A
mass spectrum is a plot of the
% relative abundance of cations that have arisen from ionisation of the sample against their
mass to charge ratio (m/z).The ionisation is the result of an electron being stripped from the atoms (molecules) present in a sample that is analysed used an instrument called a mass spectrometer. Because the
charge (z) on the cations produced in the mass spectrometer is nearly always
+1, the
ratio m/z is equal to the
mass number of the atom(s) in the cation. The
cation in largest abundance is assigned a
% relative abundance of 100%.
The abundances of others are relative to this one.
mass spec Si
In the mass spectrum of elemental silicon that is shown,
28Si has a % relative abundance of 100.0%, with
29Si being 5.1% and
30Si being 3.4%.
The sum of the %
relative abundnaces is greater than 100% (108.5% in this case)
The
sum of the
% natural abundances of the isotopes is
equal 100%.
For example the % natural abundance for chlorine is 75% 37Cl and 25% 35Cl.
Compare this with the relative abundances from the mass spectrum which would be 100% 37Cl and 33% 35Cl. Note that the relative abundances are larger than the % natural abundance.
The
% natural abundance of any isotope can be
calculated from the % relative abundances.
% natural abundance
28Si | = | % relative abundance 28Si | × | 100% |
| sum % relative abundance of all isotopes |
% natural abundance
28Si | = | 100 | × | 100% | = | 92% |
| 100+5.1+3.4 |
Thus the
% natural abundance of any isotope is
lower than its
% relative abundance in the mass spectrum.