Mass Spectrometry

• • You can use a mass spectrum showing the relative isotopic abundances of an element to work out its relative atomic mass.
  • A mass spectrum is produced by a mass spectrometer.
  • The molecules in the sample are bombarded with electrons and a molecular ion, M+ (g) is formed when the bombarding electrons remove an electron from the molecule.
  • On a mass spectrum the y-axis gives the abundance of the ions, often as a percentage.
  • The x-axis is the mass/charge ratio. This is just the molecular mass of the ion divided by its charge.
• How to find the relative atomic mass of a compound

  • To find the relative molecular mass of a compound you look at the molecular ion peak (the M peak).
  • The M peak is the one with the highest mass/charge ratio, ignoring any M+1 peak.
  • The M+1 peak accounts the the 1% of the carbon atoms that are Carbon-13.
  • The mass/charge value of the molecular ion peak is the molecular mass of the compound.

  
• Fragmentation

  • The bombarding electrons make some of the molecular ions break up into fragments consisting of the ion and a radical.
  • The fragments that are ions show up on the mass spectrum, making a fragmentation pattern. Fragmentation patterns can be used to identify molecules and their structure.
  • Only the ions show up on the mass spectrum- the free radicals are "lost" because they are uncharged.

  
• Common fragment ions 


  • It you can't remember an m/z value, you can work it out by finding the Mr of the ion.

  
• Identifying the structural formula of a molecule from its mass spectra

  • To work out the structural formula, you've got to work out what ion could have made each peak from its m/z value.

  
• Identifying the structural formula of a molecule from its mass spectra (2)

  
• Calculating the number of carbon atoms from a Mass Spectra