I have been thinking about the data presented above. These are the basic ideas I think work somehow.
First.. The ripple for the solutions may be from the cup, not material interaction with water.
From the first graph we can see how much sugar bounces around compared to the salt, I think thats mostly due to O-H bonds in the chemical structures. Then both samples were dissolved in water. I see the second graph like the pure water (or cup bottom?) drags the spectrums of the sugar/salt toward the water spectrum. It looks to me like the salt solution follows the pure water spectrum much more than the sugar solution. I don’t have the data or mathematical confidence to see if we were to take the water/salt solution spectrum and subtract the water spectrum we would come up with the salt only spectrum. Don’t know, but intuitively it looks like that statement is true. Looking at the sugar/water solution spectrum compared to the sugar only, doesn’t feel as convincing but it still looks right to me.
I’m coming out with the idea that it will be possible to subtract one substance from another in some circumstances.
Not sure if I’m right, but thats what it looks like to me.