Separation of d8 and d9 THC

[K74]

Hello,

We are using a Restek Rxi®-35Sil on a Thermo Trace GC/FID System to analyse cannabinoids. The separation is working well, except that we get peak overlap between delta 8 and delta 9 THC, which are very similar molecules. The method we use is precisely as recommended by Restek for this column (see attachment), yet we do not achieve the separation of the these molecules. I tried to optimize the method (slower Temperature increase, decrease flow rate) a bit to improve the separation, but nothing worked. Do you have an idea how to tweak the system to improve this separation? Are we overlooking something?

Thanks a lot for any input!

[Rndirk]

Do you also use hydrogen gas as carrier like in the application note?

Can you post a chromatogram that you obtained?

[K74]

Yes, we use hydrogen carrier. Please find two chromatograms attached, on the first one you can see the large CBD peak, and you can already see the THC overlap on the right. The second one shows the THC overlap zoomed in. (sorry for the low image quality, taken by phone)

[James_Ball]

What solvent are you using and what is your temperature program?

[Peter Apps]

Reducing the carrier flow rate as you have done usually makes separations worse - the peaks are further apart in time but they are also wider, so they overlap more.

Your CBD peak is overloaded - what happens if you double the split ratio ?

How much work has this column done, and when did you last do inlet maintenance ?

Peter

[James_Ball]

Reducing the carrier flow rate as you have done usually makes separations worse - the peaks are further apart in time but they are also wider, so they overlap more.

Your CBD peak is overloaded - what happens if you double the split ratio ?

How much work has this column done, and when did you last do inlet maintenance ?

Peter

Agreed, faster flow will give narrower peaks especially if using H2 carrier. Also if injecting Ethyacetate as the solvent, use a mid polarity deactivated retention gap of about 1-5m and solvent focusing starting with oven at 50c then ramping as fast as possible to 90c with a 1 minute hold before continuing the ramp will give sharper peaks. Retention gap polarity need to match polarity of the injected solvent or you will not get sharp peaks. I had to do this when injecting Ethylacetate onto a Rtx-5 column to sharpen peaks for these analytes.