Introduction to Soil Chemistry

170 spectroscopy
nitrogen containing material, enriched in 15 N, is applied to soil or a crop and
samples taken after various periods of time. The samples are digested using
kjeldahl methods as discussed earlier in Chapter 6, and ammonia is collected
and then decomposed before being injected into the mass spectrometer. A
species enriched in 15 N is a measure of the partitioning of nitrogen and its
movement in the environment. Other stable isotopes can be used in a similar
way to follow their movement through the environment in general and in soil
in particular [9,10,31–33].
8.14. CONCLUSION
Matter interacts with all forms of electromagnetic radiation, and these interactions
are used to gain information about the matter with which it interacts.
Thus, X-ray, atomic absorption, and many other spectroscopic methods are
available for the investigation of soil. X rays are used to determine the structure
of clays and identify the elements present in soil via X-ray fluorescence.
Atomic emission, adsorption, and ICP are routinely used to determine metals
extracted from soil. Ultraviolet and infrared spectroscopies along with
mass spectrometry are used to unequivocally identify compounds. NMR is
extremely powerful in determining the structure and species of various compounds
and species. A common method of analysis is colorimetry, in which the
component of interest is reacted with a reagent, which produces a colored
compound. Using the visible region of the spectrum, the intensity of the color,
as measured by a spectrophotometer, is directly related to the amount of component
present in the sample using a calibration curve. The results are related
back to the original sample.
PROBLEMS
8.1. Describe two ways to distinguish between an analytical signal and noise
in the output of an instrument.
8.2. Describe some major limitations of X-ray fluorescence as a method for
determining metals in soil.
8.3. What is spectral overlap, and how may it affect analysis of metals
extracted from soil?
8.4. List three common solvents useful in UV–Vis spectroscopy. Which of
these might be more useful in analysis of soil extracts?
8.5. Describe two ways in which spectra may be used to identify compounds.
8.6. Explain how infrared and nuclear magnetic resonance spectra are
different.