Chemistry Unit 1: 1.4: Mass Spectrometry

Quick definition:
1. The mass spectrometer determines very accurately, the relative atomic mass of an isotope!
What Happens in a Mass Spectrometer?
1. In a nutshell: atoms are converted into ions, (remember ions?) accelerated, deflected before arriving at a detector. More details? Okay then.
  1. The steps are: Ionisation, acceleration, deflection then detection.
2. Ionisation:
  1. a beam of high energy electrons is fired from an electron gun that collides with the sample inside the MS and knocks and electron from it.
    1. this forms positive (+1) ions.
      1. nearly all atoms lose 1 electron but in some cases (around 5%) 2 electrons can be lost forming 2+ ions.
3. Acceleration:
  1. the newly formed +ions are attracted to -charged plates and accelerated to a high speed.
    1. the speed they reach is proportional to their mass: the higher the mass, the slower they travel, the lower the mass, the faster they go.
      1. some ions pass through slits in the plates, this forms the ions in a beam (sounds pretty Sci-fi, doesn't it?)
4. Deflection:
  1. the beam of ions then travels into a magnetic field at right angles (90 degrees) to the direction it's travelling.
    1. the magnetic field deflects the ion beam (pew! pew!) into an arc of a circle. The deflection of an ion depends on it's m/z ratio, or mass to charge ratio.
      1. in the m/z ratio, 'm' is the mass of the ion and 'z' is the charge (usually +1)
  2. heavier ions are deflected less (as we know) than lighter ones and 2+ ions are deflected twice as much as +1 ions of the same mass.
Detection:
1. the magnetic field's strength is gradually increased so that the ions of increasing mass enter the detector one after the other.
  1. ions collide with the detector, accept electrons, lose their charge and create a current that is proportional to their abundance.
    1. Note: 'abundance' simply means 'amount of'.
2. a computer measures the strength of the current created and works out the m/z ratio. A read-out called a mass spectrum is produced - this is normally a graph.
  1. a mass spectrum
How to Calculate Low Resolution Mass Spectrometry:
1. Take the readings/mass numbers from your mass spectrum (graph), e.g. neon has two isotopes of mass numbers 20 and 22 with abundances of 90% and 10% respectively.
  1. the mass number is the y axis, the abundance (%) is the x axis.
  2. to work out the relative atomic mass of an element, you do this: (90 x 20) + (10 x 22), then you divide the answer, which is 2020, by the total mass, which is 100.
  3. the answer is 20.2 which means the relative atomic mass of neon is 20.2

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Chemistry Unit 1: 1.4: Mass Spectrometry

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