Analysis of a mass spectrum,


Nonane has a molecular formula of C9H20 and a molecular weight of 128. The
parent ion is the molecular ion at 128. There is a small peak at m/e 129, which is

due to a molecule of nonane containing one 13C isotope (i.e. 12C8
13CH20). The natural
abundance of 13C is 1.1%. Therefore the chances of a 13C isotope being present
in nonane are 9 × 1.1% = 9.9%.
The base peak is at m/e 43. This is most likely a propyl ion [C3H7]+. There are
peaks at m/e 29, 43, 57, 71, 85 and 99. These peaks are all 14 mass units apart
which corresponds to a CH2 group. The presence of a straight chain alkane is often
indicated by peaks which are 14 mass units apart (Fig. 2).

The characteristic peaks for a straight chain alkane are 14 mass units apart, but
this does not mean that the chain is being ‘pruned’ one methylene unit at a time.
Decomposition of carbocations occurs with the loss of neutral molecules such as
methane, ethene and propene, and not by the loss of individual methylene units.
For example, the daughter ion at m/e 99 can fragment with loss of propene to give
the ion at m/e 57. The daughter ion at m/e 85 can fragment with loss of ethene or
propene to give the ions at m/e 57 and m/e 43 respectively. The daughter ion at
m/e 71 can fragment with loss of ethene to give the ion at m/e 43.
There are significant peaks at m/e 27 and m/e 41. These peaks result from
dehydrogenation of the ions at m/e 29 and m/e 43 respectively. The peak at m/e
41 can also arise from the ion at m/e 57 by loss of methane.
The most intense peaks in the mass spectrum are at m/e 43 and m/e 57. The
ions responsible for these peaks [C3H7]+ and [C4H9]+ can arise from primary fragmentations
of the molecular ion itself, as well as from secondary fragmentations
of daughter ions (m/e 99 to m/e 57; m/e 85 to m/e 43; m/e 71 to m/e 43).

In mass spectroscopy, the ions responsible for particular peaks are enclosed in
square brackets. This is because it is not really possible to specify the exact structure
of an ion or the exact location of the charge. The ionization conditions used in
mass spectroscopy are such that fragmentation ions can easily rearrange to form
structures more capable of stabilizing the positive charge. For example, the fragmentation
ion at m/e 57 arising from primary fragmentation is a primary carbocation,
but this can rearrange to the more stable tertiary carbocation (Fig. 3).

decimal places and establish the molecular formula. Consider the molecules CO,
N2, CH2N and C2H4. All of these molecules have the same molecular weight of 28
and in a normal mass spectrum would produce a molecular ion of that value. In a
high-resolution mass spectrum, the molecular ion is measured to four decimal
places and so we have to consider the accurate atomic masses of the component
atoms. The accurate mass values for the ions are as follows:
CO+ = 12C16O+ Accurate mass = 12.0000 + 15.9949 = 27.9949
N2
+ = 14N2
+ Accurate mass = 28.0061
CH2N+ = 12C1H2
14N+ Accurate mass = 12.0000 + 2.0156 + 14.0031 = 28.0187
C2H4
+ = 12C2
1H4
+ Accurate mass = 24.0000 + 4.0313 = 28.0313
If the measured mass of the molecular ion is 28.0076, this would be in line with
the theoretical accurate mass for nitrogen (i.e. 28.0062). Note that the peak being
measured in the mass spectrum is for the molecular ion. This ion contains the
most abundant isotope of all the elements present. For example, the molecular ion
for carbon monoxide is made up of 12C and 16O only. There are no molecules present
containing 13C or 17O since these would occur at a higher position in the mass
spectrum. Therefore, the theoretical values for the molecular weight are calculated
using the atomic weights for specific isotopes and not the accurate atomic weights
of the elements as they occur in nature. The latter (relative atomic weights) take
the relative abundances of the different isotopes into account and will be different
in value. For example, the accurate atomic weight of the carbon isotope 12C is
12.0000 and this is the value used for calculating the accurate mass of a molecular
ion. The accurate relative atomic weight of carbon is higher at 12.011 due to the
presence of the isotope 13C.

source : book of instant notes organic chemistry

5 thoughts on “Analysis of a mass spectrum,

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