This is How Scientists Identified Marijuana’s Distinctive “Skunk” Smell

Cannabis scientists made an odorous breakthrough last year. A group of researchers finally identified 3-methyl-2-butene-1-thiol (321MBT) as the root cause of the signature skunky smell of hemp and cannabis.

Remarkably, this discovery appears to have been independently reached by two research groups at nearly the same time. While scientists from Abstrax Tech published their identification of this odorous compound in ACS Omega last November, a new article published in ACS Omega this month reveals that a second research group also made a similar public disclosure in March of 2021.

The new article details the method used by this earlier group, led by Jacek A. Koziel of Iowa State University, and considers the value of having two unique odor investigation strategies.

A timeline of discovery

As detailed in the Supporting Information for this paper, the discovery of the link between 321MBT and skunk-like cannabis and hemp began in 2015, when Koziel and the then-student Somchai Rice published a peer-reviewed article linking five chromatographic retention regions with the skunk-like smell from cannabis. While 321MBT was not identified or referenced by name here, it was one of the compounds that sits within these regions.

In early 2020, Abstraks Tech would file a provisional patent application that specifically linked 321MBT with the skunk-like odor of cannabis, but this application was not published or available online. Subsequent blogs and press releases from Abstraks Tech also did not directly mention or reference 321MBT, although the link had been discovered.

In October 2020, an interim report to the odor mitigation company Byers Scientific discovered a link between 321MBT and the skunky odor in hemp, then in March 2022, Koziel and scientists from Byers Scientific published the first formal press release specifically linking 321MBT with the odor of cannabis and hemp. The following month, Koziel gave a Plenary Lecture at the NOSE International Conference on Environmental Odour Monitoring and Control, where this discovery was also mentioned.

Then, after Abstrax Tech filed several formal patent applications with the US Patent and Trademark Office in late-2021, the Oswald et al. paper from Abstrax Tech was published in ACS Omega. This paper reported their discovery of a new family of volatile sulfur compounds, including the primary odorant 321MTB, and described the custom-built two-dimensional gas chromatography (2DGC) system used to detect it.

“While these authors were disappointed that their public disclosure was not cited in the excellent Oswald et al. article, it was reassuring that these two independent and very different approaches to the ‘skunky’ cannabis question yielded the same major conclusion,” Koziel and the Byers Scientific researchers wrote, in the new paper.

The rapid, direct screening approach

While both the Koziel and Oswald groups came to the same conclusion, it is notable that different methods were used to achieve this. The Koziel group predominantly used a rapid multidimensional−gas chromatography−mass spectrometry−olfactometry (MDGC-MS-O) based odorant-prioritization screening approach, while the Oswald group used a two-dimensional gas chromatography method with three detectors running simultaneously: a time-of-flight mass spectrometer (TOF-MS), flame ionization detector (FID), and a sulfur chemiluminescence detector (SCD).

The most significant difference between these two methods, as pointed out by Koziel and the Byers Scientific team, is the “human sensor” component of olfactometry detection.

In the method used by Koziel et al. volatile emissions are screened using a gas chromatograph connected to a mass selective detector (MSD) and solid phase microextraction (SPME) field sampler. After chromatographic separation, the sample continues to pass through the apparatus to an olfactory detector. Here, a human analyst is able to directly smell the sample and apply related odor descriptor tags to each chromatographic region using the Aromatrax data processing software package.

Using this set up, the Koziel group were able to rapidly home in on the chromatographic region most frequently associated with the odor tags “foul” or “musty” and go on to identify 321MBT as the primary odor source.

The comprehensive, indirect screening approach

The Oswald group uses a more indirect approach, which is less rapid and more labor-intensive but does provide a more comprehensive analysis.

In their technique, two-dimensional gas chromatography with a TOF-MS detector, FID detector, and SCD are used simultaneously to fully characterize and quantify the compounds present in a sample. Olfactory testing of cannabis was done with a four-person panel, the members of which were trained on common standard terpenes in cannabis to identify various odor notes. By reverse engineering various formulations of the compounds of interest found in cannabis and comparing these to the standard sample training, the panel was able to rate the formulations for pungency and this rating could be used to identify the primary odor components.

“The Oswald et al. integration of hyphenated techniques (i.e., GC × GC-TOF-MS/FID/SCD) represents an almost perfect tool for pushing the limits relative to odor compositional analysis,” the authors wrote.

These two odor screening methods also have application outside of cannabis and hemp, the authors say. Where an exhaustive approach is needed, such as in toxicology or atmospheric chemistry applications, the comprehensive Oswald et al. method is extremely useful. Where a faster odor-match is needed, such as a company trying to troubleshoot a strange odor in their product, the more direct MDGC-MS-O technique may be preferable.

Why is cannabis aroma important?

Cannabis distinctive skunk-like odor is more than just a fun little quirk, it can become a real public nuisance if uncontrolled.

“Within the cannabis industry – since it is a fairly nascent industry – a lot of the efforts in odor control really are a wet thumb in the air,” Marc Byers, president and cofounder of Byers Scientific, told Analytical Cannabis last year.

“Certainly, much more science and innovative thinking has gone into other areas of the industry – perfecting grow lights, getting the precise spectrum, the correct humidity levels, things like this.”

In one of the most well-known cannabis odor cases, residents of Sonoma County in California became so sick of the skunk smell from local cannabis grow operations that they filed a lawsuit to ban the grow operations from the neighborhood. While it may not be a traditional pollutant by any means, these smells can be extremely disruptive if not addressed.

“Although, many municipalities should have learned by now, having a couple years under their belt, that this is something they need to consider, putting rules in early on odor control,” said Byers.

Learning more about what compounds are responsible for cannabis’ smell enables the development of better air filtration systems and mitigation strategies that can neutralize the offending odor, Byers said.