1. Principles of Spectroscopy
Spectroscopy describes the interactions between electromagnetic
radiation (e.g. light) and material.
Besides a knowledge of the laws governing absorption, emission,
excitement, resonance etc., the correlation between the concentration of the
absorbent substance and the intensity of the light is extremely important
(Lambert- Beer Law).
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| Fig. 79: Electromagnetic spectrum |
These interactions are recorded and represented in the form of
spectra. The information to be gained from these spectra depends on the
properties of the substances analyzed and the nature or wavelength range of the
radiation acting on them and the different interactions that result. Fig. 79
shows the various sectors of the electromagnetic spectrum. Spectra can reveal
information about the properties of emitters (radiation sources) or of
irradiated samples or secondary emitters.
In principle the permeability of a sample to electromagnetic radiation
(transmittance) is measured as a function of the wavelength.
The difference between the radiation energy reaching the sample and
that leaving the sample is detected in the spectrum. This difference is termed
transmittance.
In process engineering, electromagnetic radiation can be used to
differentiate between raw material qualities, to describe the properties of the
end product and to control the process itself. On the basis of the interactions
between electromagnetic radiation and material a distinction is made between
emission, absorption, scatter and reflection spectra.
Emission spectra
occur with substances that are naturally luminous or whose luminosity
has been induced. Induced luminosity is caused by the fact that molecules or
atoms have been excited optically and thus taken up energy. The energy
previously taken up is emitted by the material, mainly in the form of
fluorescence and phosphorescence.
Absorption spectra
Since a body absorbs the type of radiation it emits when excited, spectra
can also be viewed from the point of view of absorption. Bodies or materials
that absorb all the radiation reaching them are called "black
bodies". This expression is in line with the everyday use of language, in
which a substance that absorbs visible radiation almost completely is termed
"black". Conversely, a body that reflects all radiation is called a
"white body". The principle of physics according to which the
radiation emitted by a body must equal the radiation absorbed is expressed by
Kirchhoff's Law.
Scatter spectra
The characteristic changes in radiation caused by scattering against
particles are documented in scatter spectra. In particular the observation and
analysis of laser-light scatter spectra in which precise measurements of
intensity make it possible to follow the movements of molecules, e.g. in
polymers, or Brownian motion, have led to a better understanding of dynamic
processes in complex substances and systems.
Reflection spectra
If a powder is irradiated with light, part of the light reaching it is
absorbed and a further part is emitted as a diffuse reflection after multiple
scattering by the powder particles and subsequently split up into a spectrum.
This is how reflection spectra are produced. The different kinds of spectra described
above can be subdivided into continuous, line, atomic, and band or molecular
spectra.
As we have already said, the uptake of electromagnetic radiation can
cause the molecules to vibrate. The vibrational spectra of pure substances and
mixtures produced in this way are as characteristic as a fingerprint. This is
especially true of near-infrared spectra, from which it is possible to
determine the identity of substances and the composition of complex mixtures
with the aid of computers.
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