Plant Raman Testing for THC and CBD: Why Sampling the Right Tissue Matters
Where a Raman instrument samples a cannabis plant can determine whether it obtains useful cannabinoid information. A 42-plant study found informative THC- and CBD-related signals in glandular-trichome-rich regions, while other leaf and stem regions did not provide the same information.
Why the wavelength and tissue mattered
Porcu and colleagues investigated Raman spectroscopy using a 1064 nm excitation laser. The longer wavelength reduced fluorescence that interfered with useful spectra at a shorter tested wavelength. The samples came from different plantations and included varying THC and CBD compositions.
The researchers compared spectral features with cannabinoid quantities established by gas chromatography with flame-ionization detection. Microscopy helped identify the glandular trichomes associated with the informative plant regions.
Identifying a chemotype is not universal quantification
Differences in characteristic bands and their relative intensities helped distinguish THC-rich and CBD-rich samples. Multivariate analysis supported differences between these groups. That is a useful analytical result, but group separation does not guarantee an equally accurate numerical THC percentage in every unknown specimen.
The spectra were not uniform throughout each plant. Measurements from trichome-rich inflorescence regions supplied information that ordinary stem and leaf regions did not. A negative or weak signal from an uninformative location therefore should not be equated with proof that the entire plant lacks THC.
What a forensic application would need
An interpretation should identify the sampled tissue, the reference chemical method, the range of compositions used for calibration and the uncertainty near the decision point being considered. Averaging selected measurement locations also raises a separate question about how well those locations represent a seized item or plant batch.
The study does not establish a universally applicable legal classification threshold. Nor does plant analysis establish how much THC a person absorbed or whether that person was impaired.
Raman spectroscopy in a plant is also a different analytical application from Raman testing in a biological fluid. Our discussion of saliva and urine Raman validation explains why the specimen matrix must remain explicit when evaluating claims about a portable instrument’s accuracy.
Primary research
Rapid In Situ Detection of THC and CBD in Cannabis sativa L. by 1064 nm Raman Spectroscopy. DOI: 10.1021/acs.analchem.2c01629.
