And, in particular, why optical emission spectroscopy?
it may happen to wonder why the sky looks blue and what is the
answer? It may be already known that sunlight is made up of all the
colours of the rainbow: red, orange, yellow, green, blue, and violet.
It may be also known that sunlight has to pass through our atmosphere
before it reaches our eyes. The gas molecules in the atmosphere break
up, or "scatter" the sunlight into its many parts. But they
scatter some parts more effectively than others. Different colours of
light have different energies or wavelengths. Red light has a long
wavelength and a lower energy, blue light has a short wavelength and
a higher energy. The gas molecules in the atmosphere scatter the
higher-energy blue wavelengths better than the red wavelengths. So
the sky looks blue.
As mentioned before, red and blue are only
two of the colours which make up the light coming from the sun. This
light can be seen as a source and as such it can be analyzed, but how
can this be done? As a source, light has a spectral range which can
be totally or selectively transmitted to an imaging system which
transfers this range to a spectrometer. This spectrometer, on
receiving the spectral range transmits it to a detector which
eventually elaborates the data.
Spectroscopy is the study of
spectra, ie characteristic wavelengths or colours. Optical emission
spectroscopy (OES) comprises several techniques that form the most
important means we have for chemical analysis.
In OES, we
measure spectra emitted by atoms and ions with optical transitions in
the wavelength range from about 100 nm to 900 nm. This range includes
the ultraviolet, and visible light (from violet at 380 nm to red at
760 nm), and the near infra-red.
With OES, we can:
the chemical composition of solids, like steel or wood or paint, and
liquids, like oil or milk, and gases, like air or car exhausts.
Knowing the content of materials, we can monitor changes in the
environment, and improve production processes and product properties,
such as strength, corrosion resistance and appearance.
deposition of layered devices used in the semiconductor industry, and
the deposition of hard coatings on tools.
study the inner workings
of atoms and ions and molecules. Knowing their electronic structure,
we can test the predictions of quantum theory and better understand
the behaviour of the chemicals that compose our bodies, our food and
the world we live in.
With spectroscopy we can determine the elements that make up the sun
and the stars. Knowing the elements in stars, we can learn much about
their age and their origins, and this helps understanding the nature
of the universe.
First published on the web:
Authors: Aranka Derzsi and Giovanni Lotito. The
text is based on a lecture given by Thomas Nelis at the first Gladnet
training course in Antwerp Sept. 2007