Spectroscopic techniques

Mass Spectrometry
1. Stages
  1. 1) Vaporisation of sample
  2. 2) Ionisation
    1. Molecules are bombarded by an electron gun
    2. form +1 ions
  3. 3) Acceleration
    1. using an electric field
  4. 4) Separation/deflection
    1. The lower the mass to charge ratio the more it will be deflected
  5. 5) Detection
    1. using computer methods
2. Time of Flight Mass Spectrometry (TOF-MS)
  1. Used to separate and to measure the relative masses of the ions produced under high-vacuum conditions
3. History
  1. 1918 by Francis W Aston, a student of J J Thompson (the man who discovered the electron
  2. Aston used the instrument to show that there were different forms of the same element. We now call these isotopes.
4. Uses
  1. Initially used to show the identity of a isotopes
  2. Now used to calculate molecular masses and characterise new compounds
5. A spectrometer
Infra-Red Spectroscopy
1. The Infra-red Spectrophotometer
  1. A beam of Infra-red radiation is passed through the sample
  2. A similar beam is passed through the reference cell
  3. The frequency of the radiation is varied
  4. The amount of radiation absorbed by a substance is compared with the radiation
  5. The results are stored and plotted
2. Key points
  1. In IR spectroscopy molecules absorb frequencies corresponding to their functional group. These absorptions are resonant frequencies i.e the frequency of the absorbed radiation matched the vibrational frequency.
  2. Energy quoted in wavenumbers. Unit= reciprocal centimetres cm-1
  3. The amount of vibration depends on...
    1. 1) Bond strength (and bond length)
    2. 2) the mass of each atom
  4. For a molecule to absorb IR, vibration with in the molecule (stretching or bending) must cause a net change in the dipole moment)
  5. Peak position depends on 1) Bond strength 2) masses of the atoms joined by the bond
  6. Strong bonds and light atoms absorb at lower wavenumbers. Weak bonds and heavy atoms absorb at high wavenumbers.
3. The Fingerprint region
  1. Called the fingerprint region because it is specific to each one
  2. The low wave spectrum of the graph (usually between 600 cm-1 and 1400 cm-1) is known as the fingerprint region
  3. This region is characteristics of each individual compound, and can be compared to a known database)

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Spectroscopic techniques

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