Agriculture Reference
In-Depth Information
CHAPTER
8
SPECTROSCOPY
(it can also be
expressed as frequency; the reciprocal of wavelength and in electron volts).
Within this spectrum visible light represents a very small part: the region of
wavelengths around 10
-5
cm. All regions of the electromagnetic spectrum have
been used to analyze environmental constituents, including those in soil. In
some cases the radiation is passed through the material being investigated and
the absorbed frequents related to the components it contains. In other cases
radiation is reflected or refracted from the sample and information is gath-
ered as a result of changes in the radiation occurring during reflection or
refraction.
How electromagnetic radiation is used to investigate the characteristics of
a sample depends on how it interacts with matter. X rays are diffracted by
atoms and thus are used to elucidate the arrangement of atoms in a material,
commonly crystals. Absorption of ultraviolet and visible light results in the
movement of electrons from one orbital to another. Electrons in double or
triple bonds and nonbonding electron pairs are most subject to this type of
interaction. Infrared radiation interacts with matter by changing its vibrational
and rotational modes. When atoms are placed in a strong magnetic field, the
spins of their electrons and protons interact with radiofrequency (RF), wave-
lengths. These methods, namely nuclear magnetic resonance (NMR) spec-
troscopy and electron paramagnetic resonance (ESR) spectroscopy, are used
to determine the environment of nuclei and electrons.
The movement of electrons between orbitals in atoms can also be used to
gain information about the elements. Energy from an outside source, such as
heat from flames, a furnace, an electrical arc, plasma; ultraviolet or visible light;
and X-rays or gamma rays, can move electrons from one orbital to another.
In this process light is absorbed, and the absorbed wavelength can be used to
determine the type and amount of element present. The other possibility is
that the excited electron will fall back to its original position, emitting a wave-
length of light specific for that particular transition. Measuring the wavelength
and the amount of light provides information as to the kind and amount of
that element present. The method of exciting the electrons varies with differ-
ent types of instrumentation, but the basic process is the same. This technique,
The electromagnetic spectrum is a continuum of wavelengths
l
