Biomedical Engineering Reference
In-Depth Information
4.2.2 Defi nitions of molecular mass
In mass spectrometry, it is important to realise that the spectrometer will
be measuring individual isotopes, so that, for instance, elements with two
or more commonly occurring isotopes will be seen as multiple peaks (e.g.
chlorine 35 Cl, 37 Cl).
The average mass of a molecule is the sum of the average atomic masses
of the constituent elements. The average atomic mass is determined by
the naturally occurring abundances of isotopes. For instance, carbon has
an average mass of 12.0107(8) but is made up of 98.93% 12 C (mass = 12
exactly) and 1.07% 13 C (mass = 13.0033548378). The average mass is
used in general chemistry but not mass spectrometry.
The monoisotopic mass is the sum of the most abundant isotopes in
each molecule. For most typical organic molecules, this means the lightest
of the naturally occurring isotopes. The monoisotopic mass will therefore
represent the biggest peaks detected in the mass spectrometer. (However,
for some heavier atoms this does not hold, for example iron (Fe) the
lightest isotope is not the most abundant.)
The fact that the mass of an isotope is not exactly the sum of its
neutrons and protons is due to an effect called the 'mass defect' and is
due to the binding energy that holds the nuclei together. Some of the
atomic mass is converted to energy according to the principle of relativity
(E = mc 2 ). Each isotope has a characteristic mass defect and this can be
used to calculate an exact mass (for instance, see Table 4.1).
Thus, by measuring the mass accurately on the spectrometer and
matching that to a theoretical calculated mass, a molecular formula may
be determined. Of course, many molecular structures are possible from a
single formula and depending on the precision of the measurement there
may be more than one molecular formula that fi ts the accurate mass. As
molecular weight increases, the number of formulae that will fi t a
measured mass will increase for any given mass accuracy.
￿ ￿ ￿ ￿ ￿
Table 4.1
Isotope
Mass
Example
12 C
12.00000
C6H5NO2
1 H
1.007825
14 N
14.003074
Monoisotopic mass
=123.032029
16 O
15.994915
 
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