Edexcel Chemistry - Topic 7: Modern Analytical Techniques I

• 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

• 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 isotope e.g. Cl-35 and Cl-37

• 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. 

• 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 groups as 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

• 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