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Mass Spectrometry

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Sir J.J. Thomson

Sir J.J. Thomson

What is mass spectrometry?

Mass spectrometry is used to display the spectra of the masses of molecules in a sample of material. It is used for determining the composition of elements, the masses of particles/molecules and for looking at the chemical structures of molecules.

Mass spectrometry was first developed by Sir J.J. Thomson, a British physicist in the early 20th century, who won a Nobel prize for physics in 1906 for his work.

The uses of mass spectrometry are:

  • Determine the relative abundance of isotopes in a certain element.
  • To get information about the chemical properties and the structures of different molecules.
  • To identify and analyse unknown compounds.
  • Many space missions have used mass spectrometers to determine the composition of solar wind and other such things.
  • Determining whether an athlete has taken steroids or other such banned drugs.
  • Analysing marine life for toxic chemical traces.

How does it work?

Mass spectrometers measure the mass-to-charge ratio of ions that are in a molecule or isotope, it uses this to then determine the mass. The following steps describe how a mass spectrometer works:

  • The sample is entered into the spectrometer via a sample inlet.
  • The molecules within the sample are then made into ions by electron impact, lazers, chemical ionisation etc.
  • The ions are then propelled into something called a mass analyser where they will be separated according to their mass-to-charge ratio by processes such as using a magnetic field or measuring how long it takes for the different ions to reach a particular detector.
  • The ions are then detected and computer software will show a mass spectrum which can then be used to determine the things mentioned earlier (such as mass or isotope abundance).
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How to read a spectrum and what it shows

The mass spectrum graph will show the abundance of all the isotopes in the element that is being investigated and the relative atomic mass of the element can be calculated from this.

The graph will show the percentage abundance as the Y-axis and the mass-to-charge ratio as the X-axis. The amount of peaks in the spectra will show the amount of isotopes of the element and the height of said peaks will show relative abundance of those isotopes.

To work out the atomic mass from this you multiply the isotope number by the relative percentage abundance of that isotope. Do this for all of the isotopes in the element and then add the figures together and divide by 100. Ta da, there you have the atomic mass of the element!

The below graph is the mass spectrum of copper (Cu).


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