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A chemist often needs a way to figure out the components of a sample. For analyzing materials that include metals, scientists often use microwave digestion. According to “Sample Preparation III: Control of pressure and temperature” from Berghof Products + Instruments (Eningen, Germany): “Today, microwave digestion is a standard procedure of sample preparation for element determination in analytical chemistry.”
In this method, a sample goes in an acid mixture, all of which gets heated to more than 200 °C. This digests the sample, turning it all into a liquid. That material can then be analyzed with various techniques, including atomic absorption and inductively coupled plasma atomic emission spectroscopy. The sample can also be turned into a solution with hotplate digestion, but that takes hours. By comparison, microwave digestion takes minutes. The speed gained must be balanced with safe practices, because the microwave heating can create some dangers, if not managed properly and in the right device.
To explore a few top-line platforms, visit the Labcompare product page. As found in shopping for other scientific instruments, it never hurts to hear from an expert, and two provide insights here.
Increase the heat
First, let’s dig a little deeper into how scientists use microwave digestion. “Microwave digestion is a sample-preparation technique used to convert a sample into a directly measurable form,” says Darren Wilson, product specialist at Anton Paar USA (Ashland, VA). “By using concentrated acids and heating in a laboratory microwave, the matrices of organic as well as inorganic samples can be quickly dissolved and brought into solution.” That preparation is required to put the sample in a form that’s easier to analyze.
A team of scientists based in Brazil used microwave digestion to prepare samples of mollusks of four species for analysis with inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry.1 The researchers intended to analyze the mollusks to ensure that they could be safely consumed. The results revealed unacceptable levels of arsenic and chromium in every species. This example shows some of the breadth of this technology.
To help scientists get even better results, Anton Paar developed the Multiwave 7000. Wilson says that this platform “represents the latest advance in microwave digestion with the concept of the microwave-heated Pressurized Digestion Cavity.”
The Multiwave 7000 can heat a sample to 300 °C. It can also go up to 199 bar in pressure. Consequently, “even difficult samples are easily digested,” Wilson notes. “With this technique, there are no minimum sample volumes, no need for sample clustering, disposable vials can be used to avoid vessel cleaning, and handling is greatly improved due to lightweight parts and simple closure.” On top of that, he notes, “A handful of built-in methods are sufficient to run a wide range of samples, practically eliminating the need for method development.”
Keep the control
Many scientists use microwave digestion in situations that require precise control. That’s just what scientists get with the Speedwave XPERT microwave digester from Berghof Products + Instruments. When asked about the key benefits of this system, Michaela Buffler, product manager at Berghof Products + Instruments, lists quite a few.
First, she points out the “extremely low running costs, due to the long lifetime of components and vessels.” She adds, “Therefore, they are not considered as consumables.”
The Speedwave XPERT microwave digester can be controlled in various ways, including with a tablet or other mobile device. (Image courtesy of Berghof Products + Instruments.)In addition, Buffler notes the high safety standards, such as making this platform a top-loader with an electronic lock. She adds that there’s “automatic regulation and abortion of processes at exothermic reactions, thanks to contactless pressure control in each vessel.”
To get the right results, scientists need a digester that’s reliably consistent. Buffler points out that microwave digesters from Berghof provide reproducibility. She attributes this to several features: “permanent, direct, and contactless temperature and pressure control; homogenous distribution of microwave radiation due to the circular design of the oven chamber; no cross-contamination thanks to minimal surface porosity of TFM-PTFE vessels.”
All of this can only be realized if a scientist can easily use the device. Buffler promises that Berghof’s microwave digesters provide “great operating comfort.” The platforms, she says, come with “full control of the instrument and monitoring of processes via touch controller, tablet, or other mobile devices.”
For regulated environments, customers can purchase additional software for this platform that is 21 CFR Part 11 compliant. With this upgrade, Buffler notes several benefits, including user management, password protection, an audit trail, and more.
More to explore
Other manufacturers also make microwave digesters. For example, Italy-based Milestone makes a variety of platforms. Its ETHOS UP fully handles up to 70 liters of sample. The company also notes that the “easyTEMP contactless sensor directly controls the temperature of all samples and solutions, providing accurate temperature feedback to ensure complete digestion in all vessels with superior safety.”
Milestone’s UltraWAVE also makes analysis easy and safe. “Loading a rack of samples into the chamber is the only manual operation in an otherwise fully automated process of microwave-assisted sample preparation,” the documentation notes. Plus, the rack can be configured for 4, 5, 8, 15, 19, 22, and 26 samples. The vials can also be made of TFM, quartz, or disposable glass. As Milestone states: “The fast assembly of the vials, the automatic closing and opening cut the handling time, reducing the labor cost up to 50%.”
Last, the UltraCLAVE’s reaction chamber can hold 40 20-milliliter disposable glass vials. This platform also offers other sample arrangements. Overall, Milestone notes, “Chemists can digest large batches of mixed sample types and even certified reference materials can be processed along with unknown samples for method validation.”
From Analytik Jena in Germany, scientists can select the TOPwave, which the company describes as providing “microwave-assisted pressure digestion” with various sample-holding vessels. The company adds that the “system works with an innovative sensor concept. All sample temperatures as well as the internal pressures are measured and displayed individually and in real time.”
Sensors also figure prominently in MARS 6 microwave digestion systems from CEM Corp. (Matthews, NC). These systems incorporate integrated sensor technology, which not only recognizes the kind of vessel being used, but also the sample number. The machine uses this information to apply the right amount of power and facilitate high-quality digests, even for the most challenging samples. More information can be found here.
With so much to consider in microwave digester options, scientists need to do some careful shopping. Clearly, some platforms aim at more industrial uses than others, but a wide range of scientific experimentation could make use of microwave digestion in processing samples. A little bit of acid and a lot of heat really changes how samples can be analyzed.
Reference
- Barbosa, I.D.S., et al. Multivariate data analysis of trace elements in bivalve molluscs: Characterization and food safety evaluation. Food Chemistry 2019 [Epub before print]; doi: 10.1016/j.foodchem.2018.02.063.
Mike May is a freelance writer and editor living in Texas. He can be reached at [email protected]