Expanding the Limits of Closed- Vessel Microwave Sample Preparation

Analytical chemists have long known that closed-vessel microwave sample preparation techniques are the most reliable way to achieve high-quality digestions—i.e., clear, precipitate-free solutions with minimal dilution, no loss of volatile analytes, and no contamination. However, as with all things, there are limits to the technology’s capability

The limits of today’s conventional microwave digestion systems are due to the available reaction vessels (Figure 1). The vessels used in these systems are made from various microwave-transparent materials, such as engineered plastics, and are shaped in ways appropriate to the temperature, pressure, and volume requirements of particular applications. Due to their limited mechanical strength and temperature resistance, these vessels naturally impose limits on the parameters of a sample digestion procedure, including temperature, pressure, process duration, and sample size.

Figure 1 - Typical microwave reaction vessels.

Conventional microwave sample processing is a mature technology. Improvements in vessel performance, or special multiple-vessel configurations to maximize throughput, can produce only incremental gains. The state of the art has reached, if not a plateau, then an increasingly gentle slope.

Removing the limits

The UltraCLAVE (Figure 2) (Milestone Inc., Shelton, CT) is an entirely different approach to closed vessel microwave sample preparation and is based on high pressure autoclave design. At its heart is a single large reaction chamber. In operation, this sample-containing chamber is prepressurized with inert gas, and then heated by a microwave field of uniform intensity. The pressurized chamber in the system serves simultaneously as the microwave cavity and the reaction vessel.

Figure 2 - UltraCLAVE system.

The individual sample containers are not pressure vessels. This approach changes the rules of microwave sample preparation by removing or greatly extending the limits associated with conventional microwave vessels. Higher temperatures, higher pressures, larger sample masses, and greater batch quantities all become possible. Even the toughest sample matrices can be digested by sustained high temperatures and pressures along with the correct chemistry. Chemists can be assured of identical temperature and pressure conditions across any number of discrete samples.

Larger volume

The UltraCLAVE reactor offers a larger volume than other commercial single microwave reaction vessels. While it is not likely to be a common use of the instrument, it is possible to process a single, very large sample in a fluoropolymer liner fitted to the entire reaction chamber (Figure 3). The 3.5-L liner can safely accommodate up to 25 g of organic sample for acid digestions and many times that sample mass for solvent extraction applications.

Figure 3 - Fluoropolymer liner for UltraCLAVE reaction chamber.

Larger batch count

The individual sample containers can be much simpler in design than in conventional microwave vessel assemblies. As a result, a higher batch count of discrete samples can be accommodated (Figure 4), and each individual sample can be of a larger mass than is possible in a conventional sealed vessel. Another benefit is the vastly reduced need for consumable vessel components.

Figure 4 - Large vessel count for high throughput.

In normal operation, the combination of prepressure nitrogen and partial pressure of process vapors ensures that boiling never occurs in individual sample containers. Loose-fitting covers on each container prevent loss of sample or cross-contamination between adjacent samples.

Several standard racks for the system are designed to hold 6–77 individual sample containers made of glass, quartz, or TFM™ (Dyneon GmbH & Co. KG, Burgkirchen, Germany). Recognizing that many laboratories have integrated particular autosampler vials into their routine sample handling, the manufacturer can usually fabricate a custom rack to accommodate customers’ vials.

Reduced acid use

The simplified sample containers in the UltraCLAVE are not required to have minimum volumes of process liquids merely to support accurate process monitoring and control. As a result, the system requires less acid per sample than other microwave labstations. In fact, acid volumes can be reduced to minimum quantities required to satisfy stoichiometry and solubility. In addition to considerable long-term cost savings, this reduces the dilution factor of the finished solutions, and can improve detection limits during analysis.

The power of mixed batches

The improvement in laboratory efficiency provided by processing samples at higher temperatures and pressures is easy to understand. At a higher temperature, decomposition takes less time. However, in many laboratories, this will not be the most significant advantage of using the UltraCLAVE.

Traditional closed-vessel microwave processing proceeds by monitoring and controlling conditions in a single reference vessel. The assumption is made that these parameters can be duplicated in all other vessels that are processed simultaneously along with the reference vessel. In order for this to be true, the sample type, sample mass, the volume of acid, and the performance of all the other vessels must be identical to that in the reference vessel. In the UltraCLAVE, the traditional process monitoring and control strategy is not necessary, since no single vessel is selected as the reference. Instead, uniform temperature conditions are achieved by positioning all the individual sample vessels in a pool of microwave absorbing fluid, a microwave “load.” It is the effective temperature of this load that is monitored and controlled throughout the heating process in the system.

