Thermo Scientific MIRAN 205B Series, Инструкция по эксплуатации

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Chapter 13 Infrared Spectroscopy Tutorial
13-17
k is a proportionality constant often known as the absorptivity. It is related to
wavelengths absorbed by particular chemicals. At a given wavelength it does not
change.
C is a measure of concentration. It represents the quantity of the gas in the sample
cell.
L is the pathlength. This is a measure of the distance that the infrared beam must
travel in order to pass through the gas in the sample chamber.
Linearity of Absorbance vs. Concentration
At low concentrations, the absorbance of a compound is typically directly proportional to
the concentration over the distance the light is allowed to travel through the sample.
Therefore, if a sample of HCl at 100 ppm yielded an absorbance of 0.14 AU and the
concentration was doubled to 200 ppm, the absorbance value (at the same pathlength)
theoretically doubles to 0.28 AU. Also, a plot of absorbance vs. concentration should
produce a straight line (Figure 13-16).
Figure 13-16. Absorbance - Theoretical Concentration Linearity
Deviations from Beer's Law
The previous section discussed Beer's Law in a general way. Figure 13-17, plotting five
concentrations of the gas shows that linearity is quite good up to 20 ppm but that above
this point the curvature becomes more pronounced.