Ensuring High Data Integrity When Measuring Heavy Metals in Cannabis Consumer Products by ICP-MS: The Importance of Comprehensive Validation Procedures

Published on: 
Cannabis Science and Technology, November/December 2023, Volume 6, Issue 9
Pages: 10-18

Columns | <b>Navigating the Lab</b>

The big four heavy metals required by the majority of states is totally inadequate, but how many are actually worthy of consideration? Here, Rob Thomas takes a look at the other metals and how ICP-MS can be used.

The lack of federal oversight with regard to heavy metals in cannabis and hemp consumer products in the US has meant that it has been left to the individual states to regulate its use. Medical marijuana is legal in 38 states, while 22 states and the District of Columbia allow its use for adult recreational consumption (1). However, the cannabis plant is known to be a hyper-accumulator of heavy metals in the soil, so it is critical to monitor levels of elemental contaminants to ensure cannabis products are safe to use. Unfortunately, there are many inconsistencies with heavy metal limits in different states where cannabis is legal. Some states define four heavy metals while others specify up to eight. Some are based on limits directly in the cannabis, while others are based on consumption per day. Others take into consideration the body weight of the consumer, while some states do not even have heavy metal limits. Some states only require measuring heavy metals in the cannabis plant or flower, while others give different limits for the delivery method such as oral, inhalation, or topical (2). In other words, it’s a fractured and dysfunctional regulatory framework, which desperately needs changing.

Pharmaceutical Guidelines

Clearly, there is a need for more consistency across state lines, particularly as the industry inevitably moves in the direction of federal oversight. The cannabis industry can learn a great deal from the pharmaceutical industry, as it went through this process almost 25 years ago when it updated its 100-year-old semiquantitative sulfide colorimetric test for a small group of heavy metals to eventually arrive at a list of 24 elemental impurities in drug products using either inductively coupled plasma-optical emission spectroscopy (ICP-OES) or inductively coupled plasma-mass spectrometry (ICP-MS) spectrochemical techniques. This new initiative also included maximum permitted daily exposure (PDE) limits, using elemental toxicological data for the different drug delivery methods (oral, parenteral, inhalation, or transdermal), based on well-established animal studies (3).

These PDE limits were defined under two different directives: United States Pharmacopeia (USP) Chapter <232> (4)and International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) Q3D guidelines for elemental impurities (5). All 24 elemental impurities with their toxicological classification are listed in Table I.

Note: The ICH, known by its full name of the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use, is an international consortium of pharmaceutical regulatory authorities around the world with a focus on scientific and technical aspects of pharmaceuticals development.

In addition, the analytical methodology to measure these elemental impurities was outlined in USP Chapter <233>, which described the analytical procedures, including the appropriate plasma spectrochemical technique (ICP-MS, ICP-OES, or equivalent), the sample digestion method, and a comprehensive set of validation protocols using well-established spike recovery procedures. These new directives meant that pharmaceutical manufacturers were required to not only understand the many potential sources of heavy metals in raw materials and active ingredients, but also to know how the manufacturing process contributed to the elemental impurities in the final drug products by implementing a risk assessment study. They did this by categorizing all 24 elemental impurities under four classes—1, 2A, 2B, and 3—which determine the toxicology priority of the elements and the likelihood of occurrence in the drug product. The different toxicity classes are described in Table II (5).

Adoption by the Cannabis Industry

These PDE limits became the basis for cannabis state regulators to set limits for heavy metals in their jurisdictions. However, for many different reasons, the majority of states did not require all 24 elemental impurities, mainly because they felt they were not representative of elemental contaminants found in the cannabis manufacturing process. There were many reasons for this, but the main justification was that cannabis consumer products were considered to be more similar to herbal medicines than pharmaceutical formulations. And as a result, they should be regulated as herbal products or dietary supplements, which historically had only required the big four heavy metals (Pb, Cd, As, and Hg) to meet compliance (6). Some states, such as Maryland, added Cr to the big four, while other states, such as Michigan, added Cr, Cu, and Ni, and New York also included Cu, Ni, Sb, Cr, and Zn. However, since these initial assessments, there has been a significant amount of compelling evidence in the public domain to suggest a wider panel of up to 20 elements might now be worthy of consideration (1).

