Short answer
Potency testing measures cannabinoid concentration in a cannabis sample. Two lab methods dominate: high-performance liquid chromatography (HPLC) and gas chromatography (GC). HPLC keeps acidic cannabinoids intact and reports THC and THCA as separate numbers. Standard GC heats the sample in the injector port, so THCA loses its carboxyl group and converts to THC. A GC run reports total THC after that conversion. Labels express flower and concentrate potency as a percentage of dry weight. Edibles are expressed in milligrams per serving.
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HPLC: the reference method
HPLC separates compounds in a liquid mobile phase. Typical steps:
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- Grind and homogenize the sample.
- Extract a weighed portion in methanol, ethanol, or acetonitrile, with shaking or sonication.
- Filter and dilute the extract.
- Inject onto a C18 column.
- Detect with UV at 220 to 228 nm or with a diode array detector.
One run quantifies delta-9 THC, THCA, CBD, CBDA, CBG, CBC, and CBN. Column temperature stays below the decarboxylation threshold, so acidic and neutral forms stay apart. Labs calibrate with certified reference standards and run blanks, duplicates, and spiked samples with each batch.
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GC: total THC after decarboxylation
GC with flame ionization detection (GC-FID) or mass spectrometry (GC-MS) vaporizes the sample in an injector port held at 250 degrees C or higher. THCA decarboxylates at that temperature. The detector sees one THC peak. GC cannot report THCA and delta-9 THC as two values unless the lab adds a derivatization step or a cold on-column inlet. GC-MS adds mass identification and is the standard tool for pesticide and residual solvent screens.
Other methods
- LC-MS/MS. Low detection limits. Used for trace and minor cannabinoids and for contaminant confirmation.
- Near-infrared spectroscopy. Fast and non-destructive. Needs a chemometric model built from HPLC reference data. Suits process control and screening, not label claims.
- Immunoassay and lateral flow strips. Field screens that return a pass or fail signal.
Sampling and sample prep
Sampling error is the largest source of potency variance. A batch is a defined lot. Labs pull multiple increments from different points, combine them, grind to a uniform particle size, and mix. The test portion runs from about 0.1 g to 1 g. Moisture is measured in parallel by loss on drying. Potency is reported on a dry-weight basis, so water content does not shift the number. A lab that skips homogenization can return a precise result for the wrong sample.
Total THC math
THCA carries a carboxyl group. Heat removes it as carbon dioxide. The leftover THC molecule weighs less than THCA. Molar mass of THCA is 358.47 g/mol. Molar mass of delta-9 THC is 314.46 g/mol. Their ratio is 0.877.
Total THC = delta-9 THC + (THCA x 0.877)
The same factor converts CBDA to CBD.
Reporting and tolerance windows
Labels list delta-9 THC, total THC, and total CBD. State regulators set a tolerance window around the label claim. The window and the penalty structure vary by state. Labs also report measurement uncertainty, which runs from about 5% to 10% relative for cannabinoids. Potency sits inside a broader compliance panel that can include pesticides, heavy metals, microbials, mycotoxins, and residual solvents.
Accreditation
ISO/IEC 17025 accreditation covers method validation, instrument calibration, reference standards, and chain of custody. It does not guarantee accuracy on a given sample. Instrument quality and sample quality both drive the final number.