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Edexcel Chemistry – Topic 7: Modern Analytical Techniques I

Mass Spectrometry: Functioning

  • A Mass Spectrometer analyses the relative molecular masses of an organic molecule and its fragments
  • A Mass Spectrometer works by a five stage process

Vapourisation – The organic molecule is turned into a gas so it can pass through the Mass Spectrometer
Ionisation – The organic molecule is bombarded by electrons causing it to fragment and become ionised
Acceleration – The ions pass through a metal plate to accelerate them through the vacuum in the Mass Spectrometer
Deflection – A high powered electromagnet, alters the projectary of the ions based on the m/z ratio desired
Detection – The ions are detected on a sensor and form a digital or hard copy of a mass spectrum

  • The system is kept under a vacuum so atoms don’t interfer with the result/movement of the ions

Mass Spectrometry: Fragmentation

  • Fragmentation occurs when organic molecules are passed through the electron beam which causes a molecule to lose an electron and split into an ion and a free radical
    M –> [M]+  +  e-
    [M]+ –> X+  + Y.
  • Only the ion is responsible for the peak
  • Both the whole molecular ion (parent ion) and its fragments are detected on a mass spectrum
  • Relatively stable ions such as carbocations (e.g. CH3CH2+) and acylium ions
    (e.g. [R-C=O]+) are common. The more stable the ion, the greater the peak intensity.
  • The peak with the highest mass/charge ratio will be the Parent ion. As the charge of the ion is +1, the mass/charge ratio is equal to the Mr.
  • Fragmentation helps to determine the structure of an organic molecule as only certain fragments that are stable can produce a peak
  • A smaller peak may be found after a strong peak due to the presence of an isotopee.g. Cl-35 and Cl-37

Mass Spectrometry: Propanal and Propanone

  • Both have the same parent ion m/z value (58)
  • Propanal has a peak at m/z value 29 ( [CHO]+ ) but Propanone doesn’t
  • Propanone has peaks at m/z values 15 ( [CH3]+ ) and 43 ( [CH3CO]+ )
  • These fragment peaks allow these two organic substances to be distinguished between.

Image result for propanal and propanone mass spec

Infrared Spectroscopy: Functioning

  • Certain bonds in a molecule absorb infra-red radiation at characteristic frequencies causing covalent bonds to vibrate
  • These absorbtions can be detected by an Infrared Spectrometer
  • Wavelengths below 1500cm-1 is the fingerprint region and contains multiple signals which are unique to a molecule so it is hard to analyse this area
  • Wavelengths above 1500cm-1 provides clearer information on functional groupsas the peaks are clearer to analyse
  • A computer can be used to analyse the IR spectra against a database of known pure compounds to identify the compound
  • Characteristic peaks caused by certain bonds include:
    C-O:  1000-1300
    C=O: 1640-1750
    N-H: 3200-3500
    O-H (Carboxylic Acids): 2500-3300 (very broad)
    O-H (Alcohols/Phenols): 3200-3550 (broad)
    The information on IR wavelengths is provided in an exam

Infrared Spectroscopy: Spectrums

  • Molecules which change their polarity as they vibrate can absorb infrared radiation e.g. C-H, C=O or O-H
  • Molecules such as H2, O2 or N2 cannot change their polarity as they vibrate so don’t appear on an IR Spectrum
  • The absorption of IR radiation by bonds in this spectroscopy is the same way that bonds in greenhouse gases absorb IR radiation
  • The IR Spectrum for Propanoic Acid is shown below