Chromatography Forum

I'm doing some chromatography of cannabis extracts, and I'm trying to find a suitable internal standard for quantitative analysis. Most references on the matter note use of Lidocaine or 4-androstene-3, 17-dione as an internal standard. Neither is easily available to me.

I'm using Methanol as a solvent, and I had the idea to simply add a small amount of Toluene as an internal standard. It seems this should work in theory, but I'm worried that due to my temperature controlled run, it may not elute completely separately from the Methanol. I'm running from 200C to 295C at a 20C/min ramp rate, 1 minute hold pre and post. Injector temperature 250, detector temperature 275. Anyone know? Or have a better idea for an internal standard in this case that's readily available and preferably cheap?

The internal standard should be as similar, chemically, as possible to the analytes of interst. This makes the chromatographic properties more similar and behavior in solution more similar. It depends on what your analytes are - and what you have on hand or can obtain. I have my doubts about toluene, both on separation from methanol at the temperature you are using (although you do not say which stationary phase) and on similarity to compunds like the ones you name.

Toluene is definitely not chemically similar to cannabinoids. I was going to adjust using the appropriate response factors for whatever compound I chose, but I suppose that would always introduce some degree of inaccuracy. The response factor of toluene is quite dissimilar from what I'm looking at, according to the data I can find. Are there considerations other than the response factor conversion for introduction of inaccuracies? You mentioned chemical similarity... Lidocaine isn't terribly chemically similar, but it has a response factor of 1 in an FID, apparently. That's pretty close to what I'm looking at.

As Don said, the ideal internal standard should be as much similar to your analytes as possible. Should have the similar partition coefficient, same stability under the GC conditions, to elute close to your analyte but to be separated chromatographically or by the mass spec detector (to have unique m/z and/or ion transitions if selected reaction monitoring is used). Usually the ideal internal standard is homologue of your analye(s) or which is more expensive option, deuterated or other isotopically labeled analogue. This is very important considerations not if you use your internal standard just to control the variability of the GC injection, but also to use it as recovery standard. If you're using it also as recovery standard it is advisable to use similar/related/isotopically labeled compound as IS in order to experience all the losses during the sample prep as your analytes do.

Best regards

Unfortunately, nearly all homologues of delta-9 THC are extremely expensive and have a grey area legality. Cannabis itself is ironically more legal for me to possess.

It seems that some labs started using CBD and CBN as internal standards, but that only works if you don't care about the CBD or CBN concentrations (which I quite specifically do). I also care about a variety of other related compounds that occur in cannabis, and thus using anything very chemically similar to THC will probably obscure peaks of interest.

Judging by what everyone is saying and what I've researched, the Lidocaine seems like my best bet. It seems to have a recovery % pretty close to THC/CBD/CBN as well.

Is it better maybe to use thebaine (morphine type alkaloid) as internal standard? Or some related compound of thebaine which are not observed in addicts plasma or urine? It is only my guess, having the structure and physical properties of your analytes in mind.

I know that thebaine can be well analyzed on nonpolar (let's say XX-5 phase) GC column, without derivatization.

Good luck

dfoil,

Alltech (now Grace) used to offer "DEA Exempt solutions" that were small quantities (plenty for what you want to do) and very reasonably priced. Lidocaine is one of them as is thebaine and procaine. Not sure if they still do since I don't interact with them all that much these days but it is worth a peek.

Not sure lidocaine is really all that similar but certainly more than your toluene.

At 200 C on a non-polar, I suspect that toluene will not sufficiently resolve from the methanol.

Best regards,

AICMM

Thanks for the information all. I'll look into the usage of thebaine and procaine as well, and I just sent an email away to Grace for some quotes on those quik-check things you mentioned. Unfortunately their website is scarce on information.