Suitability of ICP-MS

There is still much debate of what toxic metals might be included in a future federally regulated panel for cannabis and hemp, but there is no question about what analytical methodology should be used to monitor that panel. It is well-accepted that ICP-MS is the optimum technique to use for measurement purposes. The very low limits of detection and high sample throughput with ICP-MS make it ideally suited for cannabis testing laboratories. However, the technique is not without its drawbacks. It requires a skilled analytical chemist to fully understand the method development process including isotopic selection, interference removal, matrix suppression effects, internal standard selection, and sample preparation contamination issues. In other words, it requires someone who is experienced in working in the ultra-trace element environment to ensure the highest data integrity (7).

This is why when USP developed Chapter <232> PDE elemental impurities they also accompanied it with Chapter <233>, a robust and well-proven analytical method to ensure high quality data (8). However, this seemed to be an area that escaped the scrutiny of most state regulators of cannabis and hemp consumer products. They were very strict about the actual limits, based on the method of administration (oral, inhalation, or topical), but they typically didn’t propose methodology to carry out the analysis. They left it to the discretion of the cannabis testing laboratories to develop their own analytical method including quality control and quality assurance procedures. Some of the laboratories with more experienced ICP-MS analysts who were familiar with trace element analysis were implementing these safeguards and generating high quality results, but many of the other laboratories did not put an emphasis on this type of expertise and as a result were often producing questionable results, particularly for elements that are notorious environmental contaminants or are prone to loss during the sample preparation stage (9).

So, let’s take a closer look at USP Chapter <233> to better understand why it’s so important to follow this methodology to ensure data integrity and offer suggestions as to how it could be adapted for use by cannabis testing laboratories.

USP General Chapter <233>

This chapter deals with the sample preparation, analytical procedure or instrumental technique, and validation protocols for measuring elemental impurities using one of two plasma based spectrochemical techniques: ICP-OES or ICP-MS, or any other alternative technique as long as it meets the data quality objectives of the method defined in the validation protocol section (9).

Note: It was originally developed for drug products and dietary supplements but has been somewhat modified by taking shortcuts for use with cannabis-related samples. So, let’s take a closer look at each of these areas.

Sample Preparation Procedures

The selection of the appropriate sample preparation procedure will be dependent on the material being analyzed and is the responsibility of the analyst. For example, it suggests that if the sample is in a liquid form, it could be analyzed neat if it is compatible with the measurement technique or could be diluted with water and acidified if the sample is hydrophilic in nature or diluted with an organic solvent if the sample is hydrophobic. However, it recommends that the most suitable technique for solid materials is closed vessel dissolution of which microwave digestion is the most common. It emphasizes that the actual procedure should be at the discretion of the analyst but gives a typical generic method which is described here.

  • Closed vessel digestion: The benefit of closed vessel digestion is that it minimizes the loss of volatile impurities. The choice of what concentrated mineral acid to use depends on the sample matrix and its impact of any potential interferences on the analytical technique being used. An example procedure that has been shown to have broad applicability is described below:
  • Weigh accurately 0.5 g of the dried sample in an appropriate flask and add 5 mL of the concentrated acid of choice. Allow the flask to sit loosely covered for 30 min in a fume hood then add an additional 10 mL of the acid, and digest using a closed vessel technique, until digestion is complete, but be sure to follow the manufacturer’s recommended procedures to ensure safe use. Make up to an appropriate volume and analyze using the technique of choice.

Note: A 9:1 mixture of concentrated nitric acid to hydrochloric acid is typical for most cannabis infused samples (10–12).


Detection Technique

Two analytical procedures are suggested in this chapter. Where elemental impurities are typically at the parts-per-million level in the diluted sample, ICP–OES is the recommended technique. Whereas, for elemental impurities at the parts-per-billion level or lower in the diluted sample, ICP–MS is the preferred technique. The chapter also describes criteria for an alternative procedure, such as atomic absorption (AA) as long as it meets the validation requirements laid-out in this chapter. Whichever technique is used, the analyst should verify that the procedure is appropriate for the instrument and samples being analyzed by meeting the Procedure Validation requirements described below.

Note: Even though ICP-OES might be suitable for a small number of consumer products, it is well accepted that ICP-MS will be required for the majority of plant or flower materials, edibles, topical, and inhalation products, as demonstrated by the publication of a recent ASTM standardized method (13).

Validation Protocols

All analytical procedures—including ICP-OES, ICP-MS, or an alternative procedure—must be validated and shown to be acceptable, in accordance with the validation protocol. The level of validation necessary depends on whether a limit test or a quantitative determination is specified in the individual monograph. The requirements for the validation of an elemental impurity’s procedure for each type of determination are described below. Any alternative procedure that has been validated and meets the acceptance criteria is considered suitable for use.

