Terpene Analysis Approaches – Part IV

We are back at it with another blog about terpene analysis! If you did not catch my previous post, be sure check it out here before moving on. Last time, I finished up by discussing how we would move away from analyzing terpenes in standards and dive into in-matrix analysis. Well…I lied. I’m sorry, but I PROMISE, we will get there because we did get there. First, I need to cover a couple important things.

I had the opportunity to visit a cannabis lab in Santa Rosa, CA, and they let me run their GC’s for a week. Since Restek is located in PA, we are unable to currently bring cannabis into our Innovations Lab, so in order to get our hands on this sticky material we look for collaborations. Having done the preliminary work at Restek and figuring out which sampling approach I wanted to test, I could really get cracking on further method development with their team. The first thing I wanted to complete was a calibration using DI-SPME, which initially started out as a pain, but once we optimized some of the sampling parameters, we were able to gather some great data!

Under the sampling conditions that we were used in the previous blog post (see Table 1 below), we obtained the results shown in Figure 2.

Table 1. DI-SPME Parameters

It would be cumbersome to show you the results for all of the targeted terpenes, so Figure 1 gives a representation of some the terpenes of interest over the volatility range of the entire list.

Figure 1. DI-SPME Calibration Curves

The calibration range was 20 – 1280 ng/mL (ppb) and as you can see, we did not get the best results. I should note, that we were using naphthalene-d8 as an internal standard and the results were generated off of the compounds’ response factors. So, what is going on here? It looks like we’re saturating our detector, right? Guess again! We are not saturating the detector, it’s the fiber! When we analyze the data, the peaks do not have flat tops. Peaks with flat tops are indicative of saturating your mass spectrometer. So, pressed for time in the lab, we made a couple of quick changes to our sampling parameters seen in bold below (Table 2).

Table 2. Optimized DI-SPME Parameters

Under these new conditions and shifting the calibration range from 20 – 1280 ng/mL (ppb) down to 10 – 320 ng/mL (ppb), we obtained the following results (see Figure 2 below) for those same four compounds displayed previously in Figure 1.

Figure 2. DI-SPME Calibration with Optimized Parameters

Dang! Look at that improvement! We went from some embarrassing r2 values in Figure 1 to some r2 values that we can actually work with in Figure 2. Optimizing your sampling parameters and selecting an appropriate calibration range is critical for developing SPME methods. Fiber saturation is a common occurrence when doing SPME, but if you understand the parameters, you are able to overcome this issue. By changing our extraction time from 4 min to 1 min and increasing the split ratio to 250:1, we were able to go from calibration curves that were plateauing to linear curves. To see the rest of the calibration data for our terpenes of interest, refer to Table 3 below!

Linear calibrations were achieved for all compounds with an average r2 value of 0.983 in an average range of 10 – 320 ng/mL (ppb). This is a great improvement from what I have seen in published literature where an average r2 value of 0.942 was achieved for 31 terpenes in a calibration range of 50 – 1000 ng/mL (ppb) using liquid injection.[1] While our results were an improvement, we did experience some difficulties with the higher molecular weight terpenes, as well as with terpenes containing an alcohol functional group. The lower values for the heavier terpenes may be due to the SPME phase, while the terpenes containing alcohols most likely favored the water solution, making it more difficult to getting them onto the phase.

 

Overall, things are starting to look pretty good with the DI-SPME method. We still have more to come though, so stay tuned for our next blog in this series!

 

Reference

  1. Brown, A. K., Xia, Z., Bulloch, P., Idowu, I., Francisco, O., Stetefeld, J., Tomy, G. (2019). Validated quantitative cannabis profiling for Canadian regulatory compliance – Cannabinoids, aflatoxins, and terpenes. Analytica Chimica Acta. https://doi.org/10.1016/j.aca.2019.08.042

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