These changes can be highly indicative. However, it fails especially for some
carbonyls such as ketones and aldehydes, since the C¼O group exhibits the same
mass as compared to C 2 H 4 , consequently no shift can be observed.
There is one more special aspect that sometimes also provides insight into the
molecular composition, the so-called nitrogen rule. As a very simple fact, the mass of
organic molecules is commonly an even value. Consequently, fragments derived
from one bond linkage exhibit even mass values. The resulting mass spectra exhibit
an even molecular ion and are dominated by even fragment signals. Only in case of
fragment formation based on two bond linkages (e.g. eliminations) some even
fragment ions appear. This principal design of mass spectra gets reversed if nitrogen
appears in the molecule. To be accurate, if a molecule exhibits an odd number of
nitrogen atoms, the corresponding molecular mass is odd, and fragments formed by
mono linkages are even (see Fig. 5.7). The corresponding mass spectra are inverse to
most other spectra with odd molecular ions and mostly even fragment ions. This
effect is described by the ‘nitrogen rule’. Noteworthy, this rule influences also the
identification of the molecular ion. It is not necessarily the highest even m/z value
but can change under the circumstances described to the highest odd m/z value
(if visible!).
20
30
40
50
60
70
80
90
100
110
120
m/z
0
O
29
31
59 a
70
116
relative intensity
29
59
Fig. 5.6 Mass spectra of ethylpentyl ether representing preferred fragmentation at hetero atoms and
in α-position
Table 5.1 Significant ion series
Functional group
Start mass
Ion series
Alkanes
29 (C 2 H 5
+ )
43, 57, 71, 85, 99, 113, . . .
Alkenes
27 (C 2 H 3
+ )
41, 55, 69, 83, 97, 111, . . .
Ethers, alcohols
31(H 2 C¼O
+ H)
45, 59, 73, 87, 101, . . .
Ketones
43 (H 2 C-CO
+ )
43, 57, 71, 85, 99, 113, . . .
Amines
30 (H 2 C¼N
+ H 2 )
30, 44, 58, 72, 86, 100, . . .
5.1 Identification
105
carbonyls such as ketones and aldehydes, since the C¼O group exhibits the same
mass as compared to C 2 H 4 , consequently no shift can be observed.
There is one more special aspect that sometimes also provides insight into the
molecular composition, the so-called nitrogen rule. As a very simple fact, the mass of
organic molecules is commonly an even value. Consequently, fragments derived
from one bond linkage exhibit even mass values. The resulting mass spectra exhibit
an even molecular ion and are dominated by even fragment signals. Only in case of
fragment formation based on two bond linkages (e.g. eliminations) some even
fragment ions appear. This principal design of mass spectra gets reversed if nitrogen
appears in the molecule. To be accurate, if a molecule exhibits an odd number of
nitrogen atoms, the corresponding molecular mass is odd, and fragments formed by
mono linkages are even (see Fig. 5.7). The corresponding mass spectra are inverse to
most other spectra with odd molecular ions and mostly even fragment ions. This
effect is described by the ‘nitrogen rule’. Noteworthy, this rule influences also the
identification of the molecular ion. It is not necessarily the highest even m/z value
but can change under the circumstances described to the highest odd m/z value
(if visible!).
20
30
40
50
60
70
80
90
100
110
120
m/z
0
O
29
31
59 a
70
116
relative intensity
29
59
Fig. 5.6 Mass spectra of ethylpentyl ether representing preferred fragmentation at hetero atoms and
in α-position
Table 5.1 Significant ion series
Functional group
Start mass
Ion series
Alkanes
29 (C 2 H 5
+ )
43, 57, 71, 85, 99, 113, . . .
Alkenes
27 (C 2 H 3
+ )
41, 55, 69, 83, 97, 111, . . .
Ethers, alcohols
31(H 2 C¼O
+ H)
45, 59, 73, 87, 101, . . .
Ketones
43 (H 2 C-CO
+ )
43, 57, 71, 85, 99, 113, . . .
Amines
30 (H 2 C¼N
+ H 2 )
30, 44, 58, 72, 86, 100, . . .
5.1 Identification
105