FURTHER MASS
SPECTROMETRY
A guide for A level students
KNOCKHARDY PUBLISHING
KNOCKHARDY PUBLISHING
MASS SPECTROMETRY
INTRODUCTION
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MASS SPECTROMETRY
CONTENTS
• Fragmentation of molecular ions - theory
• What a mass spectrum tells you
• Molecular ions
• Fragmentation
• Mass spectra of alkanes
• Mass spectra of halogenoalkanes
• Mass spectra of aldehydes and ketones
• Test questions
• Check list
MASS SPECTROMETRY
Before you start it would be helpful to…
• recall the basic principles of a mass spectrometer
• know the different types of functional group in organic chemsitry
MOLECULAR MASS DETERMINATION
USING MASS SPECTROMETRY
Nowadays, mass spectrometry is used
to identify unknown or new compounds.
IONISATION
When a molecule is ionised it forms a
MOLECULAR ION which can also
undergo FRAGMENTATION or REARRANGEMENT to produce particles of
smaller mass.
MOLECULAR ION
FRAGMENTION
Only particles
with a positive charge
will be deflected and detected.
RE-ARRANGEMENT
FRAGMENTION
The resulting spectrum has many peaks.
The final peak (M+) shows the molecular
ion (highest m/z value) and indicates the
molecular mass. The rest of the
spectrum provides information about
the structure.
THE MASS SPECTRUM
Spectra obtained for organic molecules have many peaks. Each peak is due to a
particular fragment with a certain m/z value.
highest m/z value usually corresponds to the molecular ion
its position provides information about the molecular mass of a substance
the tallest peaks come from the most stable species
THE MASS SPECTRUM
Spectra obtained for organic molecules have many peaks. Each peak is due to a
particular fragment with a certain m/z value.
highest m/z value usually corresponds to the molecular ion
its position provides information about the molecular mass of a substance
the tallest peaks come from the most stable species
Interpretation of thousands of spectra has shown that many classes of organic
compound show characteristic fragmentation patterns due to their functional groups.
It is possible to identify the type of compound from its spectrum by looking at the ...
position of peaks
differences between major peaks
THE MASS SPECTRUM - THE MOLECULAR ION
In the spectrum of octane, a signal occurs at 114 due to the species C8H18+
80
60
40
molecular ion
114
20
Abundance %
100
The species due to the final signal is known as the molecular ion and is usually
corresponds to the molecular mass of the compound.
0
.
10
20
30
40
50
60
70
80
90
100
110 120
m/z
130
140
THE MASS SPECTRUM - THE MOLECULAR ION
80
60
40
114
20
Abundance %
100
The small peak (M+1) at 115 due to the natural abundance (about 1%) of carbon-13.
The height of this peak relative to that for the molecular ion depends on the number
of carbon atoms in the molecule. The more carbons present, the larger the M+1 peak.
0
.
10
20
30
40
50
60
70
80
90
100
110 120
m/z
130
140
THE MASS SPECTRUM - FRAGMENTATION
60
80
43
29
71
85
40
57
114
20
Abundance %
100
The rest of the spectrum provides additional information of the molecule’s structure.
Peaks appear due to characteristic fragments (e.g. 29 due to C2H5+) and differences
between two peaks also indicates the loss of certain units (18 for H2O, 28 for CO).
0
.
10
20
30
40
50
60
70
80
90
100
110 120
m/z
130
140
FRAGMENTATION PATTERNS
ALKANES
The mass spectra of simple hydrocarbons have peaks at m/z values corresponding to
the ions produced by breaking C-C bonds. Peaks can occur at ...
m/z
15
CH3+
29
C2H5+
43
C3H7+
57
C4H9+
71
C5H11+
85
C6H13+
etc.
• the stability of the carbocation formed affects its abundance
• the more stable the cation the higher the peak
• the more alkyl groups attached to the carbocation the more stable it is
most stable
tertiary 3° > secondary 2° > primary 1°
least stable
alkyl groups are electron releasing and stabilise the cation
FRAGMENTATION PATTERNS
HALOGENOALKANES
81Br
40
60
80
molecular ion contains...79Br
20
Abundance %
100
Multiple peaks occur in the molecular ion region due to different halogen isotopes.
There are two peaks for the molecular ion of C2H5Br, one for the molecule containing
the isotope 79Br and the other for the one with the 81Br isotope. Because the two
isotopes are of similar abundance, the peaks are of similar height.