By eliminating the traditional process control strategy, chemists are no longer constrained to process batches of identical samples using identical sample chemistries. To the extent that all the samples in a batch will process satisfactorily under the same temperature conditions, many dissimilar samples and chemistries can be combined in a single run in the UltraCLAVE. For example, processing four different sample types—such as soil, cement, wood chips, and vegetation—would require four separate processing cycles in a traditional microwave labstation. For the reasons described above, it requires only one cycle in the UltraCLAVE. This transformation—of a sequential process into a parallel one—may prove to be the greatest source of increased efficiency in many laboratories.

The safety of a sealed, high-pressure environment is ideal for processing dangerous samples such as hazardous waste, toxic substances, or radioactive materials. Semiautomated operation and complete computer control allow the system to be adapted for remote operation within special enclosures.

Simplicity of operation

The system engineers have operated on the principle that great power need not translate to great complexity. The UltraCLAVE is designed for clarity of interaction and simplicity of use. Loading a rack of samples into the chamber is the only manual operation required and can be done in a single motion. After that, methods are initiated with push-button simplicity (Figure 5) and automation does the rest of the work, ensuring reproducible performance from run to run. The reaction chamber is sealed, pressurized, heated, cooled, vented, and opened all under computer control. As a result, the system reduces the need for labor compared to conventional microwave systems.

Figure 5 - LabTerminal. Reaction parameters are entered via an intuitive touch-screen controller.

Sequence of operation

First, a rack of individual reaction containers is secured to the reactor cover in a single motion. (In an automated laboratory environment, even this one manual step could be adapted to occur without operator involvement.) Then, under computer control, the reaction chamber lifts up to dock with the cover, creating a pressure seal that is secured in place by steel clamps. Once the chamber is sealed, a high-performance compressor pressurizes the system with nitrogen. A microwave program, controlling temperature, pressure, and/or microwave power versus time, is then selected and initiated via the control terminal (see Figure 6).

Figure 6 - UltraCLAVE operating sequence.

During the run, integrated sensors continuously monitor and display the internal pressure and sample temperature. The UltraCLAVE software dynamically adjusts the applied microwave power in real time to precisely follow the defined temperature profile.

At the completion of the heating process, the reactor and its contents are cooled by an integrated chilling unit, causing most process vapors to condense in the reactor chamber. Once the reactor is cooled, the pressure inside the reactor is released, and the remaining process vapors are safely exhausted. Only after ambient pressure is reached will the sealing clamps release. The reactor body is then lowered, and the finished samples can be removed.

Design details and specifications

The high-pressure steel reactor has an overall volume of 4.2 L, and an effective working volume of 3.5 L with the PTFE liner installed. This is the largest-volume single reaction vessel available to microwave chemists.

Microwave energy from the system’s magnetron is introduced into the reactor through a patented microwave port. The internal geometry of the reactor is optimized for direct microwave coupling to materials in the reactor, with zero reflectance of energy back to the magnetron. This focused-multimode cavity design ensures maximum sample heating efficiency. Heat generated in the reactor is removed by a recirculating coolant system, and the stainless steel vessel remains at approximately room temperature throughout the entire heating cycle.

Operating pressures up to 200 bar (2940 psi) and temperatures up to 280 °C can be routinely achieved and sustained in complete safety. Such performance can resolve outstanding limitations in current methodology while continuing to provide all the advantages of existing microwave techniques.

