2.2    Mass Spectrometer

What is  mass spectrometer?

• A mass spectrometer is used to determine:
  1. relative isotopic mass
  2. relative abundance of isotopes
  3. relative atomic mass
  4. relative molecular mass
  5. structural formula of compounds

Determination of relative atomic mass using mass spectrometer

• Five steps:
  1. Vaporisation
     • atoms are vaporised to form gaseous atom.

• Ionisation
  • gaseous atoms are bombarded with high energy electrons to form positive ions.

• Acceleration
  • the ions are accelerated so that they have the same kinetic energy

• Deflection
  • ions are deflected by a magnetic field. The amount of  deflection depends on:
• the mass of the ion
• the amount of positive charge on it
• –   the larger the mass, the smaller the deflection.
• the higher the charge, the larger the deflection
• the two factors combine into mass/charge ratio (m/e or m/z).
• the smaller the value of m/e, the larger the    deflection

• Detection
  • the beam of ions are detected electrically
  • the data are fed into the computer and the mass spectrum is produced

Mass spectrum (How to calculate relative atomic mass, Ar from it?)

Example:

The mass spectrum of boron, B is as shown, given the relative abundances:

B-10 : 23

B-11 : 100