1.1 Atomic Structure

• Positions of Particles:

  • Proton - Nucleus
  • Neutron - Nucleus
  • Electron - Orbitals
• Relative mass of Particles:

  • Proton - 1
  • Neutron - 1
  • Electron - 1/1840
• Relative change of Particles:

  • Proton: +1
  • Neutron: 0
  • Electron: -1
• Atomic Number (Z)

  • Number of Protons in the atom.
• Mass Number (A)

  • Total number of protons and neutrons in the atom.
• No. of Neutrons = A - Z
• Isotopes

  • atoms with the same number of protons, but different number of neutrons.
• Isotopes have similar chemical properties because they have the same electronic structure.
• 4 steps in a Mass Spectrometer:

  1. Ionization
  2. Acceleration
  3. Flight Tube
  4. Detection
• The sample can be ionized in a number of ways:

  1. Electron Impact
  2. Electro Spay Ionization
• Electron Impact

  • used for elements and substances with low formula mass.
  • It can cause larger organic molecules to fragment.
• Electro Spay Ionization

  • used preferably for larger organic molecules.
  • The 'softer' conditions of this technique mean fragmentation does not occur.
• Acceleration

  • positive ions are accelerated by an electric field to a constant kinetic energy.
• KE = 1/2mv^2

  where:

  KE = kinetic energy of particle in Joules

  m = mass of the particle in kg

  v = velocity of the particle in m/s^-1
• Flight Tube

  • positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster.
  • The heavier particles take longer to move through the drift area.
  • The ions are distinguished by different flight times.
• Formula for flight time:

  t = d/v

  where:

  t = time of flight in seconds

  d = length of flight tube in meters

  v = velocity of the particle in m/s^-1
• Detection

  • the ions reach the detector and generate a small current, which is fed to a computer for analysis.
• • For each isotope, the mass spectrometer can measure a m/z (mass / change ratio) and an abundance.