Introduction to Soil Chemistry

164 spectroscopy
these make analysis of some components mixed with water difficult or
impossible.
Infrared spectroscopy is most frequently used for the identification of
organic compounds. Some organic functional groups, particularly alcohol, acid,
carbonyl, double bonds, triple bonds, and amines, have unique, strong, and
easily identifiable absorptions in the IR spectrum. Both methyl and methylene
groups are insensitive to their environments and absorb strongly in narrow
frequency ranges, making them easily identifiable and useful for quantification
(see discussion of hydrocarbon analysis below). However, since these groups
are almost always present in organic compounds, their absorptions are seldom
useful in identifying specific compounds. Carbonyl groups also have a strong,
sharp absorption that is usually the strongest in the spectrum, which also
makes their absorption behavior useful for identification.
A correlation chart giving the important and unique absorptions of various
organic functionalities can be found in many books on infrared spectroscopy.
These charts are useful, but the user needs to be familiar with the shape and
size of typical functional group absorptions before using them with any degree
of accuracy. A broad absorption between 2500 and 3650cm -1 shows the presence
of —OH. However, the absorption of alcohol —OH is very different from
that of acid, as can be seen in Figure 8.11. Another example is the carbonyl
absorption, which occurs in the 1700cm -1 region of the spectrum. It is important
to know not only that these are sharp absorptions but also that they are
the strongest absorptions in the spectrum of aldehydes, ketones, and acids as
seen in Figure 8.11. If there is an absorption in this region but it is not the
strongest absorption in the entire spectrum or is not sharp, then it is not from
a typical aldehyde, ketone, or acid but may be from a double bond, although
this is not common. Table 8.1 lists some important functional group absorptions
for the MIR region of the spectrum.
Infrared is not typically used in common soil analytical procedures,
although it has been used to investigate various soil components, the most
common of which are humus and its various subcomponents (see Figure 8.12).
It has also been used to identify soil clays, both crystalline and amorphous.
Spectra can be accessed on the Web at http://speclab.cr.usgs.gov
particularly see splib04a.
There is one case, in environmental work, where the methyl and methylene
absorptions are both useful and used. This is in the USEPA method for the
determination of total recoverable petroleum hydrocarbons (TRPHs) in a soil
or other extract. Here a supercritical carbon dioxide extraction of a
hydrocarbon-contaminated soil is made (USEPA Method 3560) (see also
Chapter 7). The extracted hydrocarbons are dissolved in tetrachloroethylene,
which is transparent in the region of the IR spectrum where methyl and methylene
groups absorb. The infrared —CH 3, —CH2—, absorptions in the
2800–3000cm -1 region are related to the amount of hydrocarbons in the soil
extract through standard curves constructed, as described above, for the
method [20–26].