Mass Spectrometry

• Mass Spectra can be used to identify the molecular mass of an organic compound and to gain further information about its structure
• When an organic compound is placed in the mass spectrometer, it loses an electron and form a positive ion, the molecular ion.
• The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound
• The molecular ion, M+ is the positive ion formed when a molecule loses an electron
• To find the molecular mass, the molecular ion peak has to be located. The molecular ion peak is the clear peak at the highest m/z value on the right hand side of the mass spectrum
• There will also usually be a very small peak one unit after the M+ peak. This is referred to as the M+1 peak, which exists because 1.1.% of carbon is present as the carbon-13 isotope e.g. propan-1-ol has a molecular mass of 60, but a small proportion of these molecules will contain an atom of carbon-13 and thus have a mass of 61
• In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation
• The other peaks in a mass spectrum are caused by fragment ions, formed from the breakdown of the molecular ion
• The simplest fragmentation breaks a molecular ion down into two species - a positively charged fragment ion and a radical. Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals are not detected
• The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures. Mass spectra can therefore be used to help identify molecules.
• So, even though 2 molecules may have the same molecular mass and the same molecular ion peak, the fragment ions found in the spectrum may be different
• Main features of a mass spectrum are:

  • molecular ion peak, M+
  • small M+1 peak
  • number of fragment ion peaks
• • m/z = 15 - CH3+
  • m/z = 29 - CH3CH2+ (C2H5+)
  • m/z = 31 - CH2OH+
  • m/z = 45 - CH3CH2O+
• the M+1 peak can be used to identify the number of carbon atoms present in the molecules of an organic compound:

  • number of carbon atoms = height of M+1 peak
  • height of M peak X 100
• drug testing in sport:

  • mass spec used to test urine
  • clenbuterol is believed to increase the development of muscle
  • considered a performance enhancing drug and is banned in athletic competitions
  • athletes who test positive - claim must have eaten contaminated meat - used to increase yields in livestock production
  • banned for use in cattle in USA and much of Europe