Calibration Curves for Cannabinoids Based on PCP Internal Standard – Medical Cannabis GC-FID

Yesterday’s ChromaBLOGraphy post titled "Possible Internal Standards for Medical Cannabis Potency Testing by GC" concerned the use of internal standards (ISTDs) for GC-FID potency testing of medical cannabis.  In that post I defined desirable characteristics for an ISTD and said that one of the benefits of ISTD use is better quantitative accuracy.  Good quantitative accuracy starts with good calibration, which I demonstrate in this post by showing a calibration curve for delta-9-THC that is based on ratios generated through PCP internal standard use.  For each standard of THC, ranging from 0.1 to 100 ng/µL, PCP (Phencyclidine) and Prazepam were added at 50 ng/µL each.  The peak area of delta-9-THC divided by the peak area of PCP represents the “Ratio” shown on the calibration curve for each concentration level.

As previously, cannabinoids were analyzed using a 15m x 0.25mm x 0.25µm Rxi-35Sil MS GC column (cat# 13820) with FID (see chromatogram below).  Evaluation of the initial calibration for cannabinoids can be done in several ways, including by visual inspection of calibration curves.  If you look below you will see the calibration curve for delta-9-THC, which stretches from 0.1 ng/µL to 100 ng/µL in the first figure.  I zoom in on the lower points in the next figure so the reader can see how close to the curve the points are, which is an indication of linear fit.

In addition to the “eyeball test”, calibration curve quality can be viewed in terms of Average Response Factor (Avg RF) and Response Factor % RSD (% RSD), which are shown in the table below.  It’s up to the individual user to determine what meets their data quality objectives, but achieving % RSD values less than 20% is generally very good for such a wide calibration range as I show here.  For the ISTD cannabinoid calibrations in this work, the % RSD values are much lower than 20%.

Correlation coefficients (CCs) are commonly used to determine linearity for calibration curves, and those are also shown in the table below.  Every cannabinoid calibration curved generated using the internal standard technique shows a 0.9998 correlation coefficient, which is excellent.

One thing to point out is that I am using split injection with our Restek Premium Precision split inlet liner with wool at a split ratio of 20.  This means that on column amounts for the calibration range are ~5 pg to 5 ng.  Not only is the FID our most linear detector, well demonstrated here, but it is a very sensitive detector, too, something shown by the last chromatogram below for ~10 pg of each cannabinoid on column.   My recommendation to those that are doing dirty samples for potency determination, whether they are chlorophyll-laden flower extracts, or lipid-rich food extracts, or feedstocks, is to take advantage of that FID sensitivity and inject as little sample as possible while still being able to do accurate quantification for all the cannabinoids of interest.  You'll keep your GC system up longer by injecting less "dirt".  Accuracy will be helped by using an internal standard, too.