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 Volume 9, No. 3, 40-42, 45.
For sample prep, modern digestion systems give chemists control, safety, speed, and versatility. Analytical instruments have evolved over the past 20 years, becoming increasingly automated. Likewise, detection limits for analyses have continued to keep pace with the advancements in equipment and instrumentation. As a consequence, sample preparation methods, such as extractions or digestions, have become the limiting factor in analytical science. The requirements for clean-room analyses have also multiplied in recent years. Seeking ever lower detection limits, scientists have pushed the boundaries of trace elemental analysis. Attention to the sample preparation stage has become an important consideration in reducing contamination, because it can have a direct bearing on the detection limit for the analysis. Recent advances in microwave technology have met the challenge of providing faster, cleaner, safer, more reproducible, and more accurate sample preparation alternatives. The sophistication of such systems has increased markedly over the last 15 years, and the technology has gone a long way toward relieving problems caused by high sample workload in the laboratory. The first steps toward relieving the sample preparation bottleneck were taken when closed vessels capable of performing digestions at elevated pressures (and thus elevated temperatures) were introduced in the mid-1970s. The combination of pressurized digestion with the precise heating control possible with microwaves was an instant success, and a new field in analytical chemistry was born. However, microwave home appliances were not constructed to withstand the rigors of acid digestion in a commercial laboratory. Problems arose with corrosion from acid vapors, as well as incomplete digestions due to heterogeneous microwave fields. The lack of feedback control parameters also increased the risk of explosion in pressurized vessels. Over the next 15 to 20 years, microwave-based sample preparation equipment evolved into a complex system fully capable of handling the dissolution requirements of the modern analytical laboratory. The technology has evolved from the early adaptations of home microwaves to an integrated system with intelligent feedback control of reaction parameters such as pressure and temperature. The microwave itself is barely recognizable compared with its home appliance cousin. Features such as integrated infrared monitoring of all reaction vessels, multilayered door construction, “snap-in” sensor connections, corrosion-resistant coating of all metal surfaces, and advanced feedback power control have transformed the instrumentation into an easy-to-use automated system with a high sample throughput, capable of keeping up with the fastest analytical instrumentation. Closed-Vessel Technology Early development work with closed-vessel microwave digestion indicated that sample types composed mostly of inorganic material (such as metals, soils, and sediments) could be digested easily without generating large amounts of gaseous byproducts. The development of closed containers—allowing microwaves to pass through and heat the digestion mixture directly—meant that much higher temperatures could be achieved, resulting in faster dissolution than conventional atmospheric pressure techniques. However, even this increase in digestion temperature was insufficient for samples that contained a large organic or carbonaceous component such as oils and solvents. The evolution of gases such as carbon dioxide from the digestion quickly reached the maximum pressure limits of the container before sufficient temperature could be achieved to accomplish the dissolution. Certain digests were also noted to continue exothermically after the application of microwave power had ceased. This led to runaway conditions that quickly caused the actuation of pressure relief mechanisms incorporated into the vessels, but left the digest incomplete. To address these and other issues, improvements in vessel technology have appeared with even more frequency than improvements in instrument design. An excellent example of the way in which instrument and vessel development has improved the preparation of certain organic sample matrices can be seen in the case of mineral oil, which is analyzed for a variety of elements. Historically, the digestion of mineral oil has been a time-consuming, open container procedure. First, the sample was charred or carbonized with sulfuric acid. Then, the oxidation was completed with nitric acid and/or hydrogen peroxide. The volume was reduced to almost dryness to remove the sulfuric acid matrix, and then the remaining sample was re-dissolved in dilute nitric acid. This procedure requires hours of almost constant operator attention and runs the risk of overheating or burning the sample, causing a loss of analyte. Great care must be taken to avoid the risk of contamination. To fully oxidize a sample of mineral oil using nitric acid alone, a sample must be heated to between 190 °C and 210 °C. Microwave-based pressurized dissolution would seem to be ideally suited to such a sample type, but early microwave vessels did not have the pressure capabilities to achieve the required temperatures. Oil samples tend to begin the dissolution procedure at about 170 °C and evolve the majority of the gaseous byproducts within a 5–10 °C temperature window. This rapid gas evolution both limited the achievable temperature and could overpower the control mechanisms of the system. Though microwave digestion was much faster than open vessel techniques, it still required several sequential steps and a relatively high level of operator involvement. Sample size was often limited to approximately 0.25 g. Recent developments in microwave instrumentation and vessel technology have extended the capabilities of the system to higher pressures or temperatures than ever before. In particular, the incorporation of advanced composite materials has greatly increased vessel pressure capability. Framed vessels constructed of advanced materials are lighter and stronger, and they cool faster and assemble more quickly than earlier models. Advances in microwave power delivery mean that the instrument will adjust the power applied precisely to the number of vessels run together. These new vessels allow control of temperatures up to 300 °C and pressure up to 800 psi. Mineral oil dissolutions can now be performed in a one-stage process with nitric acid alone. A typical digestion would be as follows:
Cool-down is automatic, and there are no charring stages. Use of sulfuric acid is eliminated, and the quality of the digestion is also improved. Up to 14 samples can be run simultaneously, and the whole process only takes 30 min. High-Temperature Digestions As an example, the digestion of
Cool-down is once again automatic. High-temperature/low-pressure digestions such as this are now routinely conducted around the world using microwave-based sample preparation equipment. Fully Sealed or Self-Venting? Organic samples that evolve large volumes of gas can achieve higher digestion temperatures, and hence a more complete digestion, if the gas is relieved while the digestion proceeds. Venting the gas in this manner removes the pressure limitations of the vessel construction from becoming a factor in the maximum temperature that can be achieved. However, great care must be taken to ensure that volatile species or analytes are not lost during the venting process. Improvements in sealed vessel technology in terms of raising the maximum allowable pressure have, to a certain extent, lessened the applicability of vessels in the “autovent” style; however, in certain circumstances, they can be a useful tool to the analyst. Vessel choice continues to depend on the type of sample being digested and the results being sought. Recently, vessels have been developed that can be run in either mode: fully sealed to all pressures or in “autovent” style in which excess pressure is automatically relieved during the digestion. All that is required to switch between modes is a simple change of vessel cover. Chemists can now incorporate both designs into their laboratories without having to purchase two different vessels. Microwave Procedures Making Inroads As scientists strive for ever lower detection limits, sample preparation techniques must inevitably continue to improve as well. These evolving needs were taken into consideration during the development of CEM’s MARS 5 System. The system comprises a microwave heating unit and proprietary sample processing vessels. Emphasis was placed on easy and convenient handling of official methods, such as U.S. Environmental Protection Agency (EPA) digestion procedures, as well as the ability to process more samples than before. EPA began promulgating microwave digestion methods in 1992. Although not all standard EPA methods have a promulgated microwave equivalent, the agency has continued to add microwave methods to its list of accepted procedures. Among those are several listed in the accompanying figure, as well as a comparison of microwave digestion versus hot plate (conventional) digestion. Additionally, the system was designed for safe operation under a wide variety of conditions, while maintaining versatility for a growing range of applications.
Copyright © 2000 American Chemical Society.
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-alumina (Al2O3) can be a time-consuming process at high temperatures and can involve the use of sulfuric and phosphoric acid. Microwave-based methods are now available that allow up to 12 samples to be processed automatically in a one-step digestion at up to 300 °C. This procedure can significantly reduce contamination issues by providing a closed, stable environment for the digestion. The following is an example of a one-step procedure of the digestion of 
