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Study Notes: The Mass Spectrometer

Mass Spectrometry (MS or Mass Spec) is used to identify organic compounds. Essentially the sample molecules are introduced into an ionisation chamber where a beam of electrons ionises the sample molecules to mainly single positively charged ions. These ions are accelerated in an electric field then passed through a magnetic field to curve their flight path. The radius of curvature depends on the mass/charge ratio of the ions. That is smaller radius for low mass and higher charge.Skip flash movie

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The magnetic field strength is varied so different ions reach the collector which measures the mass of the ions and registers their relative numbers. The result produced is in the form of a mass Spectrum.

Mass spectrum

The mass spectrometer is used to:

  1. measure the masses and abundances of naturally occurring isotopes. This data is used to calculate atomic weights:

      (m1 x %1) + (m2 x %2)
    Ar =

    Note: %1 means percentage abundance of isotope one.
  1. identify molecular formulae based on accurate masses. For instance:
  2. 30.0708 = C2H6         30.0268 = H2CO

  3. determine molecular structures based on fragmentation of the molecular ion. For instance in a sample of ethanol CH3CH2OH, the following mass peaks may be observed:

    17 = OH    14 = CH2    15 = CH3    29 = CH3CH2    46 = CH3CH2OH

Since the masses of C, H, O and N are not integers, and they will differ from each other in their decimal portions, the compound molecular weight will allow you to look up tables to find the molecular formula. For instance:

Table of compounds containing only C, H, O and N

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