Single Reaction Chamber (SRC) Digestion in a benchtop package

Transcription

1 Single Reaction Chamber (SRC Digestion in a benchtop package Revolutionizing Microwave Digestion The remarkable NEW UltraWAVE features Milestone s unique SRC technology in a fully automated benchtop package. Through advanced design and engineering, Milestone has been able to reduce the size of SRC technology, resulting in a small benchtop unit that can fit in even the smallest lab. UltraWAVE digests up to 15 samples of any matrix simultaneously. No need to batch samples of the same type into separate runs. Eliminates labor associated with assembly/disassembly of digestion vessels simply weigh the sample into a glass vial, add acid and place into the sample rack. UltraWAVE is perfect for labs that have multiple sample types to digest, or for labs that simply want the convenience of SRC technology. The superior digestion quality and minimal blank contribution of the SRC makes UltraWAVE an ideal complement to ICP-MS. The UltraWAVE with SRC technology removes the limitations of conventional closed-vessel systems and makes microwave digestion more effective and more efficient than ever before Key Benefits of Single Reaction Chamber Technology Use of Simple Vials and Racks With SRC, a large stainless steel pressure reactor acts as both the microwave cavity and reaction vessel. This reactor contains a TFM Teflon liner in which all samples are placed and subsequently subjected to the same temperature and pressure conditions. Prior to the digestion run, the reactor is pre-pressurized As the stainless steel reactor is in itself pressurized, the need for individual pressure vessels, as used in standard closed-vessel digestion, are no longer needed. Instead, samples are simply weighed into glass (disposable, quartz (reusable, or Teflon vials and loaded into a rack just like loading an autosampler! In fact the sample vials, after digestion, could be placed directly in an instrument autosampler for analysis. The photograph shows an UltraWAVE sample rack loaded with 15mL glass vials, following digestion. Loose fitting TFM caps prevent condensation from falling into the vials but still allow pressure equalization with the chamber. Sample handling is minimized, which reduces the risk of contamination a key advantage for ultratrace analysis by ICP-MS. The separate microwave run required to clean closed reaction vessels is eliminated, which greatly increases productivity. Simply throw away used glass vials, or clean used quartz vials in acid. High Temperature and Pressure Ensures Complete Digestion After the reactor is sealed and prior to the microwave run, the SRC is pre-pressurized with nitrogen. This immediately raises the boiling point of the digestion reagents inside eliminating the potential for sample cross contamination via boiling and spitting, and also prevents the loss of volatiles. The SRC operates at very high temperature and pressure conditions (up to 200 bar and 300 deg. C, so complete digestion of even the most difficult samples types is assured. The very high pressure conditions means that much higher sample weights can be digested compared to closed-vessel systems. Since all samples are, in effect, inside the same vessel, there is direct pressure and temperature control of every sample giving the best possible control of digestion conditions. There is no need to use slower and less accurate indirect temperature (IR or indirect pressure control. Digest Completely Different Samples - Simultaneously With closed-vessel digestion, a sample is chosen as a reference vessel: temperature and pressure in that sample is directly monitored and the program adjusts power to prevent an overpressure situation. This means that only samples of the same weight and type as the reference sample can be digested at the same time. With SRC there is no such limitation: since all samples are in the same vessel, any combination of sample types and acid chemistries can be digested in the same run all under the same, optimal conditions. For example, consumer products, nutraceuticals, and polymer samples can be digested simultaneously, greatly increasing the productivity of labs with multiple sample types to digest. Unlike closed-vessel digestion, there is no need to batch samples of the same matrix together for digestion. Also, reference materials can be digested at the same time as samples, even though the sample matrix is different, enabling for the first time, in-run quality control of the digestion process. 8

2 Breaking the Sample Preparation Bottleneck with a New Approach to Microwave Digestion Closed-vessel microwave digestion is widely recognized as the most effective technique for the digestion of samples for metals analysis by graphite fur-nace 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 use 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 closedvessel 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 floorstanding system capable of digesting up to 40 samples simultaneously (or 77 micro- samples. The UltraCLAVE is in rouuse 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 auto sampler 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 C. 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 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 9

