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 in 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 and to control features.
Today, laboratory microwave systems are used mostly for sample preparation, 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.
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’s 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).
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, making its benefits are more accessible.
“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 notes that different conditions may be applied to each sample in a multisample unit, all under computer control. 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 says.
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. 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.”
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 matrix-matched calibration standards, as NIST-certified calibration solutions suffice.
MAS 24 Autosampler
- For proprietary Monowave 300 microwave synthesis reactor
- Up to 24 vials of various sizes can be queued and processed in three different carousel types
- Features a fail-safe gripper driven by compressed air
- Any Monowave 300 already in use can be easily upgraded for automation with a simple upgrade kit
Microwave Peptide Synthesizer
- Boasts simple and intuitive software, an integrated computer control system and the largest touch screen interface available
- Allows user to drive reactions to as high as 300 C and up to 30 bar pressure
- Offers the option of the use of a fiber optic temperature measurement device for in-situ reaction monitoring
Microwave Digestion System
- Offers higher sample throughput and reduced overall costs with the use of disposable glass vials instead of traditional digestion vessels
- Digests up to 15 samples, of any matrix, simultaneously
- Features Single Reaction Chamber (SRC) technology, removing common limitations of sample prep
NOVAWAVE Microwave Digestion System
- Digestion tunnel system employs 12 dynamically operating micro cavities to process 12 samples simultaneously
- Features individual sample control and monitoring
- Lets users prepare up to 14 racks of 12 samples at a time
- Available in two models: stand-alone Model SA and fully automated Model FA