4 feb. 2018

Subtracting spectra to understand band positions

In this exercise (with the soy meal data in transmittance), we sort the soy meal spectra in four data-frames, by protein, moisture, fat and fiber, so in the first row we have the spectrum absorbance of the sample with the minimum value of protein, moisture, fat and fiber, and in the last one the sample with the maximum.
After, we subtract the spectra of the maximum and minimum to get the difference spectrum. Sometimes ,this spectrum difference help us to understand where are the main bands for the main constituents.
In this case is not easy due that we are working in a zone where the bands are very wide (second and third overtones), and the math treatment does not remove that overlapping (MSC). Anyway we can see some curiosities.

In the plots we see the minimum and maximum spectrum and the difference spectrum. In the next plot I overlap the difference spectra:
We have to take into account that we still have the interferences of the moisture (as example) over the other bands due that the high  range (when the spectra is sorted) is for the constituent sorted, but there are range for the other constituents. Anyway seems to be more or less clear where are the absorbance bands in this NIT zone for protein and moisture.
As a curiosity, see how the Fiber difference spectrum is the opposite than the Protein difference spectrum due to the negative correlation between these two constituents for the soy meal.
All the plots are outputs of "R" code.

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