Infra-red and mass spectroscopy


Infra-red spectroscopy

  • Molecules can absorb infra-red radiation. The energy absorbed causes the covalent bond in the molecule to vibrate
  • Only specific energies are absorbed by a particular molecule. Each absorption band is due to the vibration to a specific bond or set of 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 of vibration of bonds in specific functional groups

Uses of infra-red spectroscopy

  • To identify compounds by comparison of the patterns of peaks with a spectral databse. 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 compounds in the database
  • This is called fingerprinting. The process is used, for example to detect;
  • Traces of drugs in forensic science
  • Environmental pollutants e.g in air and water quality monitoring
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Infra-red spectroscopy

  • It is also used to measure the concentration of a substance from the strength of their characteristic absorption bands e.g. IR is 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
  • Similarly, atmospheric pollutants can be detected from their characteristic infra-red fingerprints, and their concentrations measured 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 astmosphere and how good it is at absobing infra-red radiation
  • If the concentrations of any of the compounds in the atmosphere increases, more infra-red will be absorbad and the warming effect will be greater. CO2 levels in particular have been rising as a result of increased use of fossil fuelss and this is almost certainly the origin of the global warming we are experiencing today
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Infra-red spectroscopy

  • Global warming is causing climate change, and this trend is likely to continue
  • We must take steps to try to minimise this, otherwise there will be severe consequences
  • Chemists are helping to address the problem by;
  • Making 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, e.g;

Carbon capture and storage

  • This involves reducing CO2 emission into the atmosphere by capturing CO2 from major sources such as power stations and either;
  • Injecting it 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 our dependance on fossil fuels

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Infra-red spectroscopy

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Mass Spectroscopy

  • A mass spectrometer is a device that can measeure 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 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, cruically the extent of the deflection depends on the mass of the ion
  • By varying the magnetic field, ions of any chosen mass can be deflected on a path that leads to the detector. The mass spectrumshows the relative number of ions of each mass that were detected

Analysis of isotopic composition

  • The mass spectrometer can detect atoms of different isotopes and determine the relative amounts of these isotopes in a sample. Thus, it can for example, 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 ionsied 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;
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Mass Spectroscopy

  • Very accurately the relative isotopic mass of each isotope present
  • The relative abundance of each isotope. This information then allows Ar to be calculated

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 ionsied and so fragment ions are also detected

Organic molecules can also be studied in a mass spectrometer

  • As before, when the molecule is bombarded with high energy electrons in the first instance, it will tend to lose an electron
  • The ion is formed when an intact molecule simply loses one electron is called a molecular ion
  • In the mass spectrum, this gives rise to the preak with the highest m/z value
  • The value of m/z for the molecular ion peak is equal to the Mr of the molecule
  • When dealing with larger molecules, other bits of the molecule will also drop off when it is bombarded with high energy electrons. This is called fragmentation
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Mass Spectroscopy

Relating mass spectra to organic structures

  • First, find the molecular ion peak, to allow you to deduce Mr
  • Then look for peaks corresponding to any possible fragments

Uses of mass spectrometry

  • To probe the structures of compounds
  • For detection of compounds by matching the mass spectrum to a database. The pattern of peaks in the database can be used as a fingerprint to detect the presence of a specific substance
  • The amounts of specific substances can be estimated from the sizes of their characteristic peaks
  • To detect and measure the relative amounts of specific isotopes
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