Environmental Engineering Reference
In-Depth Information
chemical identity with a spectral library established by the analytical community.
In the 1940s, before the advent of more powerful identification techniques,
UV-VIS spectroscopy was the main tool used for structural identification.
Structural analysis from electronic spectra of UV radiation yields little
structural information because of their relative simplicity. In comparison, IR
radiations are associated with molecule's rotational and vibrational sublevels. In
a molecule, there are numerous possible transitions that are quite close in energy.
As a result, it produces a continuous broad absorbance/emission band. The IR
spectra are richer than UV-VIS in structural information. Even though, IR
spectrum has diminished its major role as other recent techniques become
available, such as MS and NMR (Chapter 12). With all available structural
identification techniques, the overall utility is in the increasing order: UV
<
IR
<
NMR (Field et al., 2003).
Figure 8.11 compares the IR spectrum vs. UV spectrum for the same compound
benzene. It is noted that UV-visible spectrum typically is a plot of absorbance vs.
wavelength, whereas IR spectrum is more conveniently plotted as percentage
transmittance vs. wave number (cm
1
). The concept of absorbance and transmittance
will be introduced in the next section.
Wave number (
MS
<
n) is related to wavelength (l) as follows:
10
000
l ½m
;
n ½cm
1
¼
ð8
:
3Þ
m
The wave number has a unit of cm
1
. It is directly proportional to the frequency (n)
and hence the energy of IR. Also note that the wave number on the axis is in the
descending order and the scale is nonlinear on a typical IR spectrum. There are two
regions of the IR spectra: (a) a group frequency region
, which encompasses
radiation from about
3600
cm
1
to approximately
1200
cm
1
m
(3-8
m);
(b)
fingerprint region
in the range of 1200-600 cm
1
m
m), which is
particularly useful because differences in the structure and constitution of a
molecule result in significant changes in the appearance and distribution of
absorption peaks in this region.
From the standpoint of both application and instrumentation, the infrared
spectrum is conveniently divided into near-, mid-, and far-infrared radiations.
(8-14
m (12,800-4,000 cm
1
). The photometers
and spectrometers in this range are similar in design and components to
UV-VIS spectrometry. They are less useful in structural identification,
but found in most applications of the quantitative analysis of com-
pounds containing functional groups that are made up of H bonded to
C, N, and O. Applications include water, CO
2
, S, low molecular weight
hydrocarbons, amine nitrogen, and many other simple compounds in
industrial and agricultural materials.
Mid-infrared (MIR):
2.5-50
Near-infrared (NIR):
0.78-2.5
m
m (4,000-200 cm
1
). The mid-infrared range
is the most widely used IR as it has found use in measuring the
m
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