I feel bad making a whole thread here just to ask what seems like a very trivial question off the cuff but:
Are there cannabinoids/isomers of common compounds that don’t flouresce? What exactly is responsible for this behavior? Is it safe to assume any compound that shows up on MS but not HPLC-UV is not a cannabinoid/quinone/derivative or is that a totally improper assumption?
That’s a great question! I, much like you, have absolutely no idea as to the answer, and I eagerly await those smarter (or at least more informed) than us to fill in the blanks.
Cannabinoids tested via UV is usually done using absorbance. The cannabinoids are analyzed around 200-220 nm wavelength. My understanding is that any compound that does not absorb light around those wavelengths may not show up on a standard chromatogram.
For a fluorescence sensor you could look at something like a fraction finder from arometrix.
The explanation for the absorbance profiles of these compounds probably comes down to the amount and type of pi-conjugation. D9, D8, CBD, CBG, etc all have very similar absorbance spectra because they all have similar orientation of pi-electrons. Same thing is seen with D10, CBC, and others. CBN is distinct as well, most likely due to its unique aromatic ring which no other common cannabinoid shares. For example if someone created CBNd we would expect it to have a similar absorbance profile as CBN because of the similar aromatic ring
I suppose my question is more related to comparing LCMS or LC/QQQ data for a sample to HPLC runs. We’re seeing some peaks that are not on the HPLC chromatogram, and while we are in the process of identifying one that is a large mass fraction, it got me thinking about whether there is a possibility that we have a minor or degredation product which would not appear on the HPLC-UV result
Absolutely. Not everything absorbs light. However I’m pretty sure that because a mass spec relies on a type of ionization that isn’t universal, not everything will show up on a mass spec either.
Mass spec questions I’d direct to @kcalabs I’m not aware of anyone else on the forum actively testing with an LC-MS
As always sir, your responses are informative and well written, thank you!
YAY!!! We all learned-edddd something!!! Thanks for info, it’s interesting
is the 200-220nm range just for HPLC applications? all the TLC stuff I’ve seen was 254nm
Oh! Thank you for having such a good topic - today’s unreads had my soul being crushed.
The fast answer is yes. The longer answer is no. Organic molecules are going to be on the spectrum, so you will be able to see them. I’d check out this textbook - it goes over all the different kinds of spectra out there, how they are used, when they apply, their limitations, etc. I found it really helpful when I was doing natural product synthesis - because really you have to be able to understand the unknowns to identify them. Back in the early 2000’s I was doing flash chromatography (which didn’t really exist yet…) and identifying the “soup” people talk about on these forums. So yeah, its possible. Also they update the textbook semi-often with the new tek that’s out there, which is nice.
Beyond that - your HPLC is using UV - you can set the UV most of the time to what “range” you want to look at. And more sophisticated ones you can actually check across multiple sections of UV at once. There are other instruments that will look at completely different types of light - IR for instance or even actual fluorescence that you can see with your own eyes (aka color changes)
For what we are looking at you should be able to see just about everything. Here’s a brief paper on how this works. Since our molecules are mostly aromatic rings with tails, we should really be able to see them very clearly - as long as our separation method is working.
Ah man - I remember when my boss used to stamp on my head because he had the wavelengths of many structures memorized, and I did not. And oh the pain he would rain on me, metaphorically, when I had to run LC, IR, NMR, and MS before he would say we had enough information.
Remind me sometime to tell you about the time I was doing solid state reactions and uncovered that the “recognized” mechanism for diimide formation had been wrong for about 70 years. 
Doesn’t get any more satisfying than disproving proofs with newer technology. <3
You mean you didn’t want to read 14 more threads about whether or not d8 is a synthetic drug?
This is super interesting! I always love learning why things work and I have a fairly good understanding of ionization/mass spec detection but had no idea why things actually absorb UV.