Introduction to Soil Chemistry

156 spectroscopy
8.6. ULTRAVIOLET AND VISIBLE SPECTROSCOPY
Information about a soil extract can be obtained from the ultraviolet and
visible regions of the spectrum. These are wavelengths ranging within 190–400
and 400–900nm, respectively (wider ranges of wavelengths are available on
some instruments). The two regions of the spectrum are used very differently
in soil analysis, although they are commonly found together in UV–Vis
(ultraviolet–visible) spectrometers. The ultraviolet region can be used to
obtain spectra of soil extracts; however, it is most commonly used as a detector
for high-performance liquid chromatograph (HPLC). The visible region is
used for colorimetric analysis, discussed below.
Common solvents, such as water, acetonitirile, and heptane, are transparent
over both the ultraviolet and visible regions. Because these represent both
hydrophilic and hydrophobic solvents, a very broad range of compounds can
be easily analyzed by UV.
Adsorption in both regions involves promotion of electrons in double or
triple bonds or nonbonding pairs of electrons in elements such as those on
oxygen or nitrogen, to higher energy levels. Conjugated double bonds and nonbonding
pairs conjugated with double bonds absorb at longer wavelengths
than do isolated double bonds or nonbonding electron pairs. The more highly
conjugated a system the longer the wavelength of the maximum absorption
called the lambda max (l max) (see Figure 8.6). This means that all aromatic
compounds have strong ultraviolet adsorption bands as well as terpenes and
other conjugated systems. Unconjugated aldehydes, ketones, amides, acids,
esters, and similar compounds also have ultraviolet absorptions, although they
are not as strong.
Any extract of soil containing large amounts of organic matter may have
multiple absorption bands in the UV–Vis region of the spectrum. A UV–Vis
spectrum of a simple aqueous, low-organic-matter, soil extract is shown in
Figure 8.6. As seen in this spectrum, even a simple soil extract can have significant
absorptions that can mask other potential absorptions of interest.
Absorptions can also result in interferences with an ultraviolet and/or visible
determination of a component or the derivative of a component.
An interesting application of this region of the spectrum is the determination
of nitrate and nitrite in soil extracts. Nitrate is very soluble in water and
can be extracted using a simple water extraction procedure. Nitrate absorbs
at 210nm and nitrite are 355nm and can be quantified using these absorption
maxima (l max). Other materials extracted from soil as noted above, however,
can obscure this region, and thus caution must be exercised to determine
whether there are any interfering components in the extract [11,12,15–17].
8.6.1. Ultraviolet Sample Preparation
Samples are analyzed dissolved in a solvent, commonly acetonitrile, heptane,
hexadecane, or water. A scan of the pure solvent in the sample cell, typically
made of quartz (A in Figure 8.7), is first obtained. The dissolved sample is