Fragmentation Patterns in Mass Spectra
Fragmentation Patterns in Mass Spectra
This
page looks at how fragmentation patterns are formed when organic
molecules are fed into a mass spectrometer, and how you can get
information from the mass spectrum.
The origin of fragmentation patterns
The formation of molecular ions
When
the vaporized organic sample passes into the ionization chamber of a
mass spectrometer, it is bombarded by a stream of electrons. These
electrons have a high enough energy to knock an electron off an organic
molecule to form a positive ion. This ion is called the molecular ion - or sometimes the parent ion.
The molecular ion is often given the symbol M+ or
- the dot in this second version represents the fact that somewhere in
the ion there will be a single unpaired electron. That's one half of
what was originally a pair of electrons - the other half is the electron
which was removed in the ionisation process.
Fragmentation
The
molecular ions are energetically unstable, and some of them will break
up into smaller pieces. The simplest case is that a molecular ion breaks
into two parts - one of which is another positive ion, and the other is
an uncharged free radical.
The
uncharged free radical won't produce a line on the mass spectrum. Only
charged particles will be accelerated, deflected and detected by the
mass spectrometer. These uncharged particles will simply get lost in the
machine - eventually, they get removed by the vacuum pump.
The ion, X+,
will travel through the mass spectrometer just like any other positive
ion - and will produce a line on the stick diagram. All sorts of
fragmentations of the original molecular ion are possible - and that
means that you will get a whole host of lines in the mass spectrum. For
example, the mass spectrum of pentane looks like this:
It's
important to realize that the pattern of lines in the mass spectrum of
an organic compound tells you something quite different from the pattern
of lines in the mass spectrum of an element. With an element, each line
represents a different isotope of that element. With a compound, each
line represents a different fragment produced when the molecular ion
breaks up.
The molecular ion peak and the base peak
In
the stick diagram showing the mass spectrum of pentane, the line
produced by the heaviest ion passing through the machine (at m/z = 72)
is due to the molecular ion.
The tallest line in the stick diagram (in this case at m/z = 43) is called the base peak.
This is usually given an arbitrary height of 100, and the height of
everything else is measured relative to this. The base peak is the
tallest peak because it represents the commonest fragment ion to be
formed - either because there are several ways in which it could be
produced during fragmentation of the parent ion, or because it is a
particularly stable ion.
Using fragmentation patterns
This
section will ignore the information you can get from the molecular ion
(or ions). That is covered in three other pages which you can get at via
the mass spectrometry menu. You will find a link at the bottom of the
page.
Example 1: Mass Spectrum of Pentane |
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Let's have another look at the mass spectrum for pentane:
What causes the line at m/z = 57?
How
many carbon atoms are there in this ion? There can't be 5 because 5 x
12 = 60. What about 4? 4 x 12 = 48. That leaves 9 to make up a total of
57. How about C4H9+ then?
C4H9+ would be [CH3CH2CH2CH2]+, and this would be produced by the following fragmentation:
The methyl radical produced will simply get lost in the machine.
The
line at m/z = 43 can be worked out similarly. If you play around with
the numbers, you will find that this corresponds to a break producing a
3-carbon ion:
The line at m/z = 29 is typical of an ethyl ion, [CH3CH2]+:
The
other lines in the mass spectrum are more difficult to explain. For
example, lines with m/z values 1 or 2 less than one of the easy lines
are often due to loss of one or more hydrogen atoms during the
fragmentation process.
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Example 2: Pentan-3-one |
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This
time the base peak (the tallest peak - and so the commonest fragment
ion) is at m/z = 57. But this isn't produced by the same ion as the same
m/z value peak in pentane.
If you remember, the m/z = 57 peak in pentane was produced by [CH3CH2CH2CH2]+. If you look at the structure of pentan-3-one, it's impossible to get that particular fragment from it.
Work
along the molecule mentally chopping bits off until you come up with
something that adds up to 57. With a small amount of patience, you'll
eventually find [CH3CH2CO]+ - which is produced by this fragmentation:
You
would get exactly the same products whichever side of the CO group you
split the molecular ion. The m/z = 29 peak is produced by the ethyl ion -
which once again could be formed by splitting the molecular ion either
side of the CO group.
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Peak heights and the stability of ions
The
more stable an ion is, the more likely it is to form. The more of a
particular sort of ion that's formed, the higher its peak height will
be. We'll look at two common examples of this.
Examples involving carbocations (carbonium ions)
Summarizing the most important conclusion from the page on carbocations:
Order of stability of carbocations
primary < secondary < tertiary
Applying
the logic of this to fragmentation patterns, it means that a split
which produces a secondary carbocation is going to be more successful
than one producing a primary one. A split producing a tertiary
carbocation will be more successful still. Let's look at the mass
spectrum of 2-methylbutane. 2-methylbutane is an isomer of pentane -
isomers are molecules with the same molecular formula, but a different
spatial arrangement of the atoms.
Look
first at the very strong peak at m/z = 43. This is caused by a
different ion than the corresponding peak in the pentane mass spectrum.
This peak in 2-methylbutane is caused by:
The
ion formed is a secondary carbocation - it has two alkyl groups
attached to the carbon with the positive charge. As such, it is
relatively stable. The peak at m/z = 57 is much taller than the
corresponding line in pentane. Again a secondary carbocation is formed -
this time, by:
You would get the same ion, of course, if the left-hand CH3
group broke off instead of the bottom one as we've drawn it. In these
two spectra, this is probably the most dramatic example of the extra
stability of a secondary carbocation.
Examples involving acylium ions, [RCO]+
Ions
with the positive charge on the carbon of a carbonyl group, C=O, are
also relatively stable. This is fairly clearly seen in the mass spectra
of ketones like pentan-3-one.
The base peak, at m/z=57, is due to the [CH3CH2CO]+ ion. We've already discussed the fragmentation that produces this.
Using mass spectra to distinguish between compounds
Suppose you had to suggest a way of distinguishing between pentan-2-one and pentan-3-one using their mass spectra.
pentan-2-one | CH3COCH2CH2CH3 | |
pentan-3-one | CH3CH2COCH2CH3 |
Each
of these is likely to split to produce ions with a positive charge on
the CO group. In the pentan-2-one case, there are two different ions
like this:
- [CH3CO]+
- [COCH2CH2CH3]+
That would give you strong lines at m/z = 43 and 71.
With pentan-3-one, you would only get one ion of this kind:
- [CH3CH2CO]+
In
that case, you would get a strong line at 57. You don't need to worry
about the other lines in the spectra - the 43, 57 and 71 lines give you
plenty of difference between the two. The 43 and 71 lines are missing
from the pentan-3-one spectrum, and the 57 line is missing from the
pentan-2-one one.
The two mass spectra look like this:
Computer matching of mass spectra
As
you've seen, the mass spectrum of even very similar organic compounds
will be quite different because of the different fragmentations that can
occur. Provided you have a computer data base of mass spectra, any
unknown spectrum can be computer analysed and simply matched against the
data base.
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