Heavy metal analysis of cannabis has grown more important since marijuana and hemp were legalized for medical and recreational use in an increasing number of states in the US and worldwide.
No federal regulations exist around the safety, purity and quality of cannabis in the US, with responsibility falling on individual states to decide how to effectively regulate this increasingly common substance.
These tests vary between states, but every state requires testing of the most toxic heavy metals such as cadmium (Cd), arsenic (As), lead (Pb) and mercury (Hg). Toxic elements could potentially be introduced to plants through contamination during cultivation (uptake from water, soil or fertilizer), storage or other processing.
Cannabis plants are highly effective at sequestering soil contaminants. These sequestered metals can pose a major risk to human health due to their carcinogenic nature, or through their potential to damage cardiovascular, neurological, renal, gastrointestinal, respiratory and reproductive systems.1, 2
A wide range of cannabis products are available on the market. These include, but are not limited to, dry plants and extracts in the form of tinctures, oils, and edibles (such as cookies or gummies). This diverse range of products means that established inductively coupled plasma mass spectrometry (ICP-MS) for heavy metal analysis of food and plant materials are suitable for use in cannabis testing.3
This article outlines the analysis of two hemp plants, testing these for toxic heavy metals using the Advion SOLATION® ICP-MS, utilizing the established ICP-MS method.
High-purity ICP-MS standards (SPEX) along with trace metal grade nitric acid (Fisher Scientific) were utilized for both calibration standards and sample preparation, in order to avoid any potential contamination. Ultra-Pure water, H2O (≥18.2 MΩ–cm; from ELGA PURELAB flex) was employed in every dilution, while hemp materials were obtained from Goat Ridge Hemp Company.
Each hemp plant was homogenized and powdered prior to the dissolution process. A MARS 6 CEM microwave oven equipped with iPrep vessels was used to digest the samples.
Around 0.50 g of each sample was dissolved in a mixture of 0.90 mL of H2O, 6 ml of concentrated HNO3, 0.10 mL of gold solution (1000 µg/mL, to act as a mercury stabilizer), and an appropriate amount of premixed solution to accommodate spiked samples.
The dissolution process itself was undertaken using the MARS 6 at 200 ºC, 800 psi, and power of 900-1050 W. The process took one hour in total - 15 minutes ramping + 15 minutes holding + 30 minutes cooling. Dissolved samples were then diluted to 100 mL with ultra-pure water. This was done prior to analysis in order to reduce acid concentrations. Calibration standards were set up using 2 % HNO3.
As and Cd were calibrated from 0.08 to 10 ppb, while Pb and Hg were calibrated from 0.08 to 25 and 0.04 to 5 ppb, respectively (Figure 1). To ensure instrument quality control and ongoing calibration verification, three blanks which contained just the reagents needed for digestion and dilution were spiked with varying amounts of these elements, and measured as part of each individual batch run (Figure 1).
Figure 1. Calibration curves for As, Cd, Hg, and Pb including both calibration standards and QC samples. Image Credit: Advion
An Advion SOLATION ICP-MS (Figure 2) was connected to an ASX-560 autosampler (TELEDYNE CETAC). This combination of instruments was utilized for every measurement. SOLATION’s sample introduction system incorporates a cyclonic, temperature-controlled spray chamber fitted with a quartz torch. Analytical parameters and instrument configuration are displayed in Table 1.
Table 1. Operating conditions for Advion SOLATION ICP-MS. Source: Advion
||Quartz, 400 µL/min
|Spray Chamber Temperature
|He gas for KED mode
||0.1 sec per analyte
|Number of sweeps
|Number of readings
Figure 2. Advion SOLATION ICP-MS. Image Credit: Advion
The instrument’s Kinetic Energy Discrimination (KED) mode was utilized to measure arsenic; using He gas in the collision cell. Standard mode with no gas was used to measure the other three elements. In order to address and correct instrumental drift over time, a mixture of internal standards (In and Bi, 10 ppb Ge) were added to the unknown samples and blank, calibration standards (Table 2).
Table 2. QC recovery tests for calibration verification. Source: Advion
||Recovery of QC1
||Recovery of QC2
||Recovery of QC3
The results shown in Table 2 and Figure 1 suggest exceptional linearity with an R2 value of 0.999 for each calibration. Additionally, QC recoveries are within the accepted range of ±15 % for analytes found within the low concentration range (part per billion; ppb).
Table 3 displays the measured values of analytes present in the raw plant materials, factoring in the dilution factor applied before ICP-MS analysis took place (approximately 200-fold).
To verify the accuracy of the method used for analyzing these samples, a spike recovery test was conducted. A spike premixed solution was utilized, aiming to reach an analytical concentration of 5 ppb for Cd, As, and Pb, and 2.5 ppb for Hg per sample. Analysis with the SOLATION ICP-MS revealed that the majority of spike recoveries were all within ±15 %.
Table 3 confirms that concentrations of these particular heavy metals present in the two hemp samples analyzed are comfortably within accepted limits – which are currently 400 ppb.
Table 3. QC recovery tests for calibration verification. Source: Advion
||Sour Space Candy
||Measured conc. (ppb), n = 4
||Spike recovery (%)
||Measured conc. (ppb), n = 4
||Spike recovery (%)
The Advion SOLATION ICP-MS is an ideal instrument for multi-elemental analysis. The SOLATION delivers high sensitivity measurement of trace elements across a wide array of matrices.
Advanced and intuitive software including Quant Express, ICP-MS Express and Data Express offers users an instrument control framework that includes automated tuning processes, while making quantitative and post-analysis data processing more user-friendly and accessible.
The results of hemp measurements outlined above confirm that here, the Advion SOLATION offered the necessary sensitivity and accuracy for the analysis of heavy metal in cannabis materials.
- Järup l. Hazards of heavy metal contamination. Br. Med. Bull. 2003, 68, 167-182. 2
- Gauvin D. V., Zimmermann Z. J., Yoder J., and Tapp R. Marijuana Toxicity: Heavy Metal Exposure Through State-Sponsored Access to “la Fee Verte”. Pharmaceut. Reg. Affairs 2018, 7:1. 3
- Avula B., Wang Y-H., Smillie T. J., Duzgoren-Aydin N. S., and Khan I. A. Quantitative determination of multiple elements in botanicals and dietary supplements using ICP-MS. J. Agric. Food Chem. 2010, 58, 8887-8894.
This information has been sourced, reviewed and adapted from materials provided by Advion.
For more information on this source, please visit Advion.