This is a particularly fave topic of mine and I just vaped something potent so I took your post as a segway to present this idea. You are absolutely correct about this stuff being a complex makeup and it gets more complex as heated… you talked about thermal shifts according to compound composition.
The shift that tells the tale and then could be programmed into a computer because of the precision possible in measurement are the vacuum levels measured. Once you know the boiling point at blank off pressure for the vacuum pump for the target fraction it makes it then possible to determine when all lower fractions then have been removed so long as you can measure it accurately.
This of course assumes an ideal closed system. A sublimation apparatus like I employ is as close to an ideal system as is possible in this context. When the pressure drops micron by micron towards the blank off value of roughly ¾ of one micron at the set point temp for THC it begins collecting on the cold finger through evaporation at about 125C. At that moment without any doubt all lower boiling stuff is depleted from the boil once the vacuum hits ¾ of one micron. (In this example)
Pressure and temp are one and the same really as far as the pillar of modern physics is concerned (The Second Law of thermodynamics). It took mankind tens of thousands of years to finally codify the Second Law into our scientific foundation. Stated in purely technical terms the Second Law and pillar of modern physics states that when you put an ice cube into a hot cup of coffee that the coffee will warm up the ice but the ice will not warm up the coffee. Who knew, right? I bet burn injuries went down dramatically after they wrote that law? Of course some REALLY smart folks ran with the idea and now mankind benefits from nuclear fission… … Some though are just born genius and somehow instinctively just know that ice does not warm up their coffee but we needed Universities in on the secret so they wrote a law. Knowledge is power!
Back on topic… pressure in a closed system is not a different thing than temp from this point of view and for us the instantaneous measurement of pressure in the closed system as a whole is actually much simpler than the same measurement but expressed as temperature.
Where it gets complex has to do with chaos theory insofar as there are so many possible permutations when you consider that organic compounds are breaking down into new compounds while distilling so it makes it very, very hard to predict the exact path a temperature pressure gradient will follow by the time it reaches a known “destination” of a final isolated product. I blame it on entropy. It just sounds cooler to say that, but you are right about art.
I would in this context define an art as a flexible application of scientific procedures and “knowns” as a guideline for action in a lab rather than a set of mandates, procedures and practices. An artist learns that in theory a practice may work, but in practice the theory might not and is flexible enough to alter the practice as a refinement.