3.3.6.2 Mass Spectrometry

• Stages in Mass Spectrometry

  1. Stage 1: Ionisation - The atom is ionised by knocking one or more electrons off to give a positive ion

  2. Stage 2: Acceleration - The ions are accelerated so that they all have the same kinetic energy

  3. Stage 3: Deflection - The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected

  4. Stage 4: Detection - The beam of ions passing through the machine is detected electrically
• Particles must be ionised so they can be accelerated and deflected
• High resolution Mass spectrometry is used to calculate accurate molecular mass for molecules with the same molecular mass
• Using high resolution mass spectra, specific peaks are found for different molecules with the same molecular mass
• Mass spectrum of an element is used to calculate its relative isotopic mass and relative abundances
• The unknown compound’s mass spectrum is compared with mass spectrums of known compounds from a database to find a match. This method is called fingerprinting
• In a mass spectrometer

  1. The compound is vaporised

  2. It is then bombarded with high-energy electrons

  3. This knocks the electrons from the molecules and breaks the covalent bonds

  4. As a result, the compound is fragmented
• Molecular ion, M+

  The molecule with an electron knocked off
• Fragmentation

  The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion
• We can determine the molecular mass of a molecule by using mass spectrometry

  By locating the M+ peak, which produces the peak with the highest m/z value in the mass spectrum
• Fragment patterns give information about the structure of the compound
• Organic compounds produce a unique mass spectrum, used as a “fingerprint”
• m/z value and possible identity of fragment ion

  • 15 - CH3+
  • 17 - OH+
  • 29 - C2H5+
  • 43 - C3H7+
  • 57 - C4H9+
• The relative atomic mass for carbon-12 isotope is exactly 12 because it is by definition and is the standard/reference.
• We can use the fragments to determine the structure of an unknown compound to give its precise identity. This is because even though the molecular ion peak will have the same m/z value, the fragmentation patterns will be different.