mass spectrometry

• What happens inside a mass spectrometer?

  organic compound is vaporised and passed through the spectrometer

  some molecules lose an electron and form molecular ions

  excess energy from the ionisation makes the bonds vibrate more

  vibration causes bond to weaken

  molecular ion splits by fragmentation
• What is the symbol of molecular ion?

  M+
• In mass spectrometry how is a molecular ion represented?

  the peak with the highest mass/charge ratio
• What is the molecular mass of the molecular ion equal to?

  relative molecular mass of the compound
• What is the m/z value of CH3+?

  15
• What is the m/z value of OH- from alcohol?

  17
• What is the m/z value of C2H5+?

  29
• What is the m/z value of C3H7+?

  43
• What is the m/z value of C4H9+?

  57
• What are the advantages of using mass spectrometry?


  cheap

  small quantities of samples required
• What is the main disadvantage of using mass spectrometry?

  the sample is completely destroyed
• Interpreting mass spectra:

  • when a compound is analysed in a mass spectrometer, vaporised molecules are bombarded with a beam of high-speed electrons
  • these knock off an electron from some of the molecules, creating molecular ions
  • the relative abundances of the detected ions from a mass spectrum
  • the peak with the highest m/z value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
  • the value of m/z is equal to the relative molecular mass of the compound
• What is the M+1 peak?

  a smaller peak which is due to the natural abundance of the isotope carbon-13

  the height of the M+1 peak for a particular ion depends on how many carbon atoms are present in that molecule; the more carbon atoms, the larger the M+1 peak is