One of the most valuable pieces of information obtained from the mass spectrometer is the
relative atomic mass or relative molecular masses of the unknown sample. Obtaining this
information is a good way to help in identifying the unknown substance/substances that you are analysing.
The mass spectrum produced by the spectrometer is simply a graph of relative abundance against mass to charge (m/z) ratio. The mass spectrum for a sample of the element boron is shown opposite:
There are two peaks for the element, indicating the presence of 2 isotopes. Assuming the boron ions in the spectrometer had a charge of +1 the m/z ratio will be the mass of each isotope, in this case the masses will be 10 and 11. The abundances of each isotope is also shown, we can see that one isotope is much more common than the other, the isotopes are present almost in the ratio of 4:1.
As you can probably recall the relative atomic mass (Ar) of an element is the average mass of all the atoms taking into account the relative abundances of any isotopes of the element compared to one-twelfth of the mass of an atom of 12-C.
To calculate the relative atomic mass of boron we can use the data obtained from the mass spectrum and substitue the values into the formula below ( note: the Greel letter sigma (Σ) in the equation simply means to sum all the terms:
So using the figures from the mass spectrum we have:
You may recall from gcse science that there a several elements that are diatomic, that is they consist of molecules made up of two atoms. The diatomic elements are shown in the table below.
The mass spectrum of these simple molecules can be a little confusing to begin with, since more peaks are obtained than you might at first have expected. As an example consider chlorine. Chlorine has two isotopes, 35Cl and 37Cl. This means that we can end up with molecules with a variety of masses:
The 35Cl isotope is more common than the 37Cl isotope, they are present in the ratio of 3:1 or 75%:25%; 35Cl to 37Cl so in the mass spectrum we should expect the peak for 35Cl-35Cl to be the largest and the peak for 37Cl-37Cl to be the smallest.
You may be wondering why there are peaks at 35 and 37 when chlorine is a diatomic molecule. Remember that in the ionisation stage inside the mass spectrometer the molecules are bombarded with fast moving electrons. These electrons can and will smash molecules up in smaller parts. This is called fragmentation. The larger the molecule the more likely it is to be fragmented and broken up inside the mass spectrometer. When a molecule passes through the mass spectrometer provided that is not fragmented the peak produced with the largest m/z ratio is called the molecular ion peak, it will correspond to the Mr of the sample injected into the spectrometer. However for large molecules there may be no molecular ion peak simply because the molecule has not survived the electron bombardment from the electron gun. This is one of the advantages of electro-spray ionisation, where molecules tend to stay intact and not fragment