Comparison of Phencylidine and Prazepam as Internal Standards in Medical Cannabis Potency Analysis with GC-FID

Recently in the ChromaBLOGraphy posts below I proposed the use of Phencylidine (PCP) as an internal standard (ISTD) for cannabinoids analysis with GC-FID when using the Rxi-35Sil MS GC column.  I demonstrated that the RSD% of Average Response Factors (Avg RFs) and Correlation Coefficients (CCs) for the calibration curves generated using the ISTD technique were excellent, and that ISTD calibration/quantification could reduce quantitative error associated with sample introduction into the GC.

Possible Internal Standards for Medical Cannabis Potency Testing by GC

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

Internal Standard versus External Standard Quantification in Medical Cannabis Potency Analysis with GC-FID

You may remember that in addition to PCP, I added Prazepam to each calibration standard (see chromatogram below).  Prazepam worked about as well as PCP as an ISTD, with slightly larger RSD% values for select cannabinoids and very slightly lower CCs (see table below).  The absolute RF differences for cannabinoids using ISTDs PCP or Prazepam are explained simply by having larger peak areas for PCP versus Prazepam, a function of the better FID response for PCP.

So...should we use two internal standards for this work?  We could.  One strategy is to use PCP as an ISTD for earlier eluting cannabinoids (e.g., CBDV, THCV, CBC, CBD) and Prazepam for later eluting cannabinoids (e.g., delta-8-THC, delta-9-THC, CBN, CBG).  Why?  It’s possible that PCP response is more representative for the more volatile cannabinoids (earlier eluters) and that Prazepam is better for the less volatile compounds, although the current data set is not definitive in that regard.

My preference at this point?  Consider PCP as an internal standard for all the cannabinoids and Prazepam as a potential indicator for determining when to change the GC inlet liner.  Prazepam, as the last eluting compound and potentially the least volatile, should be an early indicator of non-volatile “dirt”, like chlorophyll,  building up over repeated cannabis extract injections and eventually leading to poor quantitative transfer of cannabinoids from the GC inlet to the GC column.  Think of Prazepam as a “control chart” compound that upon declining in area count in a continuous fashion signals the need for GC inlet maintenance.

In summary, the addition of a combination of PCP and Prazepam to every cannabinoids standard and every cannabis extract prior to analysis will lead to better potency determinations when using GC-FID.