Possible Internal Standards for Medical Cannabis Potency Testing by GC

I am often asked about internal standards for use in medical cannabis potency testing with gas chromatography.  I finally got some time in the lab to check this out and came up with a couple of possibilities after testing numerous compounds for favorable retention times versus typically analyzed cannabinoids.

Internal standards are mostly used by adding to calibration standards and sample extracts at known and consistent concentration levels immediately prior to GC of those standards and extracts.  Internal standards are especially helpful to indicate when an injection malfunction has occurred, including injecting too much or too little versus what is typical. In either of these cases, the cannabinoid compounds of interest show the same proportional peak area increase or decrease, as would an internal standard.  By using response factors (peak area of compound of interest divided by peak area of internal standard), quantitative accuracy will be better, even when relatively large errors occur in injected sample volumes.  Modern GC data processing software allows easy calibration and quantification with internal standards.

Desirable qualities in an internal standard include:

• Easy to chromatograph.
• Does not react with any compounds in standards and/or samples.
• Well separated from any compounds in standards and/or samples.
• Not expected to be present in samples before its addition as an internal standard.
• Low cost.

The 15m x 0.25mm x 0.25µm Rxi-35Sil MS chromatogram below shows that phencyclidine (PCP) and Prazepam (Przpm) meet the “well separated” criterion for internal standards, bracketing the cannabinoids shown in my last ChromaBLOGraphy post.  PCP and Przpm are extremely unlikely to be present in medical cannabis extracts.  Restek offers PCP and Przpm at DEA-exempt concentrations appropriate for internal standard addition and, at reasonable costs.

The performance of both internal standards was reliable in recent testing work, which will be demonstrated in an upcoming blog post.