Introduction by Justin Masone, Product Manager at Glass Expansion, Pocasset, MA. |
Sample introductionMost of the problems analysts encounter with ICP and ICP-MS can be eliminated by utilizing an optimal sample introduction system—too many labs overlook the importance of selecting the best nebulizer, spray chamber, and torch for their application. For example, mercury can often be challenging, as it is considered a “sticky” element, potentially leading to high carryover and long washout times. We have found that using a cyclonic spray chamber, as opposed to a Scott-style spray chamber, results in significantly faster washout times and decreased carryover; this is in addition to the increased sensitivity and lower detection limits inherent to the cyclonic design.1 Glass concentric nebulizers will provide the highest precision and sensitivity, and, for ICP-MS, a low-flow nebulizer such as the MicroMist (Glass Expansion) is ideal for high-precision ICP-MS analyses. One of the major benefits of the low-flow design is the vastly improved transport efficiency, meaning that much less sample will be consumed with little-to-no effect on sensitivity. The reduced droplet size formed by this nebulizer will lead to decreased matrix interferences (often a thorn in the side of many analysts) and a more robust plasma. Since cannabis samples can include silicates, complete digestion can only be accomplished with the addition of HF. As silicon is often not an element of interest, and since HF is an extremely deleterious acid (and requires a separate, expensive sample introduction configuration), almost all labs will settle for a partial digest using HNO3 and/or HCl, followed by filtration of the undissolved silicate particles. As an added layer of protection, an in-line sample filter can be added between the sample probe and nebulizer, which will filter out larger particulates, minimizing the downtime that often results from clogged nebulizers. |
Cannabis has been regulated for both recreational and medicinal use in Washington, Oregon, California, Nevada, Alaska, Colorado, Maine, Vermont, and Massachusetts, and for medicinal use only in Montana, North Dakota, Minnesota, Michigan, Illinois, Ohio, Pennsylvania, West Virginia, New York, New Hampshire, Rhode Island, Connecticut, New Jersey, Delaware, Maryland, Florida, Louisiana, Arkansas, Oklahoma, New Mexico, and Arizona.2
Its availability as a product for human consumption necessitates the rigorous testing of cannabis and its derivatives. Regulatory issues involve potency testing, levels of biological contaminants and pesticides, and moisture and metal content.This article will focus on the determination of metal content in cannabis.
The cannabis plant concentrates metals by absorbing them from the soil in which it is grown. These metals are used up in the plant metabolic process and, if unused, may accumulate beyond acceptable toxicity levels.
“States in which cannabis is legal have recognized the need for the analysis of the metal content of cannabis and associated materials to control exposure to these metals during cannabis human consumption,” says Dan Davis, Elemental Product Manager, Shimadzu Scientific Instruments.
Several state regulations require the analysis and quantification of four key metals—arsenic, cadmium, lead, and mercury.3 In addition to testing for these metals, certain states also require the analysis of barium, chromium, selenium, and silver.
“Since these regulations vary from state to state as to the type of metals that need testing and the limits of metal concentrations, it remains a challenge to select the appropriate analytical technique to quantify the metal levels while conforming to the nonuniformity of these regulations,” says Davis.
Sample preparation
All analytical techniques require that the cannabis material is dissolved to release the metals. The simplest dissolution method is to digest the sample with strong acids such as nitric acid and/or hydrochloric acid and, in some cases, the addition of hydrogen peroxide to increase oxidation potential.4
Dissolution destroys the molecular constituents of the digested cannabis sample and the constituent elements remain in the solution. Following dissolution, the mixture is diluted with ultrapure water to form an acid matrix solution and the digested sample is tested for metals using ICP-MS or ICP-OES.5
Analytical methods
Atomic absorption spectroscopy (AAS)
In AAS, the absorption in intensity of a specific light source is measured following atomization of the sample and is compared against a calibration curve of reference standards.6 Flame AAS is easy to use, inexpensive, and can provide sufficient throughput for a limited number of elements, provided that changes to light sources and optical method parameters are implemented when identifying and quantifying the different metals. Although graphite furnace AAS requires changing the light sources, it delivers greater sensitivity for most elements than flame AAS. While the sensitivity is acceptable, the dynamic range can be limited and can require reanalysis and dilution in order to get the sample within the calibration range.6
“AAS is an attractive method due to its relatively low cost compared to other elemental techniques,” notes Davis. “The instruments themselves as well as cost of operations tend to be far less that those used in the ICP techniques.”
