Page 280 - Handbook of Instrumental Techniques for Analytical Chemistry

270 Handbook of Instrumental Techniques for Analytical Chemistry

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carbonyl C=O stretch at 1750 to 1735 cm , but also exhibit their characteristic absorption at 1300 to
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1000 cm from the couplings of C-O and C-C stretches.
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The Aromatic Region, 910 to 650 cm The IR bands in this region do not necessarily come from the
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aromatic compounds, but the absence of strong absorption in the 910 to 650 cm region usually indi-
cates the lack of aromatic characters. The out-of-plane bending of ring C-H bonds of aromatic and het-
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eroaromatic compounds gives rise to strong IR bands in the range between 910 and 650 cm . As
previously stated, certain nonaromatic molecules such as amines and amides can also contribute ab-
sorption in this region.

Compound Identification

Since the IR spectrum of every molecule is unique, one of the most positive identification methods of
an organic compound is to find a reference IR spectrum that matches that of the unknown compound.
A large number of reference spectra for vapor and condensed phases are available in printed and
electronic formats. The spectral libraries compiled by Sadtler and Aldrich are some of the most popular
collections. In addition, spectral databases are often compiled according to application areas such as
forensics, biochemicals, and polymers. Computerized search programs can facilitate the matching pro-
cess. In many cases where exact match to the spectrum of an unknown material cannot be found, these
programs usually list the reference compounds that match the unknown spectrum most closely. This
information is useful in narrowing the search. When it is combined with the data from other analysis
such as NMR or mass spectrometry, a positive identification or high-confidence level tentative identi-
fication can often be achieved.

Quantitative
IR spectroscopy was generally considered to be able to provide only qualitative and semiquantitative
analyses of common samples, especially when the data were acquired using the conventional dispersive
instruments. However, the development of reliable FTIR instrumentation and strong computerized
data-processing capabilities have greatly improved the performance of quantitative IR work. Thus,
modern infrared spectroscopy has gained acceptance as a reliable tool for quantitative analysis.
The basis for quantitative analysis of absorption spectrometry is the Bouguer–Beer–Lambert law,
commonly called Beer’s law. For a single compound in a homogeneous medium, the absorbance at any
frequency is expressed as
A = abc (15.3)
where A is the measured sample absorbance at the given frequency, a is the molecular absorptivity at
the frequency, b is the path length of source beam in the sample, and c is the concentration of the sam-
ple. This law basically states that the intensities of absorption bands are linearly proportional to the con-
centration of each component in a homogeneous mixture or solution.
Deviations from Beer’s law occur more often in infrared spectroscopy than in UV/visible spectros-
copy. These deviations stem from both instrumental and sample effects. Instrumental effects include
insufficient resolution and stray radiation. Resolution is closely related to the slit width in dispersive IR
instruments or the optical path difference between two beams in the interferometer of FTIR spectrom-
eters. Stray light levels in FT instruments are usually negligible. Sample effects include chemical reac-
tions and molecular interactions such as hydrogen bonding. The Beer’s law deviations result in a
nonlinear relationship for plots of absorbance (A) against concentration (c). It is therefore a good prac-
tice to obtain calibration curves that are determined empirically from known standards.