Table of Contents
Advertisement
Chapter 13 Infrared Spectroscopy Tutorial
Figure 13-17. Absorbance - Actual Concentration Linearity
The apparent failure of Beer's Law at higher concentrations is most likely due to the
interaction of gas molecules with one another in the sample cell. Collisions between
molecules as well as polymerization are two phenomena thought to account for
deviations from linearity.
The SapphIRe Analyzer is programmed to make appropriate adjustments to Beer's Law at
high concentrations of gases in order to compensate for the nonlinear relationship. It does
this by including a quadratic term in the formula it uses for deriving concentration levels
from absorbance values. For certain gases, it has also been preprogrammed for two
ranges of concentration, a high and a low range.
Knowing this about the nonlinearity of Beer's Law at high concentrations, it is important
to limit analytical work to the 0 - 1 AU range in order to achieve the highest degree of
accuracy using the SapphIRe Analyzer. The ability to alter the pathlength, the effective
distance IR radiation travels through the sample cell, provides a convenient method for
keeping absorbance values within these recommended limits.
INFRARED SPECTROSCOPY SUMMARY
While there is much more to interpreting IR absorption spectra than presented in this
chapter of this instruction, the essential features of IR absorption spectroscopy are as
follows:
1. Chemical substances absorb IR radiation selectively at specific frequencies
characteristic of the natural vibration of the bonds and the molecular groups
comprising the substance. IR radiation at other frequencies is not absorbed. Each
compound's pattern of absorption frequencies, its IR spectrum, is unique. Like
fingerprints, no two are alike, and thus can be used to identify the molecules in an
13-18
Advertisement
Table of Contents
