Mass and infrared spectroscopy

• a beam of electrons is fired at an organic sample causing it to lose an electron and form a positive ion, this is called the molecular ion.
• This molecular ion is unstable and can as a result be split and fragment to form a new ion and a neutral species such as a radical.
• The M+ peak is the most prominent peak on the far right of the mass spectra and effectively tells us the relative mass of the molecular ion.
• You can have several smaller peaks surrounding the relevent peaks due to the presence of heavier or lighter isotopes.
• Some fragments are more stable than others so each fragment will NOT have the same abundance.
• The most abundant fragment from a sample is assigned a relative abundance of 100% and scientists call this the base fragment.
• In any organic compound, covalent bonds are constantly vibrating
• The main types of vibrating are stretching and bending. Stronger bonds vibrate faster than weaker bonds and if the bond is between two heavy atoms, the vibrations will be slower.
• The vibrating covalent bonds can also absorb radiation which has the same frequency as the bond vibration and for organic molecules, this radiation would be infrared.
• When bonds absorb radiation, the degree of stretching or bending increases.
• Different functional groups have different frequencies, referred to as absorptions, so can be identified and measured in wavenumbers (cm-1)
• The trough of an IR spectrum is called the peak and tells us infrared radiation has been absorbed.
• On the IR spectra, 1500cm-1 and below is called the fingerprint region. The fingerprint region is a complex series of peaks which are specific to the molecule being studied.
• Organic molecules tend to have a peak around 3000cm^-1 which is caused by the vibration of C-H bonds.
• Vibrating covalent bonds can absorb energy from the atmosphere and this takes place in gases within the atmosphere.
• • The sun emits UV and IR radiation, this passes through the atmosphere and gets absorbed by the earth's surface
  • The earth re-emits this radiation as Infrared with a longer wavelength that has the same frequency as the vibrational frequency of the bonds in greenhouse gases.
  • The vibrating bonds absorb energy and re-emit it into the atmosphere causing the temperature of the atmosphere at the Earth's surface to increase
• The combustion of fossil fuels increases the concentration of carbon dioxide in the atmosphere which leads to global warming. This has caused an effort to decarbonise human activities.
• Infrared Spectrometry has practical uses:

  • Used to monitor air pollution such as carbon monoxide or nitrogen oxide expelled from car exhausts
  • Used in breathalysers to check the levels of ethanol in the breath.