How a lab tests hemp for THC

A compliance lab dissolves ground, dried hemp in solvent and injects a tiny amount into a high pressure liquid stream that runs through a packed column. Each cannabinoid sticks to the column packing a little differently, so they come out at different times and a UV detector draws a peak for each one. Because the method does not heat the sample, THCA and delta 9 THC are measured separately and combined as total THC = delta 9 THC + 0.877 x THCA.

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Measured THCA

Percent of dry weight.

Measured delta 9 THC

Lab measurement uncertainty

Plus or minus, reported by the lab for this method.

Step through it

Sample in a vialA dried, ground hemp sample is weighed and extracted into solvent, then filtered into a vial. The autosampler draws a few microliters from each vial and injects it into the flowing stream.
Pump and solventsA high pressure pump blends acidified water and acetonitrile and pushes them through narrow steel tubing at a steady flow.
The column separatesThe column is packed with tiny particles coated in long carbon chains. Cannabinoids that cling more strongly travel more slowly, so the mix spreads into bands.
Detector and peaksAs each band exits, it absorbs ultraviolet light and the detector draws a peak. Comparing peak areas with certified reference standards turns them into percentages.
Pass or failRaise THCA to 0.40 percent and total THC passes 0.3 percent even before delta 9 THC is added. That crop would fail the hemp limit.

From peaks to total THC

Liquid chromatography runs at room or mildly warm column temperature, so the acid forms survive and show up as their own peaks. Published methods for hemp use a reverse phase C18 column, a gradient of acidified water and acetonitrile, and UV detection near 220 nm, separating ten or more cannabinoids in under ten minutes. The peak order drawn here is typical but depends on the method; peak heights are scaled for visibility.

Federal rules define total THC as delta 9 THC + 0.877 x THCA. The factor is the molar mass ratio of THC to THCA (about 314.5 / 358.5): it counts the THC that THCA would become if heated. The 2018 Farm Bill and USDA rules define hemp as Cannabis sativa with no more than 0.3 percent delta 9 THC on a dry weight basis, judged on this total.

Try it: set THCA to 0.32 percent and delta 9 THC to 0.03 percent. Total THC reads about 0.311 percent. With 0.02 percent uncertainty the range still reaches 0.3, so the result counts as acceptable; drop the uncertainty to 0 and it fails.

Measurement uncertainty

Every lab result carries a measurement uncertainty, a plus or minus band that describes how much repeated measurements could reasonably vary. USDA regulations treat a sample as an acceptable hemp THC level when the reported result with its uncertainty produces a range that includes 0.3 percent or less. Labs determine their own uncertainty for each validated method.

Sources

Fast Detection of 10 Cannabinoids by RP-HPLC-UV Method in Cannabis sativa L. (University of Bologna)Molecules (MDPI, open access)
Understanding the Relationship between THC and CBD in HempNC State Extension
7 CFR 990.1: Meaning of terms (Domestic Hemp Production Program)Electronic Code of Federal Regulations

How to cite this model

Avid Solutions International. (2026, November 11). How a lab tests hemp for THC [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/hplc-potency-testing

Last updated November 11, 2026

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