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Chapter 13 Infrared Spectroscopy Tutorial
QUANTITATIVE ANALYSIS: MEASURING ABSORPTION LEVELS
The previous section showed that the second major characteristic of an absorption peak is
the intensity of energy absorption and that the presence and intensity of IR absorption of
any given peak varies with the identity of the specific organic compound being analyzed.
Absorption also varies with the concentration of gases in the sample cell and with the
effective length of the path the infrared beam travels through the sample cell. Generally
speaking, the greater the concentration of gases and the longer the pathlength, the greater
the absorption of IR energy. Intensity of absorption is typically measured in %
Transmittance or in Absorbance Units (AU).
% Transmittance
Almost all chart recorders represent spectra as a plot of % Transmittance on the y-axis vs
Frequency and Wavelength on the x-axis. The resulting plot, Figure 13-14, is a measure
of how much IR energy has passed through the sample and reached the detector.
Figure 13-14. Definition of Transmittance and Absorbance
Transmittance is a measure of the difference between incident radiation, I
(the intensity
0
of IR energy entering a sample cell) and the transmitted radiation, I (the intensity of IR
energy leaving the sample cell and reaching the detector) or T = I/I
. We can therefore
0
express % Transmittance as: %T = (I/I
) x 100.
0
Absorbance Units (AU)
Another convenient way of representing the intensity of absorbance of infrared energy is
in Absorbance Units (AU). Like transmittance, absorbance can also be expressed as a
ratio in terms of the intensity of incident vs. transmitted IR energy: A = log
I
/I. Since
10
0
%T varies exponentially with intensity of transmitted light, a plot of absorbance vs.
concentration is linear (Figure 13-15).
13-15
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