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Chapter 13 Infrared Spectroscopy Tutorial
Relation of Transmittance to Absorbance Units
Since both transmittance (T = I
same proportion of intensity of incident vs. transmitted energy, we can substitute to solve
for the value of A in terms of T. This relationship is A = log
absorbance value at 100 % Transmittance is 0.
Summary
% Transmittance and Absorbance Units are the standard measures for
representing the intensity of absorption of IR energy.
The most accurate calibration readings with the SapphIRe Analyzer are obtained
at full scale between 0.2 and 0.6 AU, which represents a 67% - 25% transmittance
of IR energy.
AUs are derived from % Transmittance, a ratio of intensity of IR radiation
entering and leaving the sample cell.
THE BEER-LAMBERT LAW
The Beer-Lambert Law (usually referred to as Beer's Law) describes the relationship
between absorption and concentration. It states that the amount of energy absorbed at a
certain wavelength is directly proportional to the concentration of the sample and to the
pathlength over which the energy travels through the sample. A = kCL where:
A is the absorbance of the sample gas. This number represents the amount of IR
energy absorbed by the gas molecules in the sample cell.
Figure 13-15. Concentration vs. Absorbance
/I) and Absorbance (A = log
0
13-16
I
/I) are derived from the
10
0
(1 / T). Therefore the
10
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