Microwave digestion is the preparation method of choice for analyzing metals in complex mixtures. Food and environmental industries favor this technique, which is also applicable to testing materials, soil, agricultural waste products, engine oils, and biological samples. Pharmaceutical applications are growing too, as safety-conscious regulators demand trace metal analysis.
Microwaves greatly accelerate the digestion of solid analytes in acid, creating a transparent or clear solution from which all metals are liberated from their matrix. Nitric acid is the digestion medium of choice for organics, but hydrochloric, hydrofluoric, and boric acids may be used in specialized applications. Older methods, which use hot plates or oil baths as a heat source, suffer from slowness, lack of uniformity, and poor control over digestion parameters.
Recently, several vendors have introduced microwave systems that permit loading of multiple samples during each run to improve throughput, as well as to control features.
Today, laboratory microwave systems are used mostly for sample preparation— the primary focus of this article. However, microwave-assisted chemical synthesis is an emerging technique that provides many advantages over conventional heating mantles, oil baths, and hot plates.
Microwaves facilitate the synthesis of organic compounds, organometallics, and even inorganics. During chemical reactions, microwaves excite molecules in ways that simple heating cannot. Recent literature suggests that microwaves enable “green” chemistry in water- based solvents versus organics.
Unlike external heating devices that operate through the sample vessel, microwaves pass through the container and couple directly with the sample matrix, applying energy exactly where it is needed. “And when you turn it off you don’t have a large mass to cool down, only the sample,” notes Mike Collins, director of strategic marketing at CEM (Matthews, NC).
Ultimate sample digestion
Microwave sample preparation occurs in high-pressure vessels that reach up to 300° C and 1500 psi. Despite the harsh conditions, equipment has been trending toward simplicity. “You don’t want to limit the technology to trained microwave experts,” says Mr. Collins.
Today’s sample holders, for example, use snap-on caps and work with liquid handlers that interface directly with autosamplers for inductively coupled plasma analysis.
“Microwave digestion saves a lot of time during sample prep for inorganic analysis,” says Joseph A. Caruso, Ph.D., professor of analytical chemistry at the University of Cincinnati (Cincinnati, OH). Dr. Caruso, who uses a CEM microwave system, notes that different conditions may be applied to each sample in a multisample unit, all under computer control. “This saves a lot of time, particularly when you’re running samples from multiple laboratories,” says Dr. Caruso. The CEM model he uses holds up to 96 sample tubes. “You can set the samples up at the end of a work day, set up the computer- controlled parameters for each tube, come in the next morning, and the digestion is complete.”
Dr. Caruso has experimented with digestion conditions, for example lower-concentration acid and shorter digestion times. Normally, nitric acid and high temperatures oxidize carbon-containing compounds down to carbon dioxide and salts. Milder conditions allow for partial digestion of complex organic matrices and subsequent extraction or chromatographic analysis.
A typical protocol, developed by CEM for analyzing the metal content of cheese, employs 70 percent nitric acid and a CEM MARS Xpress microwave reaction system with contactless infrared temperature control. The instrument allows control of time, temperature, pressure, and digestion power for multiple samples.
One-gram cheese samples and 10 mL of acid are combined in the sample vessel, ramped to 210°C over 20 minutes, maintained at that temperature for 15 additional minutes, and cooled over 15 minutes. All samples, spikes, and blanks are diluted to 50 mL. Analysis by atomic absorption, atomic fluorescence, or inductively coupled plasma showed excellent agreement for concentrations of magnesium, potassium, calcium, and sodium across replicates.
Breaking down tough coatings
The United States Pharmacopeia (USP) recently promulgated new limits for concentrations of toxic metals (e.g., cadmium) in pharmaceutical products. USP method <233> provides a sample preparation decision tree for metal analysis, but fails to address confounding factors such as analyte losses resulting from metal ion complexation to organic ligands, incomplete digestions leading to high residual carbon contents, and high blank values.
Anton Paar’s microwave-induced oxygen combustion (MIC), a variation on the microwave-assisted digestion theme, aims to fully liberate metallic analytes by combining sample combustion and digestion in one operation. MIC dissolves even tough polymer coatings used in extended-release pills, which the company claims are not fully digested with conventional microwave technology.
“The effectiveness of MIC digestion is clearly illustrated with modern drug products, which are increasingly complex,” says Reynhardt Klopper, national sales manager for microwave products at Anton Paar (Ashland, VA). “Enterically coated drugs are protected by synthetic polymers or biopolymers that are resistant to hydrolysis and oxidation.”
Conventional microwave-assisted acid digestion fails to break down many of these coatings. As a result, some proportion of toxic metal contaminants remains sequestered and is unavailable to ICP or other analysis methods.
MIC uses a closed quartz digestion vessel, a pressurized, high-temperature reflux step, and nitric acid. According to Anton Paar, there is no need for complex matrixmatched calibration standards, as NISTcertified calibration solutions suffice.
Microwave-assisted sample preparation has come a long way since the 1980s, when systems resembled kitchen appliances more than scientific instruments. Microwave’s ability to dissolve almost any matrix, leaving target metal species behind, provides sample prep capabilities unavailable through other methods. “You can put some crazy materials in there, even metals, and after the microwaves and acid are done, you wind up with a clear liquid,” says Mr. Collins. “It’s pretty amazing what those conditions can do to a sample.”
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at firstname.lastname@example.org.