Applications

  1. Materials testing. When testing materials for RoHS Directive compliance, the UltraCLAVE can accommodate the entire range of sample types subject to the regulation, due to its flexible temperature and pressure capabilities. Its large-volume reactor and choice of sample holders make it suitable for digesting components, finished products, or raw materials. Its ability to digest mixed batches allows a variety of components to be processed at the same time.
  2. Polymers. A fundamental part of polymer QA/QC is the need to digest or extract large sample masses, since the polymer analyst typically wants to process as few samples as possible while still obtaining a representative analysis. Larger samples inevitably mean that higher pressures are generated in a closed vessel. Traditional closed-vessel techniques can successfully digest up to 1 g of polymer sample in a highpressure vessel; the most efficient high-pressure rotors can accommodate up to 10 such samples per processing run. The UltraCLAVE can digest 25–30 g, distributed among multiple vessels. As in a benchtop microwave, magnetic stirring can be used to prevent sample clumping and promote complete, trouble-free digestions.
  3. Homeland security. The sooner analyses can be completed in homeland security applications, the sooner critical decisions can be made. With the Ultra CLAVE, varied materials such as environmental samples, tissues, suspect materials, and residues can all be processed in a single run, and the speed of the instrument used for analysis will become the limiting factor, rather than the speed of sample preparation.
  4. Remediation. On a cleanup site, time in the field is money. But it is not enough to increase the turnaround time for just one type of sample. Soils, sand, cement, organics, and a host of other materials all come into the laboratory at once to be prepared for analysis. The Ultra-CLAVE can handle these samples simultaneously—in batches of up to 77 mixed samples, with minimal dilution, and rapidly.
  5. Clinical. In the cost-competitive world of clinical analysis, every penny matters. The push-button simplicity and reproducibility of microwave methods, the reduced usage and disposal of acid, and the freedom from operator exposure have always been attractive. The UltraCLAVE provides these advantages, and can simultaneously digest multiple sample types (blood, urine, feces, hair), reduce acid usage even further, and process more samples in less time, reducing a laboratory’s daily operating costs.
  6. Environmental. In a busy environmental laboratory, sample preparation is always the bottleneck that determines how much analytical work can be done. The UltraCLAVE can handle large batches of samples in a variety of types—soils, vegetation, drinking water, wipes, and swipes—with faster turnaround, with no loss of volatiles and no vapor pollution of the laboratory or environment.

Example

Goessler and Majeron1 have reported their work processing environmental and clinical Standard Reference Materials (SRMs), demonstrating accuracy, precision, and uniformity of results with large batch processing (35 or more individual samples) in the UltraCLAVE. Samples were prepared with 250 mg sample mass and 5 mL nitric acid in each sample vessel. Figure 7 shows the graphical display of the temperature–pressure–power versus time profiles. Samples were brought to the required target temperature of 250 °C and digestion was completed in less than 1 hr heating time.

Figure 7 - Graphical display of the temperature–pressure–power versus time profiles.

Figure 8 shows that the recoveries of the elements of interest are in excellent agreement with the expected values from a National Research Council of Canada (NRCC) SRM: Dorm-2. Of particular note is the quality of the data for As, Hg, and Se. One can see that there is no loss of volatile elements or cross-contamination in the UltraCLAVE when using the loose-fitting vessel covers and prepressurization technique.

Figure 8 - Recoveries of elements of interest.

Large batches of environmental samples, at least 35 or more (up to 77) at a time, can be effectively processed in the UltraCLAVE with very high quality. Figure 9 shows the recoveries from a GBW* SRM: Human Hair. Again, the data for As, Hg, and Se are in good agreement with the expected values.

Figure 9 - Recoveries from a GBW SRM: Human Hair.

High-throughput clinical laboratories can be assured of quality sample preparation using the UltraCLAVE. It is also of value to note that only 5 mL of acid was used digesting each sample. This minimized the dilution factor, decreased the consumption of expensive acids, and reduced the cost of waste disposal.

Conclusion

The UltraCLAVE performs beyond the current limitations of traditional microwave sample preparation. It offers all the advantages familiar to microwave users—speed, sample purity, reproducibility, and no pollution of the laboratory or environment—while overcoming the restrictions that have forced some laboratories to reluctantly use other digestion methods. Chemists can digest large sample masses (even of organics), large batches of mixed sample types, and difficult sample matrices requiring sustained high temperatures. Cross-contamination between samples does not occur. Extremely high throughput can be achieved. Reduced acid usage, consumables, and need for labor all provide long-term cost-savings. The instrument is safe for hazardous samples and simple to operate. It increases the capabilities of closed-vessel microwave sample preparation.

Reference

  1. Poster presentation. “35 at Once! Mineralization of Biological Materials With the Milestone AutoCLAVE.” Goessler, W.; Majeron, G. Institute of Chemistry, KFU Graz, Austria.

Dr. Borowski is Application Laboratory Manager, and Mr. Schoenfeld is a Marketing Associate, Milestone Inc., 25 Controls Dr., Shelton, CT 06484, U.S.A.; tel.: 866-995-5100; fax: 203-925-4241; e-mail: [email protected].

*GBW are reference materials produced by the National Research Centre for Certified Reference Materials.