Analytic techniques

• What happens when a molecule absorbs infrared radiation

  It makes the covalent bond vibrate more in a stretching or bending motion
• Factors affecting the amount of vibration of a bond

  • Bond strength
  • Bond length
  • Mass of each atom in the bond
• How infrared spectroscopy works

  1. Every bond has a unique vibration frequency in the infrared region of the EM spectrum

  2. Bonds absorb radiation that has the same frequency as their frequency of vibration

  3. Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound's functional group
• Peaks on an infrared spectrum

  Absorbance of energy from the infrared radiation
• Peaks required to identify a substance as carboxylic acid


  • Very broad peak at 2500 - 3300 cm-1 → O-H group
  • Sharp peak at 1680 - 1750 cm-1 → C=O group
• What happens inside a mass spectrometer

  1. Organic compound is vaporised and passed through the spectrometer

  2. Some molecules lose an electron and forms molecular ions

  3. Excess energy from the ionisation makes the bonds vibrate more

  4. Vibration causes bond to weaken

  5. Molecular ion splits by fragmentation
• Fragmentation is not predictable
• Molecular ion representation in mass spectrometry

  It is the peak with the highest mass/charge ratio
• Molecular mass of the molecular ion

  Relative molecular mass of the compound
• Molecular ion peaks of two isomers of the same compound
• Advantages of using mass spectrometry

  • Cheap
  • Small quantities of samples required
• Main disadvantage of using mass spectrometry

  The sample is completely destroyed