Introduction to Soil Chemistry

168 spectroscopy
nance (NMR) spectroscopy, it is not generally useful in the in situ analysis of
soil or soil components. This is because NMR analysis depends on a stable
uniform magnetic field in the sample. Most soils contain enough iron to alter
the characteristics of the magnetic field, thus interfering with the analysis.
Components extracted from soil can be analyzed by NMR; humus and phosphate
are two examples.
Nuclear magnetic resonance is an extremely powerful method for observing
the environment of an atom of interest. Most commonly the element
studied is hydrogen-bonded to another element, usually carbon, and is
referred to as NMR spectroscopy (sometimes called H NMR, 1 H NMR,
P NMR or proton NMR). The second is carbon, specifically 13 C, attached to
both other carbons and hydrogen. Other elements commonly measured
include fluorine and phosphorus.
Details of NMR spectroscopy will not be dealt with here but can be found
in sources cited in the Bibliography. NMR is most often carried out on liquid
samples or solutions of pure compounds in deuterated solvents, although
MNR of solids can also be obtained. Different organic functional groups,
methyl, methylene, phenyl, the hydrogen adjacent to the carbonyl carbon in
aldehydes, and organic acid group hydrogens absorb at different frequencies
and thus can be easily identified. Similarly, different 13 C environments result
in different absorptions; for instance, 13 C aromatic and carbonyl carbons have
unique absorptions.
There are also vast number of powerful NMR experiments that yield
detailed information about the structure of pure organic molecules. However,
as with other regions of the spectrum, mixtures produce spectra that are mixtures
of the components present in the 1 H NMR spectrum, such as the mixture
of toluene, hexanoic acid, and octanal as shown in Figure 8.12. This spectrum
shows the unique absorptions of each of these functional groups and also illustrates
that mixtures of compounds give spectra containing the absorptions of
all the components. Table 8.1 gives some important diagnostic adsorptions for
1 H and 13 C NMR spectroscopy.
One place where there is potentially more use for NMR in soil and environmental
analysis is in speciation of soil components. Using a broadband
NMR instrument, many different elements can be analyzed and used to differentiate
between species, for instance, PO 4 3- ,MPO4 2- and M2PO 4 - (where M
represents a metal). This potential, however, has not been exploited to any
great extent [27–30].
8.12.1. Nuclear Magnetic Resonance Sample Preparation
Because most common solvents, including water, contain protons and most
analysis involves the measurement of protons, a solvent without protons is
generally used in NMR spectroscopy. Commonly solvents in which hydrogen
has been replaced with deuterium (i.e., solvents that have been duterated) are
used; the most common is deuterochloroform. In addition, an internal standard,
most commonly tetramethylsilane (TMS), is added to the sample in the