Microwave digestion is an excellent tool that provides complete dissolution for the accurate and precise testing of heavy metals in cannabis and its products using plasma spectrochemical techniques, such as inductively coupled plasma–optical emission spectrometry (ICP-OES) and inductively coupled plasma–mass spectrometry (ICP-MS). However, there are many different options of commercially available microwave digestion systems, each with their own strengths and weaknesses. The optimum choice will often depend on the workload and sample diversity of the cannabis testing laboratory carrying out the analysis. This article describes the basic principles of the major types of microwave digestion technology and offers suggestions as to which might be the best approach based on sample matrix, digestion efficiency, sample throughput, productivity, and overall cost of analysis. The article is supplemented by real-world examples of how different microwave technologies are being utilized in cannabis testing laboratories for the digestion and measurement of heavy metals by ICP-MS.
With increased state regulations, cannabis growers are required to conduct trace metals testing to ensure a safe and high-quality product. This testing encompasses a wide variety of samples from growers and the processing industry including soils, fertilizers, plant material, edible products, concentrates, and topicals. Obtaining analytical data required to ensure quality products starts with the crucial step of preparing the sample for analysis. Reducing handling steps, eliminating outside contamination, and minimizing reagent blank contribution are all necessary for good sample preparation. It is well recognized that closed-vessel microwave digestion offers the best approach for getting your samples into solution for analysis by inductively coupled plasma–optical emission spectrometry (ICP-OES) or inductively coupled plasma–mass spectrometry (ICP-MS). However, there are basically two very different commercially available designs: rotor-based systems and single reaction chamber (SRC) technology (1). So, how do you go about selecting the optimum technology for your samples? What types of mineral acids will be best suited for your elements of interest, and what temperature and pressure will be required for the digestion process of your sample matrices? It’s only when you have a good understanding of these issues, that you can begin to look more closely at the pros and cons of the different commercially-available microwave technology. So first, let’s take a closer look at the fundamental principles of both designs.
Principles of Microwave Digestion Technology
First it should be emphasized that sample digestion of organic matrices such as cannabis products should be carried out using reagents compatible with ICP-OES and ICP-MS instrumentation. For example, the chemical or physical properties and concentration of the mineral acids used and how they affect the sample introduction nebulization processes and the potential matrix-suppression effects in the plasma should be taken into consideration. It is therefore well-recognized that the most plasma spectrochemical-friendly reagents are typically strong oxidizing agents such as nitric acid (HNO3) and hydrogen peroxide (H2O2), which are extremely efficient, but tend to generate large amounts of carbon dioxide (CO2) and various oxides of nitrogen (NOx) when they react with the samples. The microwave system and its components will therefore not only need to accommodate the high temperature required to digest all the different organic sample types, but also be able to handle the subsequent increase in pressure produced by the generation of large volumes of these gases. For some samples, the addition of small amounts of hydrochloric acid will also help to stabilize some elements particularly mercury (Hg) and the platinum group elements. However, it should be noted that if ICP-MS is being used as the analytical technique the 40Ar35Cl polyatomic species could potentially interfere with monoisotopic arsenic (As) at 75 atomic mass units (amu). This interference can be alleviated using a collision–reaction cell (CRC), but it is important to be aware of this potential problem so that the optimum instrumental conditions are used.
With rotor-based technology, microwaves are directed onto vessels containing the sample and the digestion reagents, which are placed in a rotating carousel. The digestion process is accomplished by raising the pressure and temperature through microwave irradiation, as the carousel is rotating. This increase in temperature and pressure, together with the optimum reagent, increases both the speed of thermal decomposition of the sample and the solubility of metals in solution. Rotor-based systems work extremely well for similar matrices by batching all samples together that react in the same way. By carrying out the digestion process using the same microwave power, temperature, and pressure conditions, it will ensure similar digestion quality in all positions. To increase throughput, different sized carousels can be used depending on the sample workload. However, when many different sample matrices have to be digested, productivity could be sacrificed, because each sample type has to be batched together, which unfortunately precludes different samples being digested together in the same sample run.
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- W. Lautenschlager, Milestone Srl, U.S. patent 5,270,010, 1990-06-13.
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- The National Institute of Standards and Technology (NIST) Certificate of Analysis for SRM 2711a (Montana Soil): https://www-s.nist.gov/srmors/view_cert.cfm?srm=2711A.
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How to Cite This Article
R. Boyle and E. Farrell, Cannabis Science and Technology 1(3), 30-37 (2018).