Chemists Develop Method to Detect THC Traces in Exhaled Breath

According to the United Nations, the number of people using marijuana has increased by 60% over the past decade, reaching 200 million worldwide. Historically, marijuana has been illegal in many countries, but in recent years, authorities in some countries and regions (such as Canada, Spain, Chile, the Netherlands, Australia, and 11 U.S. states) have partially or fully legalized its use. This has created a need for monitoring safe usage—especially when it comes to driving.

Recent studies have shown that in the United States alone, 12 million people were cited in 2017 for driving under the influence of marijuana. Marijuana contains over a hundred different cannabinoids, but the most psychoactive is Δ9-tetrahydrocannabinol (THC), which can range from 5% to 30% in concentration. Current roadside tests check for THC in the blood, but these methods are either time-consuming and require special training to analyze blood or saliva, or rely on a police officer’s subjective assessment of a driver’s behavior.

Recently, chemical methods for detection have been actively developed. For example, scientists have already demonstrated that the amount of THC in exhaled breath correlates with the time since marijuana use and the concentration of THC in the blood. Existing methods include lengthy mass spectrometry, optical detection of compounds formed between THC and a toxic reagent, and imprecise electrochemical detection using carbon nanotubes.

Existing Methods for THC Detection

• Mass spectrometry
• Covalent fluorescent labeling
• Chemical analyzers using carbon nanotubes

Neil K. Garg and Evan R. Darzi from the University of California have developed a simple method for detecting THC in exhaled breath, based on the electrochemical conversion of THC into the corresponding tetrahydrocannaboquinone (THC-quinone), which, unlike THC, is colored. They studied possible electrochemical reactions of these substances and selected the appropriate conditions for this reaction with the necessary yield. Their article was published in the journal Organic Letters.

How the New Method Works

Before testing the electrochemical oxidation reaction, the researchers performed oxidation using common oxidizers, since such a reaction had not been previously described. However, as early as 1911, Beam observed a violet color in an alkaline-alcoholic solution of hashish extract, an effect later attributed to the formation of colored quinones. Since then, bis(trifluoroacetoxy)iodobenzene (PIFA) has become a popular synthetic reagent for oxidizing phenols to quinones, and it was used in this study. The authors applied this reagent in an aqueous medium to THC and found that the reaction produced oxidation products with a sufficient yield. The resulting THC-quinone absorbed visible light at a wavelength of 402 nanometers, which can be detected visually or spectroscopically.

To understand why this compound is colored, the scientists calculated the orbital contributions to electronic transitions, orbital distribution, and energies using first-principles calculations. The minimum energy transition in THC corresponds to an electronic transition between the highest occupied and lowest unoccupied molecular orbitals, at a wavelength of 282 nanometers. The electronegative carbonyl bonds in THC-quinone shift the absorption peak toward the red region, making absorption in the visible range possible, which results in the violet color.

Electrochemical Oxidation Process

To determine how electrochemical oxidation of THC would occur without reagents, the chemists performed cyclic voltammetry for both THC and THC-quinone. Measurement of the oxidation potential showed irreversible oxidation at 1.02 volts for THC and at 1.71 volts for THC-quinone. When measuring the reduction potential, no reduction was observed for THC at voltages from 0 to −2.5 volts, while THC-quinone showed two reduction events at −1.02 volts and −1.75 volts, corresponding to the formation of semiquinone and hydroquinone.

In the next stage, the researchers tried applying known electrochemical oxidation conditions to a model 2,5-dialkylphenol, but instead of quinone, they obtained an insoluble polymer on the electrode. By varying the synthesis conditions, they arrived at optimal conditions—carbon electrodes and tetrabutylammonium tetrafluoroborate as the electrolyte—under which the quinone yield was 57%. They then applied these oxidation conditions to THC, using a graphite-platinum electrode pair, and achieved a 67% yield of THC-quinone.

The authors believe this yield is sufficient for detecting marijuana products in exhaled breath, but they note that this is just the beginning of developing a new analytical detection method. Future research may bring such a test system to life to help ensure road safety.

Conclusion

The researchers propose using this final reaction in electrochemical analyzers to detect THC in human breath, potentially paving the way for rapid and reliable roadside testing.