0
m/z
10
20
30
40
50
60
70
80
90
100
110 120
130
140
FRAGMENTATION PATTERNS
ALDEHYDES AND KETONES
Cleavage of bonds next to the carbonyl group (C=O) is a characteristic fragmentation
of aldehydes and ketones. A common fragment is carbon monoxide (CO) but as it is
a molecule and thus uncharged it will not produce a peak of its own. However, it will
produce an m/z drop of 28 somewhere in the spectrum.
The position of the carbonyl group influences the fragmentation pattern because the
molecular ion fragments either side of the carbonyl group
the more stable the acylium ion RCO+, the more abundant it will be and
the more abundant the species the taller its peak in the mass spectrum
FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the
molecular ion fragments either side of the carbonyl group.
O
CH3 C
•+
C4H9
MOLECULAR ION
has m/z = 100
FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the
molecular ion fragments either side of the carbonyl group.
O
CH3 C
•+
C4H9
MOLECULAR ION
has m/z = 100
O
C4H9 C+
CH3•
m/z = 85
O
C4H9
C•
CH3+
m/z = 15
Breaking the bond between the methyl
group and the carbonyl group
produces two possible ions,
depending on how the bond breaks.
Two peaks at m/z values 15 and 85 will
appear in the mass spectrum.
FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the
molecular ion fragments either side of the carbonyl group.
O
CH3 C
•+
C4H9
MOLECULAR ION
has m/z = 100
O
Breaking the bond between the butyl
group and the carbonyl group
produces two further ions, depending
on how the bond breaks.
Two peaks at m/z values 43 and 57 will
appear in the mass spectrum.
CH3 C+
C4H9•
m/z = 43
O
CH3 C•
C4H9+
m/z = 57
FRAGMENTATION PATTERNS
Aldehydes and ketones
The position of the carbonyl group influences the fragmentation pattern because the
molecular ion fragments either side of the carbonyl group.
O
Example;
CH3 C
•+
O
C4H9 C+
C4H9
MOLECULAR ION
has m/z = 100
O
CH3•
CH3 C+
m/z = 85
m/z = 43
O
O
C4H9
C•
CH3+
CH3 C•
m/z = 15
A further peak occurs at m/z = 72 (100-28) due to loss of CO
C4H9•
C4H9+
m/z = 57
IDENTIFY THE
COMPOUNDS
IDENTIFY THE COMPOUND
80
40
60
29
122
20
Abundance %
100
43
124
79 81
0
m/z
10
20
30
40
50
60
70
80
90
100
110 120
130
140
IDENTIFY THE COMPOUND
80
40
60
29
122
20
Abundance %
100
43
124
79 81
0
m/z
10
20
30
40
50
60
70
80
C3H7Br
90
100
110 120
130
140
80
105
77
60
51
120
40
43
28
20
Abundance %
100
IDENTIFY THE COMPOUND
0
m/z
10
20
30
40
50
60
70
80
90
100
110 120
130
140
80
105
77
60
51
120
40
43
28
20
Abundance %
100
IDENTIFY THE COMPOUND
0
m/z
10
20
30
40
50
60
70
80
90
C6H5COCH3
100
110 120
130
140
60
80
105 106
40
51
77
57
43
20
Abundance %
100
IDENTIFY THE COMPOUND
28
0
m/z
10
20
30
40
50
60
70
80
90
100
110 120
130
140
60
80
105 106
40
51
77
57
43
20
Abundance %
100
IDENTIFY THE COMPOUND
28
0
m/z
10
20
30
40
50
60
70
80
90
C6H5CHO
100
110 120
130
140
56 57
113
60
80
43
40
71
142
20
Abundance %
100
IDENTIFY THE COMPOUND
0
m/z
10
20
30
40
50
60
70
80
90
100
110 120
130
140
56 57
113
60
80
43
40
71
142
20
Abundance %
100
IDENTIFY THE COMPOUND
0
m/z
10
20
30
40
50
60
70
80
C10H22
90
100
110 120
130
140
REVISION CHECK
What should you be able to do?
Understand how mass spectrometry can be used to calculate molecular mass
Recall the term molecular ion and understand what information it provides
Interpret simple mass spectra
CAN YOU DO ALL OF THESE?
YES
NO
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relevant topic(s) again
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WELL DONE!
Try some past paper questions
FURHER MASS
SPECTROMETRY
The End
© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING