preferred in the
anteiso-isomer due to branching at the corresponding position but nearly impossible
for the iso-isomer. These circumstances induce an elevated 227/225 ratio for the
anteiso-isomer and a strong depletion of this value for the iso-isomer as compared to
the 227/225 ratio for the unbranched isomer.
A second example illustrating more complex mass spectrometric interpretation is
related to a specific rearrangement, the so-called McLafferty rearrangement. It is
also related to the mass spectrometric behavior of fatty acid methyl esters but
focusses solely on the functional group (see Fig. 5.9). At the methyl ester group
the aliphatic chain can be folded to a quasi-six ring and as a result a hydrogen atom
shifts from the aliphatic side chain to the carbonyl oxygen. This unstable transition
gets stabilized by elimination of an olefinic moiety and the formation of an unsaturated ester unit forming a very characteristic m/z 74 peak. This peak represents the
key fragment for identification of methyl esters. A comparable rearrangement of
10
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160 170 180 190 m/z
0
44
N
100
114
185
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
114
100
Fig. 5.7 Mass spectra of dihexylamine illustrating the nitrogen rule
106
5 GC/MS Data Evaluation