Chemists of a certain age remember their first experience with microwave chemistry either nostalgically, whimsically, or in my case, with a certain sense of dread. Drying several grams of magnesium sulfate in the same microwave oven my fellow group members used to reheat coffee did not go over well. The chemical was not particularly toxic but its splatter took the better part of a morning to remove. Subsequent comments about food and beverages tasting “a bit like magnesium...” did not help intra-group relations.
Microwave digestion works by exciting nearby water molecules to tear sample materials apart. Adding strong acids, or even a base, speeds up sample homogenization. What results is a mixture of organic materials at various stages of decomposition, and highly solubilized metal ions with uniform oxidation states suitable for analysis by inductively coupled plasma, atomic absorption, or atomic emission spectroscopy.
Scientists prefer microwaves because competing methods all have serious drawbacks. For example, ashing—where samples are burnt until only ash remains—is prone to analyte loss due to incomplete combustion. Fusion decomposition, a high-temperature technique that uses salt fluxes to solubilize samples, is labor intensive and suffers from interferences from fusion agents.
Microwave digestion solubilizes a broad variety of samples relevant to many industries, including agriculture/foods, clinical/life sciences, environmental, geoscience and mining, metallurgy, pharmaceuticals/nutraceuticals, paints and coatings, plastics, and polymers. “The only materials that would not be appropriate for microwave digestion are those that oxidize violently when acids are added,” says Leanne Anderson, technical marketing manager at CEM (Matthews, NC). “For example, explosives, propellants, and perchlorates.”
Acid selection is probably the most important factor in microwave digestion, according to Anderson. “Digesting in a closed vessel allows heating the acid above its boiling point. This increase in temperature dramatically increases the oxidation potential of the acid, allowing the use of safer acids; for example, nitric acid instead of perchloric.”
Nitric acid is most commonly employed for organic samples, including plant and animal tissues, oils, polymers, and pharmaceuticals. Sulfuric acid may be required to break up aromatic hydrocarbons. Anderson recommends that organic samples be pre-digested up to 15 minutes. Pre-digestion involves adding acid to the sample but leaving the vessel uncapped, in the fume hood, so that if the sample is prone to produce a lot of of gas it can discharge this gas before the vessel is sealed.
Anderson notes, “Inorganic samples do not contain much carbon, so they do not usually produce high pressures in digestion, though they may require higher temperatures than organic samples.”
Power and time
When microwave digestion was first introduced, control over process parameters was limited to time and power. Anderson draws the analogy to microwaving a frozen burrito: “You can program the amount of time and the power levels but you don’t know if the center is cooked or still frozen until you cut into it.” This was a problem for early microwave reactors, too, which led to the development of temperature-controlled units that have become the standard for digestion today.
By utilizing either contactless or in situ temperature measurements, the digester applies the appropriate amount of power necessary to achieve the temperature set point, in the allotted amount of time. “This approach is much safer, and more efficient than simple power and time control,” Anderson adds. A temperature sensor provides feedback for power levels from the sample temperature, without applying too much power and risking damaging the microwave vessels, or applying too little power and not fully digesting the sample. Computer control monitors each reaction (in multi-sample digesters) and records relevant parameters at every stage of the digestion, thereby providing consistent output for every sample in every run.
CEM’s MARS 6 microwave digestion system incorporates hundreds of pre-programmed methods and built-in sensors that detect which vessel set is being used, and each vessel’s required power.
Achieving digestion consistency requires that samples be homogeneous and representative of the original sample. Anderson recommends reducing the sample’s particle size, before digestion, through milling or grinding to improve contact between acid and sample. “In some instances, samples require heating to promote mixing,” she adds.
Digestion vessel geometry and materials of construction affect the efficiency of microwave digestions. Sample cup geometry may affect both digestion efficiency and sample recovery. Reduced-volume vessels, for example, may be a good choice for expensive or scarce materials, or for workflows where users seek to minimize risks associated with hot, pressurized hazardous materials.
Vessels from Parr Instrument Company (Moline, IL) use a heat-stable microwave-transparent polymer for outer containment, which also serves as a thermal insulator for the PTFE (Teflon®) sample cup. “With heating developed internally within the cup, temperatures in the outer vessel seldom exceed 50 C,” says Dr. Henry Albert, technical director at the company. The PTFE sample cup is closed with a self-sealing PTFE O-ring, which eliminates the need to preload the cup or use any tools to secure a tight seal. This also eliminates the effects of differential thermal expansion during heating and cooling cycles, while providing a chemically inert, all-PTFE wetted system. “PTFE digestion vessels exhibit unique inertness to strong acids at high temperatures,” Albert says. “The material allows energy to flow directly to the sample while also serving as an insulator to restrict heat flow from the reaction zone.”
On the negative side, PTFE has a tendency to creep or flow under pressure or load, a tendency evident even at room temperature but accentuated at higher temperatures, especially above 150 °C. PTFE creep makes achieving tight seals difficult and may lead to deformation and shorter usable life for PTFE components. PTFE is also somewhat porous, which may result in vapor migration across the cover seal and through the wall of the liner itself. Parr minimizes these issues by fabricating components from virgin PTFE molded to an optimal pressure to minimize porosity. While the amount of solute lost during normal digestion is negligible, vapor migration into the walls of the PTFE cup will occur and is unavoidable.
Lab managers who are unfamiliar with microwave digestion enjoy a range of resources for achieving consistency. “The best place to look first is the application team of your microwave system vendor,” says Anderson. Most vendors offer training courses and will advise users on specific samples and likely safety issues.
Labs should consider the suitability of the microwave system to sample type, daily throughput and workflow, and application support. Manufacturers offer many different vessel types and geometries, and these must be matched to sample and throughput as well. Higher-throughput vessels, appropriate for many EPA methods, allow digestion of large batches at moderate temperatures and pressures. “However, difficult organic samples such as heavy oils and plastics, and many inorganic samples, require temperatures and or pressures that are not suitable for this vessel type,” says Anderson, “and therefore require a higher-performance vessel to digest samples completely. For this reason, reviewing the samples you intend to run in the microwave prior to purchase can save you a lot of money.”
Daily throughput will dictate whether a sequential microwave or batch microwave offers the best workflow option for a particular lab. “If you digest a lot of the same samples every day, you will achieve better throughput in a batch microwave that can digest 24 to 40 samples at a time,” Anderson says. “On the other hand, if your laboratory prepares a few samples of various sample types, then a sequential system may be the best choice as it allows you to run any combination of samples and acids in the 24-position autosampler.”
In sequential systems, samples enter the microwave cavity with the help of a robotic arm and every sample is precisely controlled. This gives laboratories tremendous flexibility to prepare a wider range of samples. Each sample is then digested, cooled, and placed back into the rack in about 10 minutes, or a full batch of 24 samples is digested in about four hours. Operation is fully walk-away.