Note: Validation protocols are critically important for measuring heavy metals in cannabis-infused products because it’s very difficult to use one matrix as a quality control sample which is representative of all samples being analyzed.

Acceptability of Analytical Procedure

The following section defines the validation parameters for the acceptability of analytical procedure to monitor PDE limits. Meeting these requirements must be demonstrated experimentally using an appropriate system suitability procedure and reference material. The suitability of the method must be determined by conducting studies with the material under test supplemented or spiked with known concentrations of each target element of interest at the appropriate acceptance limit concentration.

Note: It should also be emphasized that the materials under test must be spiked before any sample preparation steps are performed.

Suitability of Method

To understand the suitability of the technique being used and whether its detection capability is appropriate for the analytical task, it’s important to know the PDE limit for each target element and, in particular, what the USP calls its J-value. In Chapter <233>, the J-value is defined as the PDE concentration of the element of interest, appropriately diluted to the working range of the instrument, after the sample preparation step is completed.

So, let’s take Pb as an example. The oral PDE limit for Pb defined in Chapter <232> is 5 µg/day. Based on a suggested dosage of 10 g of the drug product/day, that’s equivalent to 0.5 µg/g Pb. If 0.5 g of sample is digested or dissolved and made up to 50 mL, that’s a 100-fold dilution, which is equivalent to 5 µg/L. So, the J-value for Pb in this example is equal to 5 µg/L (ppb).

The method then suggests using a calibration made up of two standards and a matrix blank: Standard 1 = 0.5 J and Standard 2= 2.0 J. So, for Pb, that’s equivalent to 2.5 µg/L for Std 1 and 10 µg/L for Std 2.

The suitability of a technique is then determined by measuring the calibration drift by comparing results for Standard 1 before and after the analysis of all the sample solutions under test. This calibration drift should always be <20% for each target element.

It should also be pointed out that no specific instrumental parameters are suggested in this section, but only to analyze according to the manufacturer’s suggested conditions and to calculate and report results based on the original sample size. However, it does say that appropriate measures must be taken to correct for interferences, such as matrix-induced wavelength overlaps in ICP-OES and argon-based polyatomic interference with ICP-MS. For guidance, it references the use of General Chapter <730> on Plasma Spectrochemistry (14), which is a general method in the United States Pharmacopeia National Formulary (USP-NF) (15) describing both ICP-OES and ICP-MS techniques for the determination of elemental impurities in pharmaceutical materials.

The suitability of the technique and analytical procedure is then determined by running a set of validation protocols, which cover a variety of performance and quality tests, including:

  • Instrument detectability
  • Precision and repeatability
  • Specificity
  • Accuracy
  • Ruggedness
  • Limit of quantification
  • Linear range

It should also be noted that, where appropriate, certified reference materials (CRM) from a national metrology institute (NMI) or reference materials that are traceable to that CRM should be used. An example of an NMI in the United States is the National Institute of Standards and Technology (NIST) who are in the process of developing a hemp flower and a hemp oil CRM for up to 13 toxic metals (16). However, for many other cannabis and drug products it’s very difficult to find a generic matrix that is representative of all typical samples. For that reason, monitoring recoveries of spiked additions are the accepted way of compensating for the wide variabilities in sample matrices.

Instrumental Detectability

This section deals with instrumental detectability.

  • Prepare a Standard Solution of target elements at J and a matrix matched blank
  • Prepare an Unspiked Sample
  • Prepare a sample spiked at 1.0 J – Spiked Sample Solution 1
  • Prepare a sample spiked at 0.8 J - Spiked Sample Solution 2

The technique or procedure is considered acceptable when:

  • Spiked Sample Solution 1
    gives a signal intensity equal to or greater than the Standard Solution
  • Spiked Sample Solution 2 gives a signal intensity less than the Spiked Sample Solution 1
  • The signal for each Spiked Sample is not less than the Unspiked Sample

Precision and Repeatability

Prepare six separate test sample solutions and spike each one at a target concentration of 1.0 J. Acceptance criterion: RSD for the six individual samples should be <20%.


The procedure must be able to assess the impact of each target element in the presence of other components that may be present in the sample, including other target elements, matrix components, and other interfering species. It refers to USP-NF General Chapter <1225> Validation of Compendial Procedures for guidance (17).


This test is designed to assess the accuracy of the analytical method or procedure and, in particular, when samples may be above the normal calibration range.

