Closed-vessel microwave digestion is widely recognized as the most effective technique for the digestion of samples for metals analysis by graphite furnace atomic absorption (GFAA), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS). Because it operates at a much higher temperature (and pressure) than a heated digestion block, it can be applied to a far wider range of samples. With microwave digestion, the samples are enclosed; thus cross-contamination and loss of volatiles are eliminated, and use of high-quality vessel materials minimizes contamination.
For high-throughput laboratories, however, closed-vessel microwave digestion has some limitations. The digestion vessels must be cleaned prior to each use (normally by a cleaning cycle in the microwave with an acid blank). The vessels must be assembled and disassembled, which is labor intensive, and the sample solution must be transferred to another container prior to analysis, which increases handling and contamination risk. The high temperature and pressure achieved during digestion degrade the vessels over time, resulting in significant consumables costs. Finally, to ensure full control of the digestion run, the sample weight, matrix type, and acid used must be the same for every sample in the run. This means that a separate run must be performed for each sample type. High-throughput laboratories that have multiple sample types often employ several digestion systems. The technology described in this article, Single Reaction Chamber (SRC) microwave digestion (Milestone Scientific, Shelton, CT), has none of the limitations of closed-vessel digestion, and is being used increasingly in a wide variety of digestion applications.
Single Reaction Chamber microwave digestion
SRC microwave digestion was introduced commercially around five years ago with the Milestone UltraCLAVE, a floor-standing system capable of digesting up to 40 samples simultaneously (or 77 microsamples). The UltraCLAVE is in routine use in over 100 laboratories worldwide.
The SRC is a new approach to microwave digestion. As its name suggests, the SRC is a large, pressurized stainless steel reaction chamber into which all samples are placed and digested simultaneously. With SRC, the reaction vessel in effect becomes the microwave cavity, enabling the intensity and distribution of the delivered microwave energy to be optimized to the shape of the reaction vessel. This ensures even heating and therefore eliminates the need to rotate samples during the digestion run. Because the samples are placed inside a pressurized vessel, individual pressure vessels are not needed. Samples are simply weighed into glass autosampler-type vials with the appropriate digestion acid and placed in a rack similar to a rotary autosampler tray. Quartz and TFM (a high-performance PTFE copolymer) vials can also be used. Vial caps are fitted to avoid condensation from the roof of the chamber dripping into the samples (the caps are loose fitting to ensure pressure equalization with the chamber). The rack is fitted to the chamber roof; the chamber is then automatically sealed and prepressurized with nitrogen to 40 bar prior to microwave heating. This ensures that no spitting or boiling of the sample solution occurs, preventing cross-contamination or loss of volatiles. As sample temperature increases during the microwave program, so does the pressure in the chamber; therefore boiling never occurs. The chamber is lined with PTFE to prevent corrosion from acid vapors. When the program is completed, the chamber is vented and the rack removed. Samples are diluted to volume in the vials, ready for measurement.
Aside from the reduced handling and labor savings, a benefit of SRC over closed-vessel digestion is that it operates at very high temperature and pressure—up to 300 ºC and 199 bar; thus complete digestion of even the most difficult sample types is achieved. In addition, since all samples are in the same vessel, any combination of sample type (or weight) can be digested simultaneously. There is no need to “batch” digestion runs into identical sample types. Method development is largely eliminated, since a single program can be used for almost every sample type. Standard reference materials can be digested along with samples, enabling, for the first time, in-run quality control of the digestion process.
Direct pressure and temperature control of the chamber gives direct control of every sample. This eliminates the need to use slower and less accurate indirect temperature (IR) or indirect pressure control. The high pressure capability allows higher sample weights to be digested than with closed-vessel digestion, especially for high-fat-content samples that generate high pressure during digestion due to formation of CO2. Consumables costs are significantly lower: The SRC uses inexpensive disposable vials, while closed-vessel systems typically require annual replacement of all vessels, shields, and caps. The SRC also uses less acid (typically 4 mL vs 8 mL for closed-vessel digestion), which reduces consumables costs and also lowers reagent blank levels.
SRC technology in operation
Figure 1 - Flow diagram of SRC operation.
Figure 2 - Sample rack being removed following
SRC operation is extremely simple and highly automated. The process is described in Figure 1. Samples are weighed into vials, acid is added, and the samples are placed on the sample rack. The rack is fitted to the chamber top (1), which is lowered automatically into the chamber. The sample vials sit in liquid that provides a consistent “load” for the delivered microwave energy. This ensures even conductive heating and consistent conditions from run to run. The chamber clamp is secured (2), and the chamber is pressurized with N2 to 40 bar (3). The microwave program used is essentially the same for any sample type and comprises simply time to temperature (ramp time) and time at temperature (hold time) (4). When the program stops, water cooling rapidly cools the chamber, pressure is gently released, and the gases and vapors are ducted away to exhaust (5). The sealing clamp is released, and the sample rack automatically lifts clear of the chamber (6) (Figure 2). The clear digestates are simply diluted to volume and remain in the vial; no further transfer is needed.