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FURTHER MASSSPECTROMETRY

A guide for A level students

KNOCKHARDY PUBLISHING

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INTRODUCTION

This Powerpointshow is one of several produced to help students understand

selected topics at AS and A2 level Chemistry. It is based on the requirements of

the AQA and OCR specifications but is suitable for other examination boards.

Individual students may use the material at home for revision purposes or it may

be used for classroom teaching if an interactive white board is available.

Accompanying notes on this, and the full range of AS and A2 topics, are available

from the KNOCKHARDY SCIENCE WEBSITE at...

www.argonet.co.uk/users/hoptonj/sci.htm

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

MASS SPECTROMETRYKNOCKHARDY PUBLISHING

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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

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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

MASS SPECTROMETRY

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IONISATION

FRAGMENTION

FRAGMENTIONRE-ARRANGEMENT

MOLECULAR MASS DETERMINATION

USING MASS SPECTROMETRY

Nowadays, mass spectrometry is usedto identify unknown or new compounds.

When a molecule is ionised it forms a

MOLECULAR ION which can also

undergo FRAGMENTATION orRE-

ARRANGEMENT to produce particles of

smaller mass.

Only particles with a positive charge

will be deflected and detected.

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.

MOLECULAR ION

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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

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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

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

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In the spectrum of octane, a signal occurs at 114 due to the species C8H18+

THE MASS SPECTRUM - THE MOLECULAR ION

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

60

80

100

Abundance%

114

.

The species due to the final signal is known as the molecular ion and is usuallycorresponds to the molecular mass of the compound.

molecular ion

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THE MASS SPECTRUM - THE MOLECULAR ION

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.

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

60

80

100

Abundance%

114

.

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THE MASS SPECTRUM - FRAGMENTATION

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

60

80

100

Abundance%

2971

43

57

114

85

.

The rest of the spectrum provides additional information of the molecules 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).

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The mass spectra of simple hydrocarbons have peaks at m/z values corresponding tothe ions produced by breaking C-C bonds. Peaks can occur at ...

m/z 15 29 43 57 71 85 etc.

CH3+ C2H5

+ C3H7+ C4H9

+ C5H11+ C6H13

+

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

ALKANES

FRAGMENTATION PATTERNS

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HALOGENOALKANES


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.

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

60

80

100

A

bundance% molecular ion contains...79Br 81Br

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Cleavage of bonds next to the carbonyl group (C=O) is a characteristic fragmentationof 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


ALDEHYDES AND KETONES

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The position of the carbonyl group influences the fragmentation pattern because themolecular ion fragments either side of the carbonyl group.


Aldehydes and ketones

O

CH3 C C4H9

MOLECULAR ION

has m/z = 100 +

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O

CH3 C C4H9

O

C4H9 C+

CH3

O

C4H9 C CH3+

m/z = 15

m/z = 85

MOLECULAR ION

has m/z = 100

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.

+

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O

CH3 C C4H9

O

CH3 C+

C4H9

O

CH3 C C4H9

+

m/z = 57

m/z = 43

MOLECULAR ION

has m/z = 100 +

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.

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O

CH3 C C4H9

O

CH3 C+

C4H9

O

CH3 C C4H9

+

m/z = 57

m/z = 43

O

C4H9 C+

CH3

O

C4H9 C CH3+

m/z = 15

m/z = 85

MOLECULAR ION

has m/z = 100 +

A further peak occurs at m/z = 72 (100-28) due to loss of CO

Example;

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IDENTIFY THECOMPOUNDS

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IDENTIFY THE COMPOUND

122 124

29

79 81

43

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

Abundance%

20

40

6

0

80

100

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122 124

29

79 81

43

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

Abundance%

20

40

6

0

80

100

C3H7Br

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120

28

77

43

51

105

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

6

0

80

100

Abundance%

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120

28

77

43

51

105

10 20 30 40 50 60 70 80 90 100 110 120 130 1400 m/z

20

40

6

0

80

100

Abundance%

C6H5COCH3

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0

28

105 106

77

57

43

51

10 20 30 40 50 60 70 80 90 100 110 120 130 140

m/z

Abund

ance%

20

40

6

0

80

100

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0

28

105 106

77

57

43

51

10 20 30 40 50 60 70 80 90 100 110 120 130 140

m/z

Abund

ance%

20

40

6

0

80

100

C6H5CHO

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0

142

113

71

56 57

43

10 20 30 40 50 60 70 80 90 100 110 120 130 140

m/z

Abund

ance%

20

40

6

0

80

100

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0

142

113

71

56 57

43

10 20 30 40 50 60 70 80 90 100 110 120 130 140

m/z

Abund

ance%

20

40

6

0

80

100

C10H22

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REVISION CHECK

What should you be able to do?

Understand how mass spectrometry can be used to calculate molecular massRecall 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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You need to go over therelevant topic(s) again

Click on the button to

return to the menu

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WELL DONE!Try some past paper questions

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FURHER MASSSPECTROMETRY

The End

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