Microwave-acid digestion is a common sample preparation step for atomic absorption, atomic emission, or inductively coupled plasma analysis of metals. Microwave digestion takes minutes, compared with hours for conventional hot plate digestion. Because it uses high temperature and strong acids—commonly nitric and hydrofluoric—microwave digestion mineralizes any matrix. For example, EPA method 3052, based on microwave, provides total metal analysis from soil, sediments, sludge, oils, plastics, and biological materials.
“It’s a method for samples when you need to know the metal content but can’t get the species into water or other solvents,” says Jason Keith, product manager at CEM (Matthews, NC). “Unlike hot plates, microwaves heat only the sample, not the sample container.”
CEM sells two systems, the newest being the MARS6, which incorporates the firm’s One Touch™ Technology, combining advanced software, sensors, touchscreen operation, and multiple sample capability
With microwave digestion products under PerkinElmer’s (Waltham, MA) “aftermarket” business unit, product manager Mike DiVito views the technique in context rather than as an isolated prep method. “We view microwave as a way to provide customers looking for inorganic sample prep with a more complete solution” that includes collecting and preparing samples, introducing them into instruments, and data management.
PerkinElmer is in the midst of a microwave product transition due in early spring 2013. The new instrument(s) design is in response to customers demanding greater ease of use.
“They increasingly see sample prep as a bottleneck,” DiVito explains. As instruments become faster and more sensitive, sample prep must follow suit. “Producing a great sample is challenging: garbage in, garbage out.” General instrumentation trends carry over into sample prep. Users want easy setup and operation and color touch screen controllers as simple to use as consumer electronics. At the same time operators want instruments and associated labware that last, with low consumables costs and sample versatility.
Let’s take a look
Anton Paar’s (Ashland, VA) Multiwave ECO is a budgetfriendly microwave system that features pressure-activated venting on the digestion vessels, which eliminates external temperature and pressure sensors. By processing samples of up to 2g, Multiwave ECO should interest pharmaceutical companies working with USP chapters 232/233 for metal impurities. “With many products, QC labs can use the entire tablet for analysis, which simplifies sample preparation,” notes Reynhardt Klopper, product specialist at Anton Paar. Multiwave ECO processes up to 16 samples simultaneously, with a cycle time of 30 minutes (including heating and cooling).
Multiwave PRO, Anton Paar’s flagship microwave system, offers what Klopper calls “the highest temperature and pressure capabilities in the industry.” Multiwave PRO is capable of digestion, leaching, oxygen combustion, solvent extraction, drying, evaporation, and UV digestion on a single platform. Other features include wireless data transfer from sensors and simultaneous pressure monitoring of all samples.
Pharmaceutical firms will be interested in the PRO’s microwave-induced oxygen combustion (MIC) application, Klopper says. “MIC offers efficiency for analysis of complex pharmaceuticals incorporating enteric polymer coatings that inherently resist conventional acid dissolution techniques.” MIC combines both sample combustion and acid digestion in a single, closed-vessel system to effectively liberate elements of interest (metals and nonmetals) from nearly all types of combustible materials (polymers, chemicals, pharmaceuticals, etc.).
“Efficient destruction of the sample matrix using MIC ensures complete analyte recoveries and works for a wide variety of sample matrices.” The MIC accessory is a low-cost upgrade for the Multiwave PRO.
For additional resources on microwave digestion, including useful articles and a list of manufacturers, visit www.labmanager.com/microwave
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