The Benefits of ICP-MS for the Determination of Toxic and Nutritional Elements in the Cannabis Family of Flowering Plants

February 26, 2018
Table I: Microwave digestion sample preparation program
Click to enlarge, Table I: Microwave digestion sample preparation program
Abstract / Synopsis: 

This study focuses on an inductively coupled plasma–mass spectrometry (ICP-MS) sample preparation procedure and analytical methodology optimized for both toxic and nutritional elements in dried hops, a surrogate for the cannabis family of flowering plants. It shows that the wide dynamic range of the technique allows it to be used for the simultaneous determination of parts-per-billion levels of heavy metals including Pb, As, Cd, and Hg, together with high parts-per-million levels of nutritional elements, such as P, Ca, K, and Mg.

More than half of the states in the United States now allow for some form of legal use of cannabis in its various forms, including marijuana products and hemp. Its use as a medicine has been allowed in Colorado, Washington, California, Maine, Massachusetts, and Nevada for a number of years, and even traditionally conservative states like Arkansas, Florida, Montana, and North Dakota have recently enacted policy, enabling it be used for medical purposes. As a result of the widespread use of cannabis, there is not only a growing need for high-quality analytical testing of its potency, but also to assess its purity and level of contaminants (1).

One of the main areas of concern is the level of heavy metals and other elemental impurities in the cannabis plant. Metals such as arsenic, lead, cadmium, mercury, nickel, copper chromium, and aluminum can all be present in the cannabis plant because of uptake from the soil, from the use of fertilizers or hydroponic media used to enhance the growth of the plant, or possibly due to environmental pollution from industrial fallout. In addition, cannabis plants are also very efficient at extracting and accumulating metals from their environment and as a result have historically been used to clean up contaminants from toxic waste sites (2).

Cannabis growers with laboratory capabilities, who are authorized to analyze cannabinoids, can easily gain access to samples for testing purposes. However, other researchers who want to carry out scientific studies on the level of contaminants in cannabis cannot legally obtain these types of samples. Fortunately, cannabis, and humulus (hops used to make beer) belong to the same small family of flowering plants called cannabaceae. For that reason, hops are a generally accepted surrogate for cannabis because of its similar chemical and physical properties (3).

Therefore, this study focuses on a sample preparation procedure and analytical methodology optimized for both toxic elements at low parts-per-billion (ppb) levels and high concentrations of macro elements at parts-per-million (ppm) levels in dried hop samples by inductively coupled plasma–mass spectrometry (ICP-MS).

Discussion

The working range of ICP-MS makes it ideally suited for the analysis of plant materials. The ultratrace detection limits of ICP-MS permit the determination of low-level contaminants such as Pb, As, Se, and Hg, while the macro-level nutritional elements such as Ca, Mg, K, and Na can be quantified using the extended dynamic range capability of the technique, which provides up to 10–11 orders of magnitude. However, there are still a number of challenges to overcome, which makes the routine analysis of plant matrices difficult unless the sample dissolution procedure is well thought out and instrumental conditions are optimized for complex sample matrices. For example, plant matrices contain high levels of organic components which when digested can pose major problems for any ICP-MS because of the potential for blocking of the interface cones or deposition on the ion optics. For this reason, if the instrument design is not optimized for high-matrix samples, long-term stability can be severely compromised.

In addition to signal drift, the sample’s organic components, together with macro minerals, can combine with elements present in the digestion acid or the plasma argon to form polyatomic interferences. For example, chloride ions (at mass 35) combine with the major argon isotope (mass 40) to produce the argon chloride interference 40Ar35Cl+, which interferes with arsenic at mass 75. Another example is the argon dimer (40Ar40Ar+), which forms from the plasma gas and exists at the same masses as the major selenium isotopes. In addition, the major isotope of chromium at mass 52 is overlapped by 40Ar12C+, 35Cl17O+, and 35Cl16OH+ interferences generated by the sample matrix and the plasma gas. As a result, these kinds of spectral interferences have made the determination of both trace and macro elements in plant digest samples extremely challenging, unless the system is fitted with collision-reaction cell capability, which can reduce the impact of these interferences.

Experimental

Sample Preparation
Hops were purchased from a commercial store and chopped into small pieces, both to homogenize the sample and expose more surface area for increased digestion efficiency. A microwave sample preparation system (Titan MPS, PerkinElmer Inc.) using standard 75 mL polytetrafluoroethylene (PTFE) vessels was used for digestion, following the program shown in Table I. (See upper right for Table I, click to enlarge.)

Each vessel contained 0.25 g of plant material, 5.0 mL of concentrated nitric acid, 5.0 mL water, and 3.0 mL of 30% hydrogen peroxide. After digestion, the samples were diluted to 50 mL with deionized water, along with the addition of gold (Au) equivalent to 200 μg/L Au in the final solution, to stabilize the mercury.

References: 
  1. M.A. Branca, Chem. Eng. News 94(36), http://cen.acs.org/cannabis-analysis-takes-off.html (2016).
  2. “Heavy Metals and Cannabis: What You Don’t See Can Hurt You,” United Patients Group, April 6, 2015, https://unitedpatientsgroup.com/blog/2015/04/06/heavy-metals-and-cannabis-what-you-dont-see-can-hurt-you.
  3. C. Borbone, J. Yu, and N. Zabe, “Cannabis, Hemp, and Hops: Sample Comparative Results with Qualitative Strip Test Analysis for the Determination of Total Aflatoxin,” presented at the Cannabis Extraction and Analytics Poster Session at the 107th AOCS Annual Meeting & Expo, Salt Lake City, Utah, 2016.
  4. R. Thomas, Spectroscopy 17(2), 42–48 (2002).
  5. C. Bosnak and E. Pruszkowski, “The Determination of Toxic, Essential, and Nutritional Elements in Food Matrices Using ICP-MS,” PerkinElmer Application Note, 2015, https://www.perkinelmer.com/pdfs/downloads/APP_FoodAnalysisbyNexIONICPMS.pdf.