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How to Test for Heavy Metals in Cannabis

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Cannabis collects heavy metals. It “absorbs heavy metals from the soil, water and air,” says Bob Clifford, general manager at Shimadzu Scientific Instruments. These elements—arsenic, cadmium, lead, mercury and others—can trigger a range of health problems, including cancer and heart disease. Although cannabis for human consumption or smoking should be as free from heavy metal contamination as possible, that’s not always the case. When Sammy Hagar wrote “sparks fly in the middle of the night,” little did he know that this line could describe not just heavy-metal music, but the concerns of testing of cannabis for these elements.

Today’s ‘sparks’ fly around the need and necessary extent of testing cannabis for heavy metals. “Heavy-metal contamination will vary from farm to farm and different strains will have different uptake rates of metals, thus the importance of testing,” Clifford explains.

Regardless of where cannabis comes from, it can include heavy metals. “All agricultural products, including cannabis, contain some amount of heavy metals,” says GenTech Scientific. “The use of things such as fertilizers and pesticides in agricultural applications increase the abundance of these metals in soils and water.”

Beyond coming from natural sources and agricultural methods, heavy metals can infiltrate cannabis-based products in other ways. As an example, GenTech Scientific notes: “Contamination can also occur during the processing of cannabis.”

Despite the wide range of potential sources of heavy metals, it is not inevitable that every cannabis sample includes these elements—at least not at high levels. “With the correct atmosphere, clean soil and water, and limited use of pesticides and fertilizers, you can keep your levels below the acceptable safe limit,” according to GenTech Scientific. Nonetheless, this company adds, “It is important for growers to monitor the levels to ensure their products are in compliance with their state’s regulatory guidelines.” And those can vary considerably.


Where regulators do require testing cannabis for heavy metals, the list usually includes arsenic, cadmium, lead and mercury. “Other states, like Maryland, include additional elements to be tested for such as barium, chromium, selenium and silver,” Clifford says.

Testing tools

Cannabis can be tested for heavy metals in many ways, such as various forms of atomic spectrometry, including atomic absorption (AA), inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). “For AA, the method used would have to be the more sensitive graphite furnace atomic absorption—GFAA—since the flame AA method would not be sensitive enough for most elements,” Clifford explains. “Also, mercury must be measured by a method called cold vapor atomic absorption spectroscopy—CVAAS—due to poor sensitivity by AA.”

In general, flame techniques can measure elements at low parts per million, and GFAA goes down to low parts per billion. “Also, the AA method usually measures one element at a time,” Clifford says. “ICP and ICP-MS are techniques capable of measuring multiple elements simultaneously.” Nonetheless, using ICP to test cannabis for heavy metals, often requires a way to enhance its sensitivity, such as introducing the sample with an ultrasonic nebulizer (USN). “The USN can increase sensitivity up to a factor of 10,” Clifford says.

From Aeos Labs, analytical chemist Anya Engen says, “ICP-MS offers the best sensitivity and is the method used in our lab.” She adds, “The FDA and United States Pharmacopeia have standardized methods for heavy-metals analysis, which are very useful resources in the fledgling cannabis-testing industry, where regulation is slow to catch up.”

To test for arsenic, scientists often need special sample preparation, such as liquid chromatography (LC), because arsenic can be in inorganic or organic forms. The inorganic form is more toxic. “If one wants to test for the different forms, LC-ICP-MS is utilized to separate and detect the different compounds,” Clifford says. “States currently only require total arsenic, so speciation of this compound is not required.”

Tomorrow’s testing

Not all regulators even require heavy-metals testing for cannabis at the moment. “We test for heavy metals in the USA’s food supply system, enforced by FDA and USDA, so why would we not test for other consumed products like cannabis, especially when involving immune-compromised patients?” Clifford asks. “There needs to be federal oversight of testing for heavy metals, as well as pesticides, residual solvents, mycotoxins and other contaminates so testing is harmonized throughout the United States.”

When asked if any testing improvements would be useful, Engen says, “Measuring heavy-metal levels in the soil and water where cannabis is grown and correlating the data to cannabis-generated data would be helpful in illustrating an overview of possible contamination sources.” She adds, “Being in Hawaii with an active volcano, the heavy metals produced and dispersed by the volcano is a possible source of contamination in Hawaii-grown cannabis.”

Without testing cannabis for heavy metals, sparks could fly from more than guitars as customers insist increasingly on safe, tested products.