New Forensic Methods to Accurately Determine THC in Seized Cannabis

New Forensic Methods to Accurately Determine THC in Seized Cannabis

NIJ-funded researchers address the need for simple, cost-effective ways to differentiate hemp from marijuana.

The Controlled Substance Act of 1970 classified the plant cannabis, which was historically classified as either marijuana or hemp, as an illegal drug, a Schedule I controlled substance with a high potential for abuse and no FDA-approved medical use in the United States. For almost 50 years, hundreds of thousands of people were arrested and imprisoned for possessing it.

The Farm Bill of 2018 changed this straightforward classification of cannabis. The bill legalized the form of cannabis classified as hemp, while the form classified as marijuana remained illegal. The law left how to tell the difference up to law enforcement and forensic laboratories, which has proven difficult, time consuming, and expensive.

Figure 1. A comparison of marijuana and hemp showing the 0.3% THC cutoff.

Scientists use the amount of THC (one of the psychoactive components of cannabis) present in a cannabis plant to differentiate hemp from marijuana. [1] The Farm Bill defines hemp as cannabis with 0.3% or less total THC. A cannabis plant with more than 0.3% THC is considered marijuana and remains on the Schedule I substance list (Figure 1). 

The fact that many states have decriminalized the use of marijuana further complicates cannabis regulation. Currently,  the medical use of marijuana is legal in 37 states and the District of Columbia, and the recreational use of marijuana is legal in 19 states and the District of Columbia. In the remaining states, however, marijuana is still considered unlawful and must be distinguished from hemp.[2] Federal law continues to consider marijuana illegal.

As a result of the Farm Bill, forensic laboratories must measure the exact amount of THC in seized evidence to differentiate hemp from marijuana.  In 2022, more than 10% of all submissions to crime labs were to determine marijuana versus hemp. 

Thus, a new problem emerged for already backlogged crime labs. How could labs perform such precise measurements quickly and easily when few labs had the personnel, instrumentation, and protocols to do so? 

Is It Hemp or Marijuana? 

Two NIJ-supported labs addressed this problem using different types of mass spectrometry that can measure the exact amount of THC present in a sample: gas chromatography-mass spectrometry (GC-MS) and direct analysis in real time-high-resolution mass spectrometry (DART-HRMS). 

Historically, most forensic laboratories have used qualitative tests for seized cannabis samples to distinguish hemp from marijuana. Labs could confirm micro- and macroscopic  plant features, screen for THC through a colorimetric test (which changes color in its presence), or test for the presence of THC through various  gas chromatography techniques or thin layer chromatography (which separate the different components of cannabis). But none of these tests can measure the exact amount of THC present in a sample.

Dr. Walter Brent Wilson’s team at the National Institute of Standards and Technology used GC-MS to develop a simple, robust, and cost-effective method to distinguish hemp from marijuana for local, state, and federal forensic laboratories. They are expanding their work to include edibles. With support from NIJ, their research:

  • Identified methods to extract THC from cannabis samples. 
  • Quantified the amount of THC in cannabis samples using GC-MS in different scanning modes.[3] 
  • Found that single-ion monitoring mode dramatically improved sensitivity compared to full-scan mode. 

Dr. Rabi Musah’s lab from the University at Albany, State University of New York, approached the problem differently. The lab used DART-HRMS to quantify the amount of THC found in complex materials that are often difficult to study, such as edibles, beverages, and plant materials. This method can be performed directly on the sample with minimal to no sample pre-treatment. Their project:

  • Established a rapid triage approach to detect cannabis-related molecules.
  • Extracted cannabinoids (the two main ones being THC and CBD) and differentiated them using DART-HRMS.
  • Validated protocols to measure the amounts of the cannabinoids found.

The University of Albany team showed that they can use DART-HRMS to rapidly detect THC and other cannabis-related molecules in baked goods, candies, beverages, and plant materials with minimal pre-treatment steps. They also identified the limits of using DART-HRMS to detect THC in different types of samples. 

Impact on Forensic Laboratories 

Dr. Musah anticipates that the increased speed of using DART-HRMS to detect THC in cannabis samples could reduce sample testing backlogs and chemical reagent costs and streamline sample analysis protocols.

Dr. Wilson’s project created standard operating procedures for quantifying THC in federal, state, and local forensic laboratories. His research team has shared findings through webinars and publications and created training models for the Montgomery County Police Department and Maryland State Police crime labs on using GC-MS to quantify THC.

About This Article

The work described in this article was supported by NIJ inter-agency agreement number DJO-NIJ-20-RO-0009, awarded to the National Institute of Standards and Technology, and by NIJ grant number 2019-DU-BX-0026, awarded to the University of Albany, State University of New York. This article is based on the reports from those awards: “Accurate THC Determinations on Seized Cannabis Samples for Forensic Laboratories” (pdf, 26 pages) by Walter Brent Wilson, Ph.D. and “Research to Develop Validated Methods for THC Quantification in Complex Matrices by High-resolution DART-MS-Focus on Edibles and Plant Materials,” (pdf, 18 pages) by Rabi A. Musah, Ph.D.

Sidebar: How is Mass Spectrometry Used in Cannabis Analyses?

Mass spectrometers allow scientists to calculate the exact molecular weight of a substance and identify known and unknown compounds.

One type of mass spectrometry, gas chromatography-mass spectrometry (or GC-MS), combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, food and flavor analysis, and identification of unknown samples.

Another type of mass spectrometry, direct analysis in real time-high-resolution mass spectrometry (or DART-HRMS), combines the features of direct analysis in real time with high-resolution orbitrap mass spectrometry to identify different substances within a test sample. Scientists can use DART-HRMS to obtain exact mass measurements rapidly with high-resolution mass spectrometers without having to use a separation step. Applications of DART mass spectrometry include pharmaceutical investigations, forensic studies, quality control, and environmental studies.

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