  • Prepare standard solutions containing target elements at concentrations ranging from 0.5 J to 2.0 J using suitable calibration or reference materials.
  • Run calibration using calibration standards.
  • Prepare samples under test by spiking at concentrations from 0.5 J to 2.0 J before any sample preparation is carried out.

The technique or procedure is considered acceptable when the spike recovery of three replicates at each sample concentration should be 70–150%. If available, CRMs should also be used to confirm accuracy of the method.


The effect of random events on the analytical precision of the method shall be established by performing the “Repeatability” test on (1) different days, (2) with different instrumentation, or (3) with different analysts. Note: Only one of these three experiments are required to demonstrate ruggedness.

Acceptance criterion: RSD should be <25% for each element.

Limit of Quantification and Linear Range

The limit of quantification (LOQ) and linear range capability is demonstrated by meeting the Accuracy requirement described earlier.

It’s also worth mentioning that with pharmaceutical manufacturers, federal regulators can inspect them at any time and ask to see the data to confirm their products are compliant with the PDE limits defined in Chapter <232>. And if not, they must show evidence that they are following the validation protocols defined in Chapter <233>. The US Food and Drug Administration (FDA) even has the power to either fine that company or in extreme cases shut them down if there is flagrant abuse of the process. It is highly unlikely that the majority of cannabis testing laboratories are carrying out these exhaustive validation protocols, but even if they are, do state regulators have the time and the inclination to be holding them to the same high standards as pharmaceutical regulators? . . . I suspect not!

Final Thoughts

There’s no question that the cannabis industry is facing some serious challenges. Lab shopping has driven many testing laboratories out of business, and as result consumers are not sure whether certificates of analysis (COAs) can actually be believed (18). For that reason, they are turning to the lower cost illicit market, where regulations are lapse and testing is often non-existent. Nowhere is this more evident than in the testing of heavy metals. In a recent ASTM symposium on contaminants, a number of researchers reported finding up to 10 toxic metals in unused vape oils and 21 elemental contaminants in cannabis flower samples (19). There is also a mountain compelling evidence in the public domain that confirms this (20).

So clearly the big four heavy metals required by the majority of states is totally inadequate, but how many are actually worthy of consideration? ICP-MS does not care how many elements you throw at it. It can measure the periodic table in a few minutes, with very little penalty on the cost of analysis when measuring four, 20, or even 40 analytes per sample (21). So, it is clear that a federally regulated suite of heavy metals is most probably going to include a wider panel than just the big four. For that reason, comprehensive quality control procedures are going to be critically important to ensure data integrity. Pharmaceutical regulators realized this by writing a very detailed chapter on the ICP-MS analytical procedure, including exhaustive validation protocols. The cannabis testing community in conjunction with state regulators should start implementing the protocols defined in USP Chapter <233>, if they haven’t begun to already because this will be the standard by which they will be assessed when federal oversight of the industry eventually becomes a reality.


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  2. Pruyn, S.A., Wang, Q., Wu, C.G., and Taylor, C.L., Quality Standards in State Programs Permitting Cannabis for Medical Uses, Cannabis and Cannabinoid Research, March 28, 2022, (accessed Oct. 15, 2023),
  3. Thomas, R. and Destefano, A., Understanding Sources of Heavy Metals in Cannabis and Hemp: Benefits of a Risk Assessment Strategy, Analytical Cannabis, July 21, 2022 (accessed from website Oct. 10, 2023),
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About the Guest Columnist

Robert (Rob) Thomas is the principal scientist at Scientific Solutions, a consulting company that serves the educational needs of the trace ele­ment user community. He has worked in the field of atomic and mass spectroscopy for almost 50 years, including 24 years for a manufacturer of atomic spectroscopic instrumentation. Rob has written over 100 technical publications, including a 15-part tutorial series entitled, A Beginner’s Guide to ICP-MS. He is also the editor and frequent contributor of the "Atomic Perspectives" column in Spectroscopy magazine, as well as serving on the editorial advisory board of Analytical Cannabis. In addition, Rob has authored six textbooks on the fundamental principles and applications of ICP-MS. His most recent book is entitled A Practical Guide to ICP-MS and Other AS Techniques, which was published in September 2023. Rob has an advanced degree in analytical chemistry from the University of Wales, UK, and is also a Fellow of the Royal Society of Chemistry (FRSC) and a Chartered Chemist (CChem).

How to Cite This Article:

Thomas, R., Ensuring High Data Integrity When Measuring Heavy Metals in Cannabis Consumer Products by ICP-MS: The Importance of Comprehensive Validation Procedures, Cannabis Science and Technology20236(9), 10-18.