ABB MB3000 Series Installation And User Manual page 60

Introduction
For liquid or solid samples, where the absorption bands are generally broader than
the spectral resolution, it is expected that the absorbance will increase linearly with
increase in optical path length. Hence a plot of absorbance vs pathlength should be
linear. This is called the photometric linearity. As the absorbance increases, the
transmittance reduces in a non-linear way. For example at absorbance 1, the
transmittance is 10%, at absorbance 2, the transmittance is reduced to 1% and at
absorbance 3, the transmittance is further reduced to 0.1%. It is expected that the
absorbance vs pathlength will be quite linear for a small absorbance range from
example from 0 to 0.3 A. As absorbance range increases, the linearity becomes
increasingly challenging.
With an FTIR, spectra are recorded as raw spectra first that are converted to
transmittance by dividing by an open beam reference spectrum. In the raw spectra
normally the noise level is constant. It becomes weighted according to the spectral
response of the reference in transmittance. When converted to absorbance the noise
will increase rapidly with level of absorbance because of the non-linear
transformation of the logarithm. At high absorbance the fidelity and noise behavior
of the spectral response near zero intensity becomes critical.
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Appendix C Spectroscopy Background
Figure C- 2
AA003700-01 rev. G. 2.0
shows a