If I have a spectrum (as is, no background subtracted) and want to subtract a background spectrum, is a simple subtraction or logarithmic function? Does it matter if the spectrum is in A or %T? For reference, I have two spectra that are in absorbance. I tried simple subtraction and I think it worked, but I'd like to gain better knowledge behind the actual theory.
You may want to check: https://www.researchgate.net/post/FTIR_peaks_Absorbance_or_Transmittance2
Thank you for your help, Carlos.
I found the answer on this site: https://chem.uiowa.edu/sites/chem.uiowa.edu/files/people/shaw/JSG%20-%20How%20To%20FTIR%20-%20141027.pdf
I had to set my raw sample data as I and my background as Io. Then used A=-ln(I/Io) in order to remove the background from my spectrum. I always thought it was a simple subtraction, but found out that didn't work when I tried to subtract the background data from my sample data.
The transmission spectrum is not linearly proportional to the concentration of the sample. The intensity is logarithmically proportional to the concentration. Therefore, you cannot do simple spectral subtraction. On the other hand, the absorbance spectrum holds Beer-Lambert Law and is linearly proportional to the concentration of the sample. Thus, as long as you used transmission technique and convert the spectrum into the absorbance mode, you can do straightforward subtraction. If, however, you use non-transmission technique, such as ATR, your subtraction is only approximate unless you correct the optical distortion using the exact optical theory, that is based on the Maxwell equation.
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