Introduction to Soil Chemistry

154 spectroscopy
extract components such that the metal is protected from the heat source.
Other interferences such as changes in the viscosity of the extract that can
affect the accuracy of analysis are also possible, although they are less
common.
Whenever soil samples are being analyzed by any atomic spectroscopic
method, it is essential to make sure that no interfering, overlapping wavelength
elements are present in the soils. If they are then steps must be taken to correct
for these interferences.
8.5.1. Excitation for Atomic Emission
For the alkali-earth metals, as noted above, a simple flame of almost any type
can be used to excite the metals. However, in order to determine a wide range
of metals, it is common to use either an acetylene-air or acetylene-nitrous
oxide flame as the source of energy to excite the atoms. The burner is long
with a slot in the top and produces a long narrow flame that is situated endon-end
to the optics receiving the emitted light.
Light given off by the excited electrons falling back to ground state is passed
through slits, isolated using a grating, adjusted to the analytical wavelength,
and the amount of light is measured using a photomultiplier or other light
detecting device. The wavelength of the light specifies the element present
while its intensity is directly related to the amount present.
Introduction of sample into the flame is accomplished by Bernoulli’s principle.
A capillary tube is attached to the burner head such that gases entering
the burner will create suction. The sample is thus aspirated into the burner,
mixed with the gases, and passed into the flame. Heating in the flame excites
electrons that emit light of distinctive wavelengths as they fall back to their
original positions in the elements’ orbitals. Because a number of electrons can
be excited and there are a number of orbitals into which they can fall, each
element emits a number of different wavelengths of light particular to that
element. In the case of atomic emission, one of these, usually the most prominent
or strongest, is chosen as the primary analytical wavelength.
Generally the higher the temperature of the sample, the more sensitive will
be the analysis. Thus, in addition to the two types of flames discussed above,
a third excitation source, which is not a flame but a plasma from an inductively
coupled plasma (ICP) torch, is used (see Figure 8.5). Argon support gas is
seeded with free electrons that interact with a high-frequency magnetic field
of an induction coil gaining energy and ionizing argon.The reversing magnetic
field causes collisions that produce more ions and intense thermal energy
resulting in high-temperature plasma into which the sample is introduced.
While the first two flames are used both for emission and absorption spectroscopy,
ICP is used for emission spectroscopy. The three are arranged in
order of increasing temperature. Both acetylene—air and acetylene—nitrous
oxide can be used in the same instrument and the flames can be adjusted to
be oxidizing or reducing to allow for increased sensitivity for the element