The Molecular Ion (M+) Peak
The Molecular Ion (M+) Peak
This
page explains how to find the relative formula mass (relative molecular
mass) of an organic compound from its mass spectrum. It also shows how
high resolution mass spectra can be used to find the molecular formula
for a compound.
Using a mass spectrum to find relative formula mass
The formation of molecular ions
When
the vaporised organic sample passes into the ionisation 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.
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.
The
molecular ions tend to be unstable and some of them break into smaller
fragments. These fragments produce the familiar stick diagram.
Fragmentation is irrelevant to what we are talking about on this page -
all we're interested in is the molecular ion.
Using the molecular ion to find the relative formula mass
In
the mass spectrum, the heaviest ion (the one with the greatest m/z
value) is likely to be the molecular ion. A few compounds have mass
spectra which don't contain a molecular ion peak, because all the
molecular ions break into fragments. That isn't a problem you are likely
to meet at A'level.
For example, in the mass spectrum of pentane, the heaviest ion has an m/z value of 72.
Because
the largest m/z value is 72, that represents the largest ion going
through the mass spectrometer - and you can reasonably assume that this
is the molecular ion. The relative formula mass of the compound is
therefore 72.
Finding
the relative formula mass (relative molecular mass) from a mass
spectrum is therefore trivial. Look for the peak with the highest value
for m/z, and that value is the relative formula mass of the compound.
There are, however, complications which arise because of the possibility
of different isotopes (either of carbon or of chlorine or bromine) in
the molecular ion. These cases are dealt with on separate pages.
Using a mass spectrum to find a molecular formula
So
far we've been looking at m/z values in a mass spectrum as whole
numbers, but it's possible to get far more accurate results using a high
resolution mass spectrometer. You can use that more accurate
information about the mass of the molecular ion to work out the
molecular formula of the compound.
Accurate isotopic masses
For
normal calculation purposes, you tend to use rounded-off relative
isotopic masses. For example, you are familiar with the numbers:
1H | 1 | |
12C | 12 | |
14N | 14 | |
16O | 16 |
To 4 decimal places, however, these are the relative isotopic masses:
1H | 1.0078 | |
12C | 12.0000 | |
14N | 14.0031 | |
16O | 15.9949 |
The
carbon value is 12.0000, of course, because all the other masses are
measured on the carbon-12 scale which is based on the carbon-12 isotope
having a mass of exactly 12.
Using these accurate values to find a molecular formula
Two simple organic compounds have a relative formula mass of 44 - propane, C3H8, and ethanal, CH3CHO.
Using a high resolution mass spectrometer, you could easily decide
which of these you had. On a high resolution mass spectrometer, the
molecular ion peaks for the two compounds give the following m/z values:
C3H8 | 44.0624 | |
CH3CHO | 44.0261 |
You can easily check that by adding up numbers from the table of accurate relative isotopic masses above.
Example 1 | |||||||||||||||||||||
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A gas was known to contain only elements from the following list:
The gas had a molecular ion peak at m/z = 28.0312 in a high resolution mass spectrometer. What was the gas?
SOLUTION
After
a bit of playing around, you might reasonably come up with 3 gases
which had relative formula masses of approximately 28 and which
contained the elements from the list. They are N2, CO and C2H4. Working out their accurate relative formula masses gives:
The gas is obviously C2H4.
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