Infra-red Spectroscopy

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  • Infra-red Spectroscopy
    • Molecules can absorb infra-red radiation - the energy absorbed causes the covalent bonds in the molecules to vibrate
      • Only specific energies (corresponding to specific wavelengths or wavenumbers) are absorbed by a particular molecule
        • Each absorption is due to the vibration of a specific bond or set on bonds
          • An infra-red spectrum shows the wavenumber ranges for the absorption bands of a substance. By comparison of many compounds, it has been possible to assign bands at certain positions to vibrations of bonds in specific functional groups
    • Uses of infra-red spectroscopy
      • To identify compounds by comparison of the pattern of peaks with a spectral database
        • The peaks with wavenumbers less than about 1600cm-1 especially, are mainly due to excitation of complex vibrations involving many atoms.
          • The pattern of these may be unique to a specific compound, so this is called the fingerprint region of the spectrum. A computer can search for an exact match between this pattern and those recorded for known patterns
            • This is called fingerprinting and is used to detect for example; traces or drugs in forensic science and environmental pollutants e.g. in air and water quality monitoring
      • To measure the concentration of a substance from the strength of their characteristic absorption bands e.g. used in modern breathalysers
        • The instrument measures IR absorption at a chosen wavelength where ethanol absorbs strongly, but other normal breath components do not
          • The instrument is calibrated to relate the extent of IR absorption to the ethanol concentration in the breath
      • Atmospheric pollutants can similarly be detected from their characteristic infra-red fingerprints and their concentrations determined from the strength of these absorption bands
    • Infra-red radiation and climate
      • When infra-red is emitted at the earth's surface, some of it will be absorbed by molecules in the atmosphere instead of escaping directly into space. This is called the greenhouse effect and it tends to warm the atmosphere
        • The contribution of any individual substance to the greenhouse effect depends on the concentration of the substance in the atmosphere and how good it is at absorbing infra-red radiation
          • For example: O2 and N2 are abundant in the atmosphere but don't absorb UV, so are not greenhouse gases
            • H2O (O-H bond vibrations excited), CO2 (C=O vibrations) and CH4 (C-H vibrations) all make substantial contributions to the Earth's greenhouse effect
              • If the concentrations of any of these compounds in the atmosphere increases, more infra-red radiation will be absorbed and the warming effect will be greater
                • CO2 levels in particular have been rising as a result of increased use of fossil fuels and this is almost certainly the origin of global warming
      • Global warming is causing climate change. Chemists are helping to address the problem by;
        • Making the measurements of changing atmospheric composition and developing the theory that has helped demonstrate that climate change is real and almost certainly driven by human activity
        • Investigating solutions such as;
          • Carbon capture and storage (CCS)
            • This involved reducing CO2 emission into the atmosphere by capturing CO2 from major sources such as power stations, and either;
              • Injecting into deep geological formations where it can be trapped beneath rock
              • Injecting it into the deep ocean where it can slowly dissolve
              • Reacting it with natural oxide minerals to form stable carbonates
          • Developing alternative energy sources, to reduce dependence on fossil fuels
          • Chemists are continuously monitoring the composition of the atmosphere and the amounts of CO2  and other greenhouse gases that we are releasing into it. This helps us check that we are on course to meet targets for reducing emissions such as those agreed under the Kyoto protocol
    • Mass Spectrometry
      • A mass spectrometer is a device that can measure accurately the masses of single ions
        • The ions are usually generated by bombarding a sample with high energy electrons which can knock an electron out from the individual atoms or molecules, leaving them as positive ions
          • The positive ions are formed into a beam which then passes through a magnetic field. This deflects the ions from their original course and, crucially, the extent of deflection depends on the mass of the ion
            • By varying the magnetic field, ions of any chosen mass can be deflected onto a path that leads to a detector. The mass spectrum shows the relative number of ions of each mass that were detected
      • Analysis of Isotopic Composition
        • A mass spectrometer can detect atoms of different isotopes and determine the relative amounts of these isotopes in a sample. It can provide the information needed to calculate accurate Ar values
          • A sample of an element is heated to high temperature to generate individual gaseous atoms. These are then ionised and passed into the mass spectrometer which allows the masses of individual ions to be measured very accurately
            • If different isotopes are present, the mass spectrometer allows us to measure;
              • Very accurately the relative isotopic mass of each isotope present
              • The relative abundance of each isotope
                • This information then allows Ar to be calculated
                  • Very accurately the relative isotopic mass of each isotope present
      • Mass spectrometry of molecular substances
        • If a substance consisting of molecules is injected into a mass spectrometer, the molecules are first ionised by loss of an electron and some of these ionised molecules are then carried through the mass spectrometer and detected
          • In addition, some of the molecules will usually fall apart as soon as they have been ionised and so fragment ions are also detected
            • Organic molecules can also be studied in a mass spectrometer
      • Uses of mass spectrometry
        • To probe the structures of compounds;
          • m/z for the molecular ion peak gives the Mr value
          • The fragmentation pattern can give clues as to the structure
        • For detection of compounds by matching the mass spectrum to a database. The pattern of the peaks can then be used as a fingerprint to detect the presence of a particular substance
        • The amounts of specific substances can be estimated from the sizes of their peaks
        • To detect and measure the relative amounts of specific isotopes. This is useful, for example, in carbon dating


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