Mass Spectrometry

1
Mass Spectrometry
2
Gas Chromatogram of Flavor Compounds
3
Definition
A mass spectrometer bombards a substance under
investigation with an electron beam and
quantitatively records the result as a spectrum
of positive ion fragments. This record is a Mass
Spectrum. A mass spectrum is a presentation of
the masses of the positively charged fragments
vs. their relative concentration. Separation of
the positive charge ion fragment is on the basis
of mass. (Mass/Charge)
4
Mass Spectrum of Pentane
43
100
80
42
60
Intensity
57
41
40
27
20
72, M
39
29
10
20
30
40
50
60
70
m/z
5
Gas Chromatography and Mass Spectrometer
6
Mass Spectrometry
7
Essential Features of Mass Spectrometer
(1) Sample Inlet System GC inlet system - The
samples separated by gas chromatography are
introduced into the ion source of mass
spectrometer.
8
(2) Ion Source The stream of vaporized sample
molecules from sample injection system entering
the ion source interact with the beam of
electrons to form positive ions. The electron
beam is emitted from a hot filament. (3)
Accelerating Chambers The positive ions are
pushed out of the source by relatively small
repeller potential, and then accelerated by a
large potential difference (1 to 10KV - a strong
electrostatic field) between the first and second
accelerating slits.
9
Mass Spectrometry
10
(4) Analyzer (Ion Separation) The positively
charged ions from the ion source can be separated
according to the respective masses of the ions
magnetic deflection in a magnetic field by
varying either the magnetic field applied to the
analyzer tube. The mass which passes through the
exit slit is dependent upon the radius (r) of the
ion path in the magnetic field, the magnetic
field strength (B, gauss) and the ion
accelerating potential (V, volt) is defined by
the fundamental equation m/z 4.82 x 10-5
B2 r2 /v
11
Mass Spectrum of Pentane
43
100
80
42
60
Intensity
57
41
40
27
20
72, M
39
29
10
20
30
40
50
60
70
m/z
12
(4) Ion Collector The positive ions striking the
collector produce a flow of ions proportional to
the ion abundance. (5) Recorder The
amplified ion currents are measured on a
computer.
13
Ion Fragmentation Mechanisms
e

C
H
2e
3
C
H
and
C
H
C
H
and

C
H

3

3
2

3
14
Fragmentation of 2-Methylpentane



Facile
m/z 43
Facile



C
H
3
More difficult
m/z 71
C
H
3



C
H
C
H
C
H
C
H
C
H
3
2
2
3
m/z 57
15
Mass Spectrum of 2-Methylpentane
43
100
80
60
Intensity
40
71
20
57
86, M
10
20
30
40
50
60
70
80
90
m/z
16
Mass Spectrum of Pentane
43
100
80
42
60
Intensity
57
41
40
27
20
72, M
39
29
10
20
30
40
50
60
70
m/z
17
2-Chloroethylphenyl ether
94
100
H
C
l
107
80
O
94
60
Intensity
40
156
107
20
20
40
60
80
100
120
140
160
m/z
18
Fragmentation of Benzene
Benzene (C6H6) has substantial m/z 79 and m/z 80
peaks. The molecular ion is considered to be at
mass 78.
19
Fatty Acids
Molecular ion peak of a straight chain
monocarboxylic acid is weak but usually
discernible. The most characteristic peak
(sometimes the base peak) is at m/z 60 due to
McLafferty rearrangement.
H

C
H
R

O
H
C
C
H
R


2
C
H
C
O
H
2
C
H
2
H


H
O



O


C
H
O
H
O
C

C
H
2
C
H
2
20
Fatty Acids
21
Methyl - Ester of Fatty Acids
The mass spectrum of a methyl - ester is very
similar to that of corresponding carboxylic acid.
The methyl ester is more volatile than the free
fatty acids and therefore the easier to examine
m/z 74 Corresponding to the m/z 60 peak of
fatty acid is usually base peak or predominant
22
McLafferty Rearrangement
H
R
C
C
H
C
R

2
2
2

O


C
H
C
H
O
C
2
3
C
H
2
H
H
H


O


O




O




C
H
O
C
C
H
O
3
C
3
C
H
O
C
C
H

3
2
C
H

2
C
H
2
23
Methyl Ester of Fatty Acids
24
Benzene Compounds
O
H

C
H
O
H
2

-
CO
-
H


m/z
107
m/z
108
7 memberd ring

H
H
H
H
-
H
2

H
H
Benzonium ion
H
Phenyl ion
m/z
79
m/z
77

C
H


C
H

6
7
6
5
25
Benzene Compounds
Tropylium ion
CH3
-
CH3

H





H
CH3
CH3
m/z
91
H
H
CH2
H
CH2
H
CH2
CH2CHR




H
CHR
H
H
H
H
H
H
H
McLafferty Rearrangement
m/z 92
26
Mass Spectrum of n-Octylbenzene
Tropylium ion
27
Naphthalene
28
Unique Peaks of Mass Spectra of Some Compounds
29
Mass Spectrum of Unknown Compound
30
Mass Spectra of Unknown Compounds
91