Cannabis Heavy Metal Testing

Cannabis is known for its phytoremediation capability which is an ability to bio-accumulate toxic heavy metals out of the soil and into plant material. I would suggest reviewing the following paper by Girdhar et al. that describes the hyperaccumulatory and phytoremediation capability of Cannabis sativa (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4235884/). This increased protensity to uptake heavy metal makes it important to analyze the toxic heavy metal content in products made from cannabis.

Metal analysis is done through a process of microwave digestion and introduction into an Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The sample is first quantitatively weighed in a vial and a combination of nitric and hydrochloric acid or concentrated 30% hydrogen peroxide are added. This vial then goes into a pressurized reaction vessel to minimize foaming and cross contamination. Microwaves are applied to heat the sample under strong acidic conditions and the resulting product is a liquid that can be safely injected into the nebulizing chamber of the ICP-MS instrument without concern for clogging. I have heard that entire vape cartridges could be liquefied using this technique, which is a testament to its effectiveness of being able to digest ordinary cannabis products. The nebulized solution goes into a spray chamber to minimize background signal noise before it is aspirated into an inductively coupled argon plasma that is around 10,000K or 17,540 degrees Fahrenheit at its hottest point. This temperature at its hottest point is around equivalent to the surface temperature of the sun. This breaks all of the bonds in the sample resulting in the mixture’s atomized form which is also ionized in the argon plasma full of electrons and ionized argon. The resulting positively charged metal ions are attracted towards an entrance cone with a negative potential and are separated by mass to charge ratio in a quadrupole based mass spectrometer, similar to ions being separated in a mass spectrometer for pesticide, terpene, or residual solvent analysis. The concentration of lead, arsenic, cadmium, and mercury are determined based on the signal response of the sample compared to the signal response of a calibration curve with certified reference materials.

The construction of Inductively Coupled Plasma torch.
A: cooling gas tangential flow to the outer quartz tube
B: argon gas flow to form plasma
C: flow of carrier gas with sample
D: induction coil which forms the magnetic field where electrons are introduced and collide with argon creating ionized argon atoms and more electrons
E: force vectors of the magnetic field
F: the argon plasma.

Currently, the state of Oklahoma has established the following acceptable ranges for toxic heavy metal content in cannabis products:

  • Lead – max limit < 1 ppm (µg/g)
  • Arsenic – max limit < 0.4 ppm  (µg/g)
  • Cadmium – max limit < 0.4 ppm (µg/g)
  • Mercury – max limit < 0.2 ppm (µg/g)

How do these levels compare to other states regulating the testing of heavy metals? Some states such as Oregon do not require heavy metal testing. California differentiates between inhalable cannabis products and “other” cannabis product when mandating limits.

California

  • Lead (inhalable cannabis) – max limit < 0.5 ppm
  • Lead (other cannabis) – max limit < 0.5 ppm
  • Arsenic (inhalable cannabis) – max limit < 0.2 ppm
  • Arsenic (other cannabis) – max limit < 1.5 ppm
  • Cadmium (inhalable cannabis) – max limit < 0.2 ppm
  • Cadmium (other cannabis) – max limit < 0.5 ppm
  • Mercury (inhalable cannabis) – max limit < 0.1 ppm
  • Mercury (other cannabis) – max limit < 3.0 ppm

Colorado has established the following requires for cannabis testing:

  • Lead – max limit < 10 ppm
  • Arsenic – max limit < 10 ppm
  • Cadmium – max limit < 4.1 ppm
  • Mercury – max limit < 2.0 ppm

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