Agriculture Reference
In-Depth Information
with cellulose or another suitable diluent. Very thin layers of soil may also be
prepared and used for quantitative analysis.
Other limitations involve both the mass absorption coefficient of soil com-
ponents and secondary and tertiary excitation. The mass absorption coefficient
can be calculated and used to correct fluorescence determinations if the exact
composition of the material being analyzed is known. This is not possible in
soil. Secondary or tertiary excitation occurs when X rays emitted by an
element other than the one of interest cause emission of fluorescence of the
element of interest. These potential sources of error are possible in any soil
analysis using X-ray fluorescence.
Some of the these limitations, but not all, can be overcome by making an
extract of the soil and determination of the elemental composition of the
extract. This approach can eliminate or minimize problems associated with the
limitation of X-ray fluorescence analysis to a particle surface [7,8].
8.5.
ATOMIC SPECTROSCOPY
Excitation of electrons in an atom promotes them to a higher energy level,
and when they fall back to their original level, they release energy of the same
wavelength as the energy absorbed. When this energy is in the visible range
of the electromagnetic spectrum, it gives rise to what is termed a
line spec-
trum
, which consists of discrete wavelengths, or lines of light unique to each
element that are absorbed or given off. Early chemists used these unique lines
to identify new elements as they were discovered. Samples of a new (or
thought to be new) element were put in an electrical arc and the light emitted
dispersed using prisms and recorded using photographic film. This is the orig-
inal spectrometer.
Although such instruments as described above are available, they are not
typically used in soil analysis. Today samples are most often aspirated into a
flame or torch and the diagnostic wavelength detected and quantified by
photomultipliers. Modern spectrometers are different because of the use of
many different methods of heating samples and the range of wavelengths
available. Today, because of increased sensitivity of instrumentation and detec-
tors, more of the spectrum is available for this type of analysis, typically both
the visible and the ultraviolet regions. Thus wavelengths ranging from 200 to
900 nm commonly can be used for the analysis of elements present.
The basic idea of exciting electrons and isolating unique, diagnostic, wave-
lengths and measuring the amount of light emitted is useful for routine mea-
surement of soil. This is called the
emission mode
(EM). Potassium, one of the
three most important plant nutrients, and sodium, which poses problems in
some arid soils, along with calcium are easily and routinely measured using
the EM mode, which is also commonly used to determine the concentration
of these elements in blood. An instrument capable of determining elements in
both the EM and atomic absorption (AA) modes is shown in Figure 8.4. Using
