Absorption spectroscopy

  • it uses the fact that each atom has a different electron shell and thus all absorb different wevelengths
  • a spectrometer consists of a few parts:
    • source
      • it is a source of white light
      • the source should have a broad span of white light and should be as stable as possible
      • some sources also generate electromagnetic radiation of shorter or longer wavelengths beyond visible light
    • monochromator
      • it is a set of three parts: entrence slit, dispersion device, exit prism
      • the white light passes through the entrence slit
      • it then hits a prism which seperates the white light into its different components
      • a narrow band of wavelengths is passed through the exit slit
      • the prism then turns to pass different wavelenghts onto the exit slit
    • sample
      • the light from the monochromater then passes through our unknown sample
    • detector
      • it detects whether light was or wasn’t passed through the sample and records it into a computer file
  • the results is called an absorption spectrum
Absorption spectrum of benzene
Absorption spectrum of benzene

Beer-Lambert law

  • Beer-Lambert law determines the realtionship between absorbance and concentration
    • whilst absorption spectroscopy gives us a qualitative result, Beer-Lambert law gives us a qantitative result
  • formulation:

$$A=\epsilon{lc}$$

  • where:
    • $A$ is absorbance, dimensionless
      • it is the result of absorption spectroscopy
    • $\epsilon$ is the molar mass coefficient, [$dm^3\cdot{mol^{-1}}\cdot{cm^{-1}}$]
      • it varies from wavelength to wavelength
    • $l$ is the length of the cuvette
    • $c$ is the concentration
  • concentration can be thus calculated as:

$$c=\cfrac{A}{\epsilon{l}}$$