Environmental Engineering Reference
In-Depth Information
Owing to the coupling of vibrations, the position
of an absorption peak related to a given organic
functional group cannot always be specifi ed exactly;
usually some range of wavenumbers are associated
with each functional group (Skoog
et al.
1998).
Vibrations can be classifi ed into two basic catego-
ries: stretching and bending. When the infrared radi-
ation is absorbed, the associated energy is converted
into these types of motions. A stretching vibration is
characterized by a continuous change in the intera-
tomic distance along the axis of the bond between
two atoms. Bending vibrations involves a change in
the angle between two bonds. There are of four types
of bending: scissoring, rocking, wagging, and twist-
ing (Skoog
et al.
1998; Pavia
et al.
2001).
Vibrational infrared absorption involves discrete,
quantized energy levels. Although rotational fre-
quencies of the entire molecule are not infrared
active, they frequently couple with the vibration
modes in the molecule to give additional fi ne struc-
ture to these absorptions. These combinations lead
to the commonly observed broad bands rather than
discrete lines in the infrared spectrum (Hsu 1997;
Pavia
et al.
2001).
stretching vibrational bands that occur in the middle-
infrared region of 3000-1700 cm
−1
give rise to over-
tones or combinations, which are the absorption
bands observed in the near-infrared region. The
bonds usually involved are C-H, N-H, and O-H.
What appears in the near-infrared region of the spec-
trum is the result of vibrations of light atoms that
have strong molecular bonds. Weak chemical bonds
or bonds involving heavy atoms have a low vibra-
tional frequency; thus, their overtones will not be
detectable in the near-infrared. Consequently, the
most observable overtones and combination bands
in the near-infrared are the result of chemical bonds
containing hydrogen attached to atoms such as nitro-
gen, oxygen, or carbon; that is, the chemical struc-
tures that are common in many organic compounds.
Moreover, these weak overtone bands are more sen-
sible to their environment than the fundamental
mode of the same vibration. A slight perturbation in
the bonding produces small changes in the funda-
mental mode, but great frequency shifts and ampli-
tude changes in the near-infrared (Wetzel 1983). The
near-infrared spectrum has several regions that are
sensitive to the environment of the absorbing mole-
cules and to the number of molecules present, allow-
ing for quantitative measurements. However, the
combination bands and spectral overtones do not
occur in distinct absorption peaks; instead, several
overlapping peaks are observed. Thus, to extract
information on chemical compounds from the spec-
tral response of a sample it is necessary to perform
a calibration. The near-infrared spectral data of the
samples must be correlated to other chemical data,
obtained by different methods, by an appropriate
statistical relation to make it possible to predict
the chemical constituent of interest from the near-
infrared spectra of unknown samples (Wetzel 1983;
Korsman
et al.
1999).
Near infrared spectroscopy (NIRS) needs minimal
or no sample preparation. The near-infrared region
has been extensively used in quantitative analysis of
3.5.2 Infrared applied to sediments
To give a better description of application and
instrumentation, the infrared spectrum is conven-
iently divided into near-, mid-, and far-infrared radi-
ation. Table 3.1 shows the rough limits of each
infrared region.
The far-infrared region of the spectrum is particu-
larly useful for studies of vibration absorptions of
inorganic solids (especially semiconductors) and for
investigation of pure rotational absorption by mol-
ecules that present permanent dipole moments, such
as O
3
and H
2
O in the gaseous state (Skoog
et al.
1998).
The near-infrared region has several applications
in the study of sediments. Some of the fundamental
Table 3.1
Infrared spectral regions.
Near infrared
Middle infrared
Far infrared
12.800-4000 cm
−
1
4000-200 cm
−
1
200-10 cm
−
1
Wavenumber (
û
)
Wavelength (
λ
)
0.78-2.5
μ
m
2.5-50
μ
m
50-1000
μ
m
