Cannabis Concentrates Part II: We're Heading to Space!

Welcome back! It’s been a little while since my last blog, but in that time, we’ve been doing some interesting things regarding cannabis research. Last time, I discussed that we would be analyzing residual solvents in cannabis concentrates and today, I’m going to show one of the methods that we’ve been working on.

Before we get into the sample preparation techniques that we’ve been focusing on, let’s go over some initial things. First, we need to select a column that has the capability of resolving these light weight, volatile compounds (see Table 1). An easy way to do this is through Restek’s Pro EZGC Chromatogram Modeler. After placing our compound list into EZGC, we saw that the program gave us numerous selections with the first column being the Rtx-502.2. However, since our future work includes terpene analysis, we decided to go with the Rxi-624Sil MS, 30 m x 0.25 mm x 1.40 µm (cat. # 13868). This column has better high temperature stability and it also lines up perfectly with what Amanda chose for her method using Full Evaporation Technique (FET).

Now that we have our column selected, we can move on to the main event! When thinking about how to approach this problem (analyzing residual solvents) we started out by using Jason Herrington’s favorite method: KISS. For those who haven’t heard of KISS, it stands for Keep It Simple, Stupid (sorry, no Gene Simmons here). We wanted to keep everything as simple as possible to make your life in the lab as stress free as possible. To do that, we thought that the easiest approach to this analysis would be through headspace (HS). Headspace samples are prepared by taking a sample and adding it to a dilution solvent in a HS vial (we’re using a 20 mL vial). Depending on the analytes of interest, an inorganic salt (Sodium Chloride, Sodium Sulfate, etc.) can be added to the vial as well to help lower the partitioning coefficient of the more polar compounds. For more information on HS analysis, please check out Restek’s Headspace Technical Guide!

In our current method, which we will call HS-Syringe-GC-MS, we are taking the headspace from a 6 mL sample. All standards (STDs) were prepared as follows: 3 g sodium chloride (NaCl) was measured into a 20 mL amber headspace vial (cat. # 23086) with screw top cap (cat. # 23090). 6 mL of deionized (DI) water was then added to the vial. All STDs then received internal standards. Lastly, STDs were capped and vortexed at 3000 rpm for 10 seconds, inverted, then vortexed again for 10 seconds at 3000 rpm. Luckily for us, we are using the CTC PAL RTC rail system, so the vial equilibration and injection are automated. Once equilibrated, 500 µL of the headspace is drawn into a gastight syringe and injected into the gas chromatograph.

 

*Note: If you are not using a rail system, don’t fret! This method can easily be done with a manual setup.

 

For more detailed specs, please refer to Table 2 & 3, which has our rail and GC-MS parameters.

 

Now that we’ve gone over how we are preparing our samples, let’s take a look at the chromatography (Figures 1 & 2):

 

 

This total ion chromatogram (TIC, Figure 1) shows that we are able to resolve all 19 compounds of interest. Some of the polar compounds that have higher partitioning coefficients are a little more difficult to see. However, in the extracted ion chromatogram (EIC, Figure 2), you can see that we were in-fact able to drive them out of the water and into the headspace. The only compounds that are not baseline resolved are the Toluene-d8 and Toluene. The resolution value for these two compounds is 1.38, which is resolved enough for accurate integration. Since Toluene-d8 is our internal standard, we’ll probably be changing our internal standard in the near future, so that Toluene is completely baseline resolved.

 

 

I know we threw a lot at you here, so I think this is a good place to take a break! We do have more to show you, but we don’t want to give it to you all at once. We want to leave you hungry for more! To recap, we were able to take a very simple approach to this analysis by using the HS-Syringe technique and with the help of EZGC, we were able to resolve all of our compounds with hardly any difficulty. In addition, this method can also be done with a manual HS-Syringe and run on a GC-FID. Like I said, we have much more to cover, so stay tuned for our other HS approach in our next blog!