Reconstruction: Residual vs Dextrose

I tried to explain in several posts how the Residual Matrix that remains after apply a Principal Components Algorithm can show us the residual spectra so we can see what else is in an unknown sample analyzed in routine which can not be explained by the Principal Components model.

In this plot I show the residual for three samples which has an ingredient which was not in the model that we build with samples of different batches of a certain formula.

We can correlate the residual spectra with a database of ingredients to have an idea of what could be the ingredient more similar to that residual.

I compare the residual with the spectra of dextrose (in black), and the correlation is 0,6, so it can be a clue that dextrose can be in the unknown sample analyzed.