Mass Spectrometry

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First step

The atoms must be vaporised before entering the mass spectrometer.

A vacuum must be created in the mass spectrometer.

The ions produced have to be able to run through the mass spectrometer without hitting air molecules.

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Stage 1: Ionisation

The vaporised sample is passed into the ionisation chamber.

An electrically heated metal coil gives off electrons which are attracted to the electron trap. The electron trap is a positively charged plate.

The atoms/molecules in the sample are bombarded with a stream of electrons and some of these collisions have enough energy to knock out one or more electrons.
This forms positive ions.

This happens for all atoms - even ones which would normally form negative ions or no ions at all.
The atoms must be positive ions in order for mass spectrometry to work.

The majority of the positive ions formed will have a charge of +1 because it is more difficult to remove electrons from a positive ion.
These positive ions are forced into the rest of the mass spectrometer by the ion repeller. The ion repeller is another metal plate carrying a slight positive charge.

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Stage 2: Acceleration

  • Two negatively charged plates are places either side of the mass spectrometer
  • There are 3 small slits between the two which the positive ions go through
  • As the negative force is equal on either side, the positive ions go straight through the slit, rather than towards the negatively charged plates (which would happen if there was only one plate as opposites charges attract).
  • This accelerates the positive ions so they all have the same kinetic energy.(
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Stage 3: Deflection

  • The positive ions are deflected by a magnetic field
  • The amount of deflection depends on two things:
    • The lighter the ion, the more it will be deflected
    • The more the ion is charged (A.K.A. the more electrons that were knocked off in the ionisation stage), the more it will be deflected

These two factors create the mass/charge ratio (written as m/z or m/e).


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Stage 4: Detection

The beam of ions is detected electrically.

Only ions which pass through all 3 slits in the acceleration stage and are deflected around the whole spectrometer will make it to the ion detector. The other ions collide with the walls where they pick up electrons to be neutralised and eventually removed by a vacuum pump.

When an ion hits the detector, an electron jumps from the metal onto the ion in order to neutralise it. That leaves a space amongst the electrons in the metal, and the electrons in the wire move to fill the gap.

A flow of electrons in the wire is detected as an electric current which can be amplified in order to be recorded.

More ions = greater current

The ions which were not deflected towards the detector can be detected by changing the force/strength of the magnetic field.

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Relative abundance


This shows a graph of a typical mass spectrometry result.

The vertical axis is labelled 'relative abundance' or 'relative intensity'. It is related to the current recieved by the chart recorder. The greater the current, the more abundant/intense the ion.

The most commen ion has a mass/charge ratio of 98.

This graph shows that this particular particle consists of 7 different isotopes.

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Calculating atomic masses

The relative sizes of the peaks on a mass spectrometry graph shows a measure of the relative abundances of the different isotopes.

Multiply each of the m/z ratios of isotopes by their relative abundances. Add these values together and divide by 100 if the abundance is given as a percentage, or divide by the sum of the abundances.

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Example of calculating atomic masses

Example: ( m/z = 10 relative abundance = 23 m/z = 11 relative abundance = 100 (10*23) + (11*100) = 1330 1330/(23+100) = 10.8 10.8 is the relative atomic mass of the element.

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