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Unpacking the USDA’s Hemp Testing Framework

By Alexander Beadle

Published: Nov 23, 2020   
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Just over a year ago, the US Domestic Hemp Production Program was established by the US Department of Agriculture (USDA), following the release of an interim final rule governing the production of hemp in the United States. Since then, a total of 53 state and tribal hemp production programs have been green-lighted by the federal agency under the program.

The purview of this interim final rule also included a number of important guidelines relating to the testing of hemp and the quantification of THC in hemp material – guidelines which form the basis of today’s national hemp testing framework.

The legal background surrounding hemp testing

The passage of the Agriculture Improvement Act of 2018, also known as the 2018 Farm Bill, mandated that the USDA establish a national regulatory framework for legal hemp production in the United States.

In addition to this regulatory framework, the 2018 Farm Bill required the USDA to establish a procedure for testing the THC content of hemp, “using post-decarboxylation or other similarly reliable methods.”

As the definition of hemp is a legal distinction based on whether a cannabis plant contains less than 0.3 percent THC by weight, the testing guidelines for the determination of THC in hemp are central this federal guidance.

Despite the clear importance of accurate chemical analysis, hemp testing labs are currently not required to be ISO accredited under the rules of the Hemp Program. Current testing guidelines say that while ISO accreditation is not mandated, the USDA still “strongly encourages adherence to the ISO 17025 standard.”

“The Interim Final Rule published on October 31, 2019, requested comment on a potential future ISO requirement for hemp testing laboratories,” a USDA spokesperson told Analytical Cannabis when asked for comment.

“USDA received comments based on this request and is currently analyzing the submissions.”

The essential hemp testing procedure

The current testing guidelines for identifying THC in hemp include a seven-step general sample preparation and testing procedure for laboratories to follow.

  1. A sample is received by the laboratory.
  2. The sample is dried to remove the majority of the moisture present. Samples should be dried to a consistent loss, normally with a final moisture content of around 5-to-12 percent moisture content.
  3. The dried sample is milled through a wire screen no larger than 1.5 x 1.5 millimeters in order to discard impurities such as twigs or stems.
  4. The sample is separated into one testing specimen and one “retain” specimen.
  5. Moisture content is determined, or the sample is dried to a consistent weight.
  6. Chemical analysis/testing is performed on the testing specimen using such “post-decarboxylation or other similarly reliable methods.”
  7. Total THC should be calculated on a dry weight basis, and test results reported on a dry weight basis.

Though the guidelines set out this multi-step process, they do not actually prescribe any specific analysis method for any part of this process. They do stipulate that whatever method is chosen, the laboratory must meet the AOAC International standard method performance requirements (SMPR) for selecting an appropriate method.

“Hemp must be tested using post-decarboxylation or other similarly reliable analytical methods where the total THC concentration level reported accounts for the conversion of delta-9- tetrahydrocannabinolic acid (THCA) into THC. Testing methodologies currently meeting these requirements include those using gas or liquid chromatography with detection,” said the USDA spokesperson.

While the USDA does not prescribe or outline a specific analysis method for determining THC concentrations, the consensus standards organization AOAC International has developed a method – the Official Method of Analysis (OMA) 2018.11 – for detecting and measuring cannabinoids in hemp in a way that fulfills the requirements put forth by the interim final rule.

“As per the statutory language in the Farm Bill[…], what’s required there in that statutory language is reporting the percent total THC on a dry weight basis,” explained Dr Holly Johnson in a recent Analytical Cannabis webinar. Johnson is the chief science officer for the American Herbal Products Association and chairs the AOAC International’s Cannabis Analytical Science Program (CASP) working group.

“The two current Official Methods of Analysis both report[ed] on an as-is basis the concentration of the cannabinoids. So, the stakeholders in CASP decided we needed some additional steps to make a standard method that was specifically for hemp materials that would comply with the requirements for reporting on a dry weight basis.”

The OMA 2018.11 relies on a liquid chromatography-diode array detection (LC-DAD) technique with optional mass spectrometric detection to detect and quantify the cannabinoids present in hemp, including THC.

Why post-decarboxylation analysis? Why report on a dry-weight basis?

The simplest answer to many procedural questions on hemp testing, such as ‘Why are labs asked to report results on a dry weight basis?’ is ‘because of the Farm Bill.’

When asked about the need to report THC percentages on a dry weight basis, a spokesperson for the USDA reiterated that “the dry weight requirement was stipulated by the 2018 Farm Bill.” Similarly, about the decision to set the upper THC limit at 0.3 percent rather than 0.2 percent, as is commonly used in the European Union, the wording of the Farm Bill was once again given as an answer.

But consider that perhaps some of the requirements put forward by the testing guidelines and by the statutory language of the Farm Bill are a matter of compliance testing, and not pure quality control. When results are reported on an as-is basis, laboratories receiving “wet” samples will fail to compensate for the amount of moisture in the sample if results are reported as-is. This can dramatically increase errors and cause the resulting determinations of cannabinoid content to be grossly inaccurate. Requiring that results be reported on a dry weight basis could perhaps be one way to limit this sort of variance in testing.

The language requiring labs to use “post-decarboxylation or other similarly reliable methods” for analysis is a recognition of the fact that THCA present in the sample will decarboxylate to form THC over time, or when heated. By requiring methods that either initiate decarboxylation, such as gas chromatography, or that are able to accurately analyze both THCA and THC, such as liquid chromatography based methods, this ensures that the THCA content in samples can be adequately accounted for.

This article originally appeared in Analytical Cannabis' Technologies and Techniques for Cannabis Testing eBook in September 2020. 

Alexander Beadle

Science Writer

Alexander Beadle has been working as a freelance science writer since 2017 and has covered the cannabis industry for Analytical Cannabis since 2018. He has also written for our sister publication, Technology Networks, and the cannabis industry consultant firm Prohibition Partners, among others. Alexander holds a Master's in Materials Chemistry from the University of St. Andrews, where he won a Chemistry Purdie scholarship, and conducted research into zeolite crystal growth mechanisms and the action of single-molecule transistors.


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