Microwave digestion is an extremely useful sample preparation method in trace metal analysis. The process uses microwaves to heat up a sample that is combined with concentrated acid, breaking down solids into solutions for analysis.
While hot block digestion has been widely used for this purpose, it comes with disadvantages such as airborne contamination and poor volatile compound recovery. Microwave digestion offers advantages over this method, but it must be carried out correctly. When devising a microwave digestion method, there are several factors to consider, both from a practical and safety standpoint.
Microwave digestion sample preparation and its importance
How exactly does microwave digestion work? This method involves first loading a measured amount of sample material into a vessel and adding a concentrated acid. Acids typically used include hydrochloric acid, sulfuric acid, nitric acid, and hydrogen peroxide. As Daniel Hudgins, lead scientist at Anton Paar, explains, “the vessel is either sealed directly or placed in a larger pressure vessel depending on the design of the microwave.” Then, microwave energy is applied, which results in rapid heating of the sample and concentrated acid.
“Because the vessel is sealed, it is capable of reaching very high pressures, which allow for access to temperatures in excess of the standard boiling point of the acids,” says Hudgins. “This elevated temperature helps to increase the oxidative potential of the acid and tends to accelerate the decomposition of the sample materials.” A typical microwave digestion process can be completed in just 20-40 minutes. The closed vessel also helps ensure that no volatiles can escape during the decomposition process.
“When devising a microwave digestion method, there are several factors to consider, both from a practical and safety standpoint.”
Once sufficient time has elapsed for complete digestion, the next steps are to remove the microwave power, cool and decant the sample, and complete trace analysis with a suitable technique such as ICP-OES or ICP-MS. This sample preparation method can be used in a range of application areas including environmental, food, pharmaceutical, agricultural, and chemical.
According to Hudgins, microwave digestion offers several primary advantages over other methods, such as open vessel hot block digestion. Higher temperatures within the vessel allow for more complete digestions of more challenging samples and ultimately create a better analyte for continued analysis. What’s more, microwave heating is nearly instantaneous, rapidly accelerating the digestion process. Indeed, two major complaints with hot block digestion include long run times and poor digestion quality.
Other advantages include avoiding the loss of volatile elements, such as mercury and lead, and lowering risk of contamination from external sources. There are also safety advantages to using a closed vessel as the analyst is not exposed to corrosive acid fumes.
Requirements and considerations in microwave digestion
Microwave digestion requires specialized equipment and a well-planned process. As Hudgins notes, you need to consider the required temperature for digestion, sample size, and throughput.
One disadvantage of microwave digestion is the risk of cracking or explosion of the tubes housing the samples. Since you’re dealing with aggressive reagents, vessels must be chemically inert and able to withstand the increased pressure. Molded TFM™ PTFE is a popular choice for microwave digestion tube construction as it’s transparent to microwaves and resistant to chemical vapors. Plus, it offers high mechanical stability and is easy to clean.
Even with the right equipment, you need to find out if there are any acids that should be avoided. You’ll also need to consider which final analysis technology you’ll be using as this will influence the types of acid you can use.
Of course, when dealing with concentrated acid at high temperatures and pressures, proper safety precautions are vital. The rapid heating can cause spontaneous exothermic reactions resulting in an additional increase in temperature and pressure. Hudgins explains that there are several areas to consider in terms of safety, mainly concerning the microwave digestion device and its capabilities and features.
“Microwave digestion requires specialized equipment and a well-planned process.”
It’s crucial to be aware of the maximum operating pressure and maximum design pressure of the device, and whether or not it has mechanical interlocks that protect the operator from a vessel rupture. Hudgins notes that you also want to determine what form of temperature control is used to prevent overheating and how accurate it is. Temperature and pressure should be monitored throughout the process, typically achieved using built-in sensors.
External safety factors should be considered, too. Aside from wearing appropriate personal protective equipment, Hudgins says that operators should ensure that the device is well shielded to prevent microwave leakage and that the unit is properly vented to remove hazardous vapors from the work areas. Of course, manufacturers’ instructions should be followed closely. For example, adhering to the recommended filling volume and using the correct procedure for loading and unloading samples are best practices.
Microwave digestion has proven to be a highly valuable sample preparation method in trace metal analysis. It offers rapid and complete digestion and avoids issues such as contamination and volatile loss. The method isn’t foolproof, however, and it’s important to take into consideration the factors discussed above to ensure the process is efficient and safe.