Evolution in the legislation regarding the production and consumption of cannabis and related products for medicinal and recreational uses has led to emerging regulations regarding the potency, terpene profiles, and impurity content of such products. Among the impurities, or contaminants, that require testing are metals. Because of their toxicity and carcinogenicity, most jurisdictions are, at a minimum, requiring testing for arsenic, lead, cadmium, and mercury. Metals accumulate in plant material through normal metabolic processes. Furthermore, some plants are capable of hyperaccumulating metals to concentrations well above those in the soils and waters that nourish them. Some such metals are beneficial or nutritional in nature, whereas others show varying levels of toxicity. As such, testing for metals, particularly ones that are toxic, are important for products destined for human consumption. Quantitation of metals within cannabis materials can be accomplished through a variety of analytical techniques. Such amenable technologies include atomic absorption (AA) spectroscopy, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS). As with all aspects of analytical instrumentation, each method has its advantages and disadvantages. Here, we present an overview of analytical methodologies, challenges facing the analyst, and notes on regulatory stipulations.
Inductively Coupled Plasma-Optical Emission Spectrometry
Inductively coupled plasma-optical emission spectrometry (ICP-OES) represents the “middle-of-the-road” for elemental analysis with a price-point and performance intermediate to AA and ICP-mass spectrometry (MS). ICP-OES operates under the principle that adding energy to an element will cause an electron to attain a higher energy level. When that electron falls back to its initial level, the element emits a quantum of light of known wavelength. The wavelengths of light correspond to an individual element whereas the intensity, or brightness, of the light corresponds to concentration. Compared to AA, ICP-OES offers several advantages when it comes to cannabis testing, though with increased performance comes some potential drawbacks.
First, it is a true, simultaneous, multielement technique. Characteristic wavelengths of emission lines for multiple elements are diffracted and detected simultaneously, precluding the need for running sequential analyses for each element. Additionally, with many instruments, an entire spectrum is collected at once, allowing the user to retroactively analyze a dataset for elements, even if they were not included in the initial method.
Secondly, detection limits are improved compared to flame AA, with sensitivity for most elements in the range of low parts per billion. Detection limits can be improved for most elements by approximately one order of magnitude through the use of an ultrasonic nebulizer to increase nebulization efficiency. However, even when using an ultrasonic nebulizer, some cannabis laboratories have found that after the samples have been prepared and diluted, the required sensitivity is pushing the limit of the instrument’s fundamental capabilities.
Lastly, ICP-OES is more amenable to complex or dirty matrices than ICP-MS, allowing for easier analysis of brines, oils, and samples with high levels of total dissolved solids. Despite similar sample introduction systems, ICP-OES benefits in this regard from not physically pulling ions into the instrument, like ICP-MS. In the latter technique, high salts and other complex matrices can build up in the system interface region, high concentrations of easily ionized elements (such as Na, K) contribute to the space-charge effect, and high concentrations of matrix elements can contribute to polyatomic and isobaric interferences.
In contrast to AA, ICP-OES systems are more expensive to purchase and operate. They require the use of argon gas at a rate of ≥10 L/min to maintain the plasma as well as specialized and consumable glassware for sample introduction and analysis, such as nebulizers, spray chambers, and torches. Some ICP-OES instruments maintain a flow of argon even when the instrument is in standby mode to purge the optical bench.
The emerging leader in metals analysis in cannabis products is ICP-MS because of its high sensitivity and rapid sample throughput resulting from near-simultaneous multielement data acquisition. ICP-MS benefits from using the highly energetic ICP as an ion source coupled with the mass specificity and rapid switching of a quadrupole for mass filtering.
An ICP-MS system uses an ICP source that is similar to that used in ICP-OES, but in this case, it is using the argon plasma to form cations rather than to generate light from atoms in the sample. The cations are then pulled into the mass analyzer portion of the instrument by differential vacuum, where they are mass-filtered using a quadrupole, allowing for rapid switching between masses transmitted to the detector and high mass specificity.
The high sensitivity of ICP-MS instrumentation, with detection limits often in the range of tens- to single-digit parts per trillion range, can be attributed to the efficiency with which an ICP generates cations for almost all elements on the periodic table. Such sensitivity allows for detection of trace elements in a variety of matrices, even ones that are very dilute. This high sensitivity enables analysts to reduce interfering compounds from the matrix through dilution and to analyze a variety of sample types.
As an analytical technique, ICP-MS has historically suffered from the stigma of being a complicated, esoteric technique. However, most modern instruments are coupled with software that provides for a comparable ease of use, with automated instrument tuning, method setup, and post-run analysis. Furthermore, many instruments also use collision or collision-reaction cell technology to reduce the effect of polyatomic interferences on analyte ions.
The two primary downsides to ICP-MS, like ICP-OES, are relatively high cost of ownership and susceptibility to interferences when analyzing complex matrices. Like an ICP-OES system, an ICP-MS system will consume argon at a rate of >10 L/min, as well as employing the use of consumable glassware and interface cones. Furthermore, in high-salt or otherwise complex matrices, ICP-MS can suffer from degradation of signal, interferences from matrix elements, or the space-charge effect, in which easily ionized and abundant elements defocus the ion beam of low-concentration analyte elements. However, because of the high sensitivity of ICP-MS, dilution of samples can help resolve matrix-related problems while still preserving analyte concentrations in levels that are quantifiable.
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How to Cite This Article
A.P. Fornadel, D.L. Davis, R.H. Clifford, and S.A. Kuzdzal, Cannabis Science and Technology 1(1), 36-41 (2018).