[JeffMear]

Hello, I'm not to savvy on the technical aspects of HPLC and I was wondering if someone could help me. I'm writing a story on medical marijuana, specifically the testing of marijuana through use of GCMS and LCMS. Labs in the medical marijuana states are using these methods to break down the percentages of certain cannabinoids to reach a greater understanding of the plant. My question is: When they gather percentages of cannabinoids by evaluating the peaks would this percentage relate to total weight of the sample? If tests show a sample to be 20% THC does that mean the 20% by weight of the flower is THC or is it saying that it's 20% THC in relation to the other compounds being searched for? It just logically seems that a marijuana flower couldn't be made up of 20% THC by weight compared to the total sample. Labs are saying that if a sample tests at 27% THC that there is .27 grams of THC for every gram of marijuana. Is this even possible to conclude using the results from a HPLC/ms or GC/MS? I know this is a bizarre subject but any help would be greatly appreciated.

Thanks, Jeff


Edit> I didn't realize I was in the student section. If this needs to be moved go ahead, thanks.

[JGK]

The 20% will refer to the fact hat THC accounts for the total peak area recorded in the chromatogram under the those specific method conditions.

You cannot equate it to a percentage by mass unless you can quantify each peak against a pure standard. Two Peaks produced may have similar heights or areas but if the the response/unit concentration is not identical for each compound producing that peak they will not represent the same mass

[JeffMear]

The 20% will refer to the fact hat THC accounts for the total peak area recorded in the chromatogram under the those specific method conditions.

You cannot equate it to a percentage by mass unless you can quantify each peak against a pure standard. Two Peaks produced may have similar heights or areas but if the the response/unit concentration is not identical for each compound producing that peak they will not represent the same mass

First of all, thank you.

So if you had a pure standard for THC but not all of the other compounds would you be able to get a % by mass or would you need a pure standard for all compounds found in a sample??

[fledarmus]

It depends on the detection method you are using and on the scientific rigor you are applying to the process.

In theory, the only way to determine a true mass percent would be to develop standard curves for the component corresponding to each peak you detect in your chromatogram. This would mean running a pure sample of that component at various concentrations to determine the peak area corresponding to that concentration and preparing a graph (or mathematical function, which is easier these days) of peak area vs. concentration in the sample. If this is done individually for each component of your mixture, then when you inject your mixture, you should be able to read the concentration of each component in the mixture by reference to the standard curve for that component.

In practice, similar compounds have similar properties, and many components are measured by reference to other components rather than standard curves unless you are working in highly regulated lab situations like forensics or pharmaceuticals.