Transcript Mass Spectrometry - La Salle University

Mass Spectrometry
Only Cations are Detected
Ionization to Radical Cation
Molecular Ion (m+)
Glossary
• Molecular ion - The ion obtained by the loss of
one electron from the molecule (m+)
• Base peak - The most intense peak in the MS,
assigned 100% intensity
• Radical cation - positively charged species with
an odd number of electrons
• Fragment ions - Lighter cations (and radical
cations) formed by the decomposition of the
molecular ion. These often correspond to stable
carbcations.
• m/z - mass to charge ratio
Methanol
Octane, m+ = 114
m-71
-71 (base)
Base peak
-57
-43
-29
-15
CH3CH2CH2CH2CH2CH2CH2CH3
m+ = 114
m-57
m-43
m-29
m+
Isooctane, no molecular ion
CH3 CH3
CH3CCH2CHCH3
CH3
m+ = 114
loss of .
(isobutyl)
CH3
CH3C +
CH3
m/z = 57
Decane
2-Methylpentane
Effect of Branching in
Hydrocarbons
Isotopes
• Mass spectrometers are capable of separating and
detecting individual ions even those that only differ by
a single atomic mass unit.
• As a result molecules containing different isotopes can
be distinguished.
• This is most apparent when atoms such as bromine or
chlorine are present (79Br : 81Br, intensity 1:1 and 35Cl :
37Cl, intensity 3:1) where peaks at "M" and "M+2" are
obtained.
• The intensity ratios in the isotope patterns are due to
the natural abundance of the isotopes.
• "M+1" peaks are seen due the the presence of 13C in
the sample.
Bromomethane
1-Bromopropane
2-Chloropropane
(3-Chloropropyl)benzene
CH2
CH2CH2Cl
base peak m/e 91 (m-63)
Chloroacetone
O
+
m/z = 57
CH3CCH2
.
- Cl
O
CH3CCH2Cl
m+
= 92
m+2 = 94
(3:1 ratio)
.
- CH3
.
+
O
CCH2Cl
- CH2Cl
O
CH3C
m/z = 77,79
+
m/z = 43 (base)
+.
O
3-Pentanone
+
CH2CH3
C
CH3CH2
. loss of
CH3CH2C=O
CH2CH3
m/z = 57
m/z = 86
m-29
base
m+
+.
O
2-Pentanone
+
CH2CH2CH3
C
CH3
. loss of
CH3C=O
CH2CH2CH3
m/z = 43
m/z = 86
m-43
base
m+
m-15
(E) 2-Hexene
2-methyl-2-pentene
and
2-hexene
1-Butanol
CH3CH2CH2CH2OH
+.
- H2O
CH3CH2CH
m+ = 74
+
CH2OH
m/z = 56
+
CH2=OH
m/z = 31
m-18
CH2
+
.
1o Alcohols: Loss of H2O
CH3
+.
.
+CH
2
loss of H
Toluene
+
m/z = 92
m/z = 91 tropylium ion
m-1
m+
+
CH2CH2CH3 .
+
CH2
. loss of
CH2CH3
Propylbenzene
+
m/z = 120
m/z = 91
m-29
m+
CH3
H3C
C
Isopropylbenzene
H
+
+.
CH3
CHCH3
loss of
.CH
3
+
m/z = 120
m/z = 105
m-15
m+
n-Butylbenzene
m/e 91
CH2CH2CH2CH3
m/e 92
3-Methyl-1-penten-3-ol
m/z = 71
OH
m+
4-Methyl-1-penten-3-ol
m/z = 57
OH
m+
McLafferty Rearrangement
link to SDBS
+.
O
H
O
H
m+ = 86 H
H


+.

H transfer from carbon
results in loss of a neutral alkene
O
H
+.
+
H
m/z = 44
McLafferty Rearrangements in
Alkyl Benzenes

CH2

CH2
+.
CH2
loss of CH 3CH=CH2
H
CHCH3
H 
H
m/e 92
m+ 134
- propyl
.
CH2
+
m/e 91
+.
Benzamide
2-Octanone
H


+. loss of
.
C6H13

+
O
O
m+ = 128
m/z = 43
+.
loss of pentene
via McLafferty
O
H
m/z = 58
2-Octanone
H


+. loss of
.
C6H13

+
O
O
m+ = 128
m/z = 43
+.
loss of pentene
via McLafferty
O
H
m/z = 58
Decanoic Acid
p-Chloroacetophenone
Methyl Octanoate
2,4-Dimethoxyacetophenone
Cyclohexanone
High Resolution Mass Spectrometry
Determination of Molecular Formula
CO
N2
C2H4
all show m + at 28
CH2N
exact mass
CO 27.9949
N2 28.0062
C2H4 28.0312
CH2N 28.0187
Isotope Ratios Can Help to
Determine Molecular Formula
MF
CO
N2
C2H4
MW
28.0
28.0
28.0
Relative intensities (%)
M
M+1
M+2
100 1.12
0.2
100 0.76
---100 2.23
0.01
Comparisons of Molecular Weights
and Precise Masses
MF
C3H8O
C2H8N2
C2H4O2
CH4N2O
MW
60.1
60.1
60.1
60.1
exact mass
60.05754
60.06884
60.02112
60.03242
Determine the Formula
fragment finder
rel. intensity (%)
Molecular mass m+1 m+2
110
111 112
100
6.96 0.60
exact mass = 110.0376
Determine the Formula
rel. intensity (%)
Molecular mass m+1 m+2
118
119 120
100
7.45 4.55
Subtract Sulfur’s contribution
fragment finder
rel. intensity (%)
subtract sulfur (32)
Molecular mass m+1 m+2
118
119 120
100
7.45 4.55
86
100
87
88
6.67 0.15
Determine the Molecular
Formula
rel. intensity (%)
Molecular mass m+1 m+2
154
155 156
100
15.41 3.77
Compound gives four signals in
the C-13 NMR spectrum
rel. intensity (%)
Molecular mass m+1 m+2
m+4
190
191 192
194
100
6.48 130.77 31.81