Despite the low cost and potential for low detection limits, AAS suffers from issues with sample throughput and the need to use a hollow cathode lamp for each element. The number of elements per sample is thus limited by the number of lamps that the instrument can hold. In addition, quantitation of each element requires its own analysis; certain elements are not very sensitive to AAS and their analysis may require the use of a hydride vapor generator to form volatile hydrides prior to sample injection.
“The need to use a hydride vapor generator is one of the drawbacks of AAS as an additional piece of laboratory equipment as well as reagents are called for, thus adding to the cost,” says Davis. “For labs which are testing only a few elements, the necessity to use a hydride vapor generator may not be an issue and these labs may benefit from the use of AA as their metals analytical technique.”
Inductively coupled plasma-optical emission spectroscopy (ICP-OES)
ICP-OES uses an argon plasma for atomization of the digested sample and excitation of the atomized elements. It allows the simultaneous analysis of more than 70 elements in approximately one minute per sample with a much greater linear dynamic range. ICP-OES instruments cost about 2–5 times more than AAS instruments,6 but the technique has a lower capital cost than ICP-MS and is somewhat easier to use.5
ICP-OES with an ultrasonic nebulizer (USN-ICP-OES) and a low-flow mini-torch was investigated as a lower-cost alternative to ICP-MS.7 The results of the analysis show that USN-ICP-OES is a suitable alternative to ICP-MS; the four key elements tested were within the target analysis and the system was capable of achieving good sensitivity and accuracy at the desired element concentration levels.
“When it comes to cannabis testing, ICP-OES offers a price point and performance intermediate to AAS and ICP-MS, “concludes Davis.
Inductively coupled plasma-mass spectrometry (ICP-MS)
ICP-MS uses an argon plasma for atomization of the digested sample and excitation of the atomized elements.6 It can be used for the quantitative analysis of the four key elements in cannabis samples following acid digestion. Although instrument costs are higher for ICP-MS than for ICP-OES, ICP-MS offers greater sensitivity and is more suitable for ultratrace-level analytes.7
Conclusion
AAS, ICP-MS, and ICP-OES are all suitable for trace-metal screening of medicinal and recreational cannabis. However, each technique has advantages and disadvantages. To summarize:
- AAS: Lowest operating costs but low sample throughput
- ICP-MS: Greatest analytical sensitivity but highest capital costs
- ICP-OES: Lower in cost than ICP-MS and higher sensitivity than AAS
For more information on ICP-OES and ICP-MS techniques that can be applicable to metal testing in cannabis, visit http://www.geicp.com.
References
- Thomas, R. Practical Guide to ICP-MS: A Tutorial for Beginners. Third edition. CRC Press: Boca Raton, FL, 2013; ISBN 9781466555433.
- https://thecannabisindustry.org/state-marijuana-policies-map/
- http://www.ncsl.org/research/health/state-medical-marijuana-laws.aspx
- http://www.cannabissciencetech.com/metals/metals-cannabis-and-related-substancesregulations-and-analytical-methodologies
- http://www.cannabissciencetech.com/metals/determination-multiple-metals-cannabis-samples-using-icp-ms-and-icp-oes
- https://www.cannabisindustryjournal.com/column/instrumentation-for-heavy-metals-analysis-in-cannabis/
- https://www.shimadzu.eu/sites/default/files/analysis_of_heavy_metals_in_cannabis_by_usn_icp_oes.pdf
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com.