Thermonicolet Omnic Software User's Guide 6.1 (PDF) - Charles E ...

Subtracting spectra
Use Subtract in the Process menu whenever you want to subtract one spectrum
from another. Subtract is commonly used to remove spectral features of solvent
residues or pure components from the spectrum of a mixture of compounds.
How does spectral
subtraction work?
When you use Subtract to subtract one spectrum from another, the software
calculates data point by data point the difference between the two.
According to the Beer-Lambert law, the spectrum of a sample that is a mixture of
two materials (components A and B) is the sum of the spectra of the two materials.
If you subtract the spectrum of one pure component (for example, B) from the
mixture spectrum, the result is a spectrum of the other pure component (A). This
can be expressed by the equation (A + B) - B = A.
In most cases spectra are not subtracted on a one-to-one basis, since the components
do not have the same concentration and so their intensities do not match. For
example, a mixture may be 20% component A and 80% component B, or it might be
90% A and 10% B. If you first scale a reference spectrum of component B so that its
intensities match those in the mixture spectrum, you can then cleanly subtract out the
peaks due to component B, leaving only peaks due to component A.
Consider this example: You have used a fixed-path cell to collect a sample
spectrum of a 10% solution of a compound in an organic solvent. You decide to
collect a reference spectrum of the pure solvent using the same cell so that you can
subtract the reference spectrum from the sample spectrum to produce a spectrum
of the compound alone. If you subtract the spectra without scaling, the result will
exhibit overcompensation for the solvent peaks (giving negative peaks) since the
“reference” is 100% solvent while the “sample” is only 90% solvent. The answer
is to first scale the reference spectrum to 90% so that the subtraction result will be
“clean.” The scaling factor is called the “subtraction factor.”
Algebraically, the subtraction works like this:
Sample - Reference * Factor = Result
212 Thermo Nicolet