Introduction to Soil Chemistry

148 spectroscopy
although technically applicable to all elements, is most sensitive and therefore
most commonly used in the detection and quantification of metals.
All analytical methods that use some part of the electromagnetic spectrum
have evolved into many highly specialized areas of use and ways of extracting
information. The interaction of X rays with matter represents an excellent
example of this diversity. In addition to straightforward X-ray absorption, diffraction,
and fluorescence, there are a wide host of other techniques that are
either directly X-ray-related or come about as a secondary result of X-ray
interaction with matter such as X-ray photoemission spectroscopy (XPS),
surface-extended X-ray absorption fine-structure spectroscopy (SEXAFS),
Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques,
to name only a few [1,2].
No attempt will be made to thoroughly investigate all these X-ray or other
specialized spectroscopic techniques. Only the main, common, or routine
methods and instrumentation used in soil analysis will be discussed.
8.1. SPECTRAL OVERLAP
Spectra can be obtained as either absorption, where the material of interest
absorbs defined wavelengths of radiation, or emission spectra, where the material
of interest emits definite wavelengths of radiation. All compounds and
elements absorb and emit numerous wavelengths or bands of electromagnetic
radiation. If two compounds or elements are placed in a beam of electromagnetic
radiation, all the adsorptions of both compounds will be observed. Or if
the same mixture is excited, all bands emitted from both compounds will be
present in the emission spectrum. This leads to the possibility of three types
of interference: two different compounds absorbing in the same place in the
spectrum, two different compounds emitting in the same place in the spectrum,
and one compound emitting while another is absorbing in the same place
in the spectrum. This latter case, however, is usually only observed in atomic
and X-ray spectroscopy. One or more of these types of interference can be
present in an analytical procedure, especially when applied to soil and soil
extracts.
In situations where all the components of a mixture are known and all their
characteristics fully understood, correction for any or all of the types of interference
described above can be made. However, in soil and soil extracts it is
generally impossible to identify all the components and fully understand all
their characteristics. Thus, it is especially important to rule out, compensate
for, or eliminate all possible types of interference when carrying out a soil
analysis.This is generally accomplished by isolation of the component of interest
from most, if not all, of the soil matrix components with which it is associated
(see Chapter 7) [3].