aromatic hydrocarbons. As mentioned, aliphatic mass spectra are characterized by
numerous and systematic occurring peaks with higher intensities at lower masses.
On the contrary, mass spectra of aromatic hydrocarbons exhibit only very scarce
peaks but with high intensities dominantly for the molecular ion as illustrated for
some aromatics in Fig. 5.4.
However, often not solely aliphatic or aromatic moieties exist in organic molecules but both types of hydrocarbon moieties appear together. A close look on the
corresponding mass spectra allows to distinguish the individual structural units.
Interestingly, at the connection of the aromatic and aliphatic units not the direct
bond is the preferred point of fragmentation but the proximate aliphatic bond. The
formed methylene-substituted aromatic ions are stable as the result of enhanced
mesomerism. For phenyl substitution the corresponding benzyl fragment at m/z 91 is
characteristic. In Fig. 5.5 this preferred fragmentation is obvious by comparing the
base peaks of a methylated and an ethylated aromatic hydrocarbon. For the first one,
the aromatic pattern remains, whereas in the latter case a preferred separation of the
methyl group is obvious that produces a base peak at M-15 m/z.
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
29
43
57
71
85
99
141
127
113
155
239
268
m/z
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
29
43
57
71
85
99
113
127
141 155
183
268
m/z
239
141
71
113
183
253
253
Fig. 5.3 Mass spectra of 6-propylhexadecane and pristane with specific fragmentations
5.1 Identification
101
numerous and systematic occurring peaks with higher intensities at lower masses.
On the contrary, mass spectra of aromatic hydrocarbons exhibit only very scarce
peaks but with high intensities dominantly for the molecular ion as illustrated for
some aromatics in Fig. 5.4.
However, often not solely aliphatic or aromatic moieties exist in organic molecules but both types of hydrocarbon moieties appear together. A close look on the
corresponding mass spectra allows to distinguish the individual structural units.
Interestingly, at the connection of the aromatic and aliphatic units not the direct
bond is the preferred point of fragmentation but the proximate aliphatic bond. The
formed methylene-substituted aromatic ions are stable as the result of enhanced
mesomerism. For phenyl substitution the corresponding benzyl fragment at m/z 91 is
characteristic. In Fig. 5.5 this preferred fragmentation is obvious by comparing the
base peaks of a methylated and an ethylated aromatic hydrocarbon. For the first one,
the aromatic pattern remains, whereas in the latter case a preferred separation of the
methyl group is obvious that produces a base peak at M-15 m/z.
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
y
t
i
s
n
e
t
n
i
e
v
i
t
a
l
e
r
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
29
43
57
71
85
99
141
127
113
155
239
268
m/z
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
29
43
57
71
85
99
113
127
141 155
183
268
m/z
239
141
71
113
183
253
253
Fig. 5.3 Mass spectra of 6-propylhexadecane and pristane with specific fragmentations
5.1 Identification
101