The subject of this issue is Δ9-tetrahydrocannabinolic acid (THCA), which occurs naturally in the plant. THCA needs to be heated so it changes into THC, the active form that gets you high. All cannabinoids occur naturally in their acid forms, that’s just how their enzymes make them. The difference between THCA and THC is a carboxy group. Upon smoking, cooking or vaping heat gets rid of the carboxy so THCA gives off CO2, loosing about 12 percent of its weight in the process.

Why does this matter for lab testing? Because THCA is heavier than THC, and lab results are given in mass percent.

The root of the confusion is the fact that different lab techniques give inherently different potency values. Depending on the lab, the analysis machine might use one of two separation methods: gas chromatography (GC) or liquid chromatography (LC).

GC uses temperatures high enough to completely decarboxylate all the cannabinoids in the mixture. This means GC does the decarboxylation for you, giving you only one reading for THC.

This makes GC useless for testing edibles because you need to be able to tell the difference between inactive THCA and active THC…

Stuff like this is confusing to me, and I don’t pretend to understand it all.  There’s a New Mexican dispensary that combines both THC and THCA results in one listed percentage, which if I understand it correctly, is not how to determine the THC value in bud.  And yet, New Mexicann is the only dispensary to carry the Americans for Safe Access PFC label, which makes it even more confusing.  For more info:

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