Microwave Digestion Details

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 Microwave Digestion Details

There’s more than one way—far more than one— to prepare a sample with this technology

In many analytical projects, scientists start by getting rid of what they don’t want—that is, the thing that a scientist wants to measure is surrounded by, engulfed in, or connected to something that is just in the way. One way to get at the important component is through microwave digestion.

This technology builds on a longstanding approach to preparing samples. As explained by Cezar Bizzi of the Universidade Federal de Santa Maria in Brazil and his colleagues: “Sample preparation has a long history when thinking about the conversion of solid samples to representative solutions using concentrated acid solutions and high temperature.”1 Microwave digestion is a more recent version of this technique.

In most cases, microwave digestion consists of putting a sample in a strong acid and applying microwaves. With the sample in a closed container, the microwaves subject the sample to increasing heat and pressure, which puts the item of interest in solution. Then, that component can be quantified with various detection methods.

As Michaela Buffler, product manager at Berghof Products + Instruments (Eningen, Germany), says, “Microwave digestion breaks down the sample matrix, leaving the elements of interest for spectroscopic analysis.”

An academic example

At the University of Alabama in Huntsville, the Reliability and Failure Analysis Laboratory includes a microwave digestion lab station. The scientists there use microwave digestion to prepare samples for analysis with atomic absorption spectroscopy, ion cyclotron resonance, and inductively coupled plasma-mass spectrometry (ICP-MS). This lab reports on some of the benefits of the technology by writing: “One of the reasons is the time saved with using a microwave technology compared to other heating block techniques. Using the microwave will decrease digestion time from hours to minutes.”

The University of Alabama lab uses microwave digestion to heat ionic liquid mixtures. “Having the ability to monitor the heat of the liquid using the temperature probe guarantees that the solution is at the proper temperature, and by using the microwave we have consistent heating throughout the solution as a whole,” the lab’s website notes.

One company’s pair of platforms

Berghof offers two microwave digestion platforms: the Speedwave Entry and the Speedwave Xpert.

According to the company’s website, the Speedwave Entry makes a good fit with a wide range of applications. In environmental applications, for example, this platform can be used in preparing sediment and wastewater samples. In agriculture, the device works with grains and nutrients. In biological studies, it works well with blood and tissue samples. That’s just a sampling of where the Speedwave Entry makes a good fit with workflows.

The Speedwave Xpert can prepare samples for more than 600 applications. (Image courtesy of Berghof Products + Instruments.)

Part of the ease of operation of a microwave digester depends on the parts, especially the digestion vessel. The Berghof website points out that its “digestion vessels consist of only three components and can be opened and closed manually without the need for special tools.”

For versatility, the platform can be equipped with rotors that include 10 or 24 pressure vessels, which hold 60 and 10 milliliters, respectively.

Safety features should also be considered, especially depending on the sample and reagents being used. Berghof reports that both Speedwave models have been optimized for improved safety. The website points out that “if the rupture disc breaks, the integrated gas collection system safely collects all nascent gases and discharges them without risk. Thanks to the constant evacuation of the oven chamber during operation, possible emissions are effectively removed.”

The Speedwave microwave digesters use a collection of sensors, including the one shown here. (Image courtesy of Berghof Products + Instruments.)

Berghof developed the Speedwave Xpert  for more difficult sample preparations in over 600 applications. For example, the company recommends this platform for preparing active ingredients, as well as raw and final materials in the cosmetics and pharmaceutical industries. In materials science, this platform works well with alloys, ceramics, metals, and more. In the energy industry, scientists can use this instrument with coal and oil samples.

Some of the unique features of this platform include what Berghof describes as patented sensor technology, adding that the “optical measurement method guarantees users full control over the reaction.” The company also points out that the “instantaneous temperature and pressure measurement directly in the vessels allows spontaneous reactions to be intercepted, thus ensuring a seamless reaction.”

With this platform, like the Speedwave Entry, the Xpert can use different vessel options. One setup includes 24 pressure vessels that each hold 40 milliliters, and another carries 12 that each hold 60 milliliters. One container setup uses eight pressure vessels that can hold up to 100 milliliters, and Berghof recommends this setup for “difficult to digest, inorganic samples such as oxides, metals, and alloys.” A 12-pressure-vessel setup that accommodates 100 milliliters per vessel is best for “food and environmental samples such as soil, sludge, and sewage.”

Adding automation

CEM (Matthews, NC) makes the Discover SP-D, which the company describes as “the world’s only automated sequential microwave acid digestion system.” The output from this platform is a clear fluid that is suitable for analysis with atomic absorption or mass spectrometry, as well as ICP-MS. As the company notes: “Since each vessel can have its own method, you can mix and match samples and acids within the automation rack, providing your laboratory great flexibility.”

In addition to a variety of other interesting features, this microwave digester can also be automated. “To increase the throughput of a Discover SP-D, the Explorer Autosampler is available,” the CEM website notes. “This is an advanced robotic arm that moves your samples from a rack, to the microwave cavity for digestion, then back to the rack.”

To get the most throughput, though, the process also needs to be fast. According to CEM, its “microwave cavity produces a highly defined and extremely homogeneous microwave field. This cavity design enables the system to process samples faster and more reproducibly than batch-style systems.”

Overall considerations

A scientist in search of a microwave digester can explore many options. Beyond the few mentioned here, many more exist. As an example, Aurora (Vancouver, Canada) offers the Transformer series. The company points out that the platforms in this series come “with a toploading, pressure-resistant, heavy-duty oven chamber, safeguarding the operator under the most strenuous of circumstances.” Plus, scientists can select from different setups.

Other companies offer platforms with their own take on the vessel. As an example, Milestone (Shelton, CT) claims that its “patented UltraWAVE Single Reaction Chamber (SRC) technology transcends traditional closed- and open-vessel digestion, offering significantly greater digestion capabilities for even the most difficult sample types.” The key to this system “is a Teflon-lined 1-liter stainless steel reaction chamber, which serves both as a microwave cavity and a reaction vessel.”

To find the best microwave digester platform for a specific lab, it depends on the number of samples that need to be analyzed, the volume, and the required temperature. The specific applications also come into play. This is an area where thinking ahead makes a big difference.

Mike May is a freelance writer and editor living in Texas. He can be reached at [email protected]