3 from run to run. The chamber clamp is secured (2, and the chamber is pressurized with N 2 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. Miniaturization of SRC technology The UltraWAVE (Milestone (Figure 3 is a miniaturized, benchtop version of the UltraCLAVE, approximately the same size as a conventional closed-vessel microwave system. The cabinet is stainless steel and the work area is surrounded by a thick acrylic screen that raises and lowers automatically with the top of the SRC chamber. Interlocks prevent operation unless the chamber clamp is in position and secured. Its 1500-W microwave power source delivers microwave energy to a 1-L chamber (3.5 L in the UltraCLAVE that can hold ml vials (or mL vials for microsamples. Although the sample capacity is lower than the UltraCLAVE, the smaller chamber size of the UltraWAVE means that cooling time is significantly quicker. Efficient water cooling of the chamber means that sample cooling is also faster than with closed-vessel systems, despite the higher digestion temperature. Maximum operating temperature is 300 ºC and maximum pressure is 199 bar; thus complete digestion of even the most difficult sample types is achieved. The chamber is fitted with a sealed PTFE liner, which prevents corrosion due to acid vapors, allowing any type of digestion acid or mixture to be used. Up to 1.25 g of organic sample can be digested in the 15-mL vials; for higher sample weights a range of sample racks with larger vials is available. Vials are offered in glass, quartz, or TFM. Figure 1: Flow diagram of SRC operation. Table 1 Digestion of HDPE polymer; closed-vessel with high-pressure rotor vs UltraWAVE Closed-vessel with two high-pressure rotors (12-position UltraWAVE with two racks (15-position Weigh 12 samples, add acid, assemble vessel, load 24 min Weigh 15 samples, add acid and cap, place vial in rack 10 min into rotor (0.25 g sample + 8 ml HNO 3, 2 ml H 2 (0.25 g sample + 4 ml HNO 3, 1 ml H 2 Microwave program: 20 min to 210 ºC; hold for 20 min 40 min Microwave program: 10 min to 210 ºC; hold for 20 min 30 min Cool down to 65 ºC 20 min Cool down to 65 ºC; depressurize chamber 15 min Vessel disassembly 12 min Vessel disassembly N/A Total run time 96 min Total run time 55 min Subsequent runs (assemble 2nd rotor during previous 72 min Subsequent runs (assemble 2nd rotor during previous 45 min run run Total number of runs in an 8-hr shift 6.3 Total number of runs in an 8-hr shift 10.4 Total number of samples digested in an 8-hr shift 76 Total number of samples digested in an 8-hr shift 157 Table 2 Digestion of plant material; closed-vessel with high-capacity rotor vs UltraWAVE Closed-vessel with two high-capacity rotors (24-position UltraWAVE with two racks (15-position Weigh 24 samples, add acid, assemble vessel, load 35 min Weigh 15 samples, add acid and cap, place vial in rack 10 min into rotor (0.25 g sample + 8 ml HNO 3, 2 ml H 2 (0.25 g sample + 4 ml HNO 3, 1 ml H 2 Microwave program: 20 min to 180 º; hold for 10 min 30 min Microwave program: 10 min to 180 ºC; hold for 10 min 20 min Cool down to 65 ºC 30 min Cool down to 65 ºC; depressurize chamber 12 min Vessel disassembly 15 min Vessel disassembly N/A Total run time 110 min Total run time 42 min Subsequent runs 75 min Subsequent runs 32 min (assemble 2nd rotor during previous run (assemble 2nd rotor during previous run Total number of runs in an 8-hr shift 6 Total number of runs in an 8-hr shift 14.7 Total number of samples digested in an 8-hr shift 144 Total number of samples digested in an 8-hr shift

4 Sample throughput comparison: UltraWAVE vs closed-vessel digestion When measuring sample throughput in microwave digestion, the time required for the sample weighing, acid addition, vessel assembly, microwave program, cooling time, and vessel disassembly is combined. Assuming two rotors are used, the second rotor can be loaded while the previous run is in progress. Table 1 shows the comparison of sample throughput between closed-vessel digestion and the UltraWAVE for the digestion of high-density polyethylene (HDPE polymer samples. HDPE requires a high-pressure rotor for closed-vessel digestion typically a 12-position rotor is used. With no vessel assembly or dis assembly and a faster cool-down time, the UltraWAVE is two times more productive than closed-vessel digestion. Table 2 shows a similar comparison, this time with a highcapacity 24-position rotor, for the digestion of plant material. Despite the higher-capacity closed-vessel rotor that can be used with this application, the UltraWAVE is still 1.5 times more productive. Figure 4 shows a graphical representation; note that vessel uncapping time is eliminated with the UltraWAVE. Figure 5 is a graphical comparison of the number of samples that can be digested in an 8-hr shift by the two techniques. It should also be noted that, while 40-position rotors are available in closed-vessel digestion, they are very rarely used at full capacity since the ramp to temperature takes much longer due to the load on the microwave, and in some cases the target digestion temperature would not be reached. The above comparisons do not take into account the vessel cleaning time required with the closed-vessel system. For ICP- OES analysis, vessels need to be cleaned daily, and for ultratrace analysis by ICP-MS they should be cleaned prior to every run, ideally by a microwave run with acid blank. The glass vials used with the UltraWAVE are simply discarded after the sample has been analyzed. The comparison also does not consider the extra productivity of the UltraWAVE due to run mixed batches of samples a major benefit for laboratories that digest multiple sample types. Figure 2: Sample rack being removed following digestion. Study of vial contribution to blank levels To confirm the absence of crosscontamination, and to check the blank contribution from glass vials, quartz and glass vials containing the reagent blank (4 ml of high-purity HNO 3 were placed in a mixed batch digestion run of toys, consumer products, and pharmaceutical samples. The vials were not precleaned prior to use. Table 3 shows the concentrations of four key analytes in the blanks, measured by collision cell ICP-MS. In all cases, the blanks were at the low- to mid-ppt level. Cross- contamination did not occur, and for these analytes, disposable glass vials are clearly suitable for use. Figure 3: UltraWAVE SRC microwave digestion system. new Table 3 Blank levels in a mixed batch (data from UltraCLAVE Vessels Arsenic (µg/l Cadmium (µg/l Mercury (µg/l Lead (µg/l Quartz (reusable Min < Max Aver Glass (disposable Min. < 0.01 < 0.01 < Max Aver. <

5 Figure 4: Graphical comparison of run time. Figure 5: Graphical comparison of number of samples digested in an 8-hr shift. Tim Michel Applications Manager, Milestone Inc., 25 Controls Dr., Shelton, CT 06484, U.S.A. Summary SRC technology changes the traditional work flow of microwave digestion by removing the limitations of closed-vessel systems. Sample throughput is up to two times higher, with significant labor savings due to reduced vessel handling and cleaning. No batching of samples into runs of the same matrix is required, which is a major benefit for laboratories that digest multiple sample types. Method development is largely eliminated, and the high temperature and pressure capability means that superior digestion quality is achieved for the more challenging samples. Consumables costs are significantly lower, and digestion acid usage is reduced by 50%. Finally, the small size of the UltraWAVE makes it as convenient to install as a closed-vessel system. Milestone sub/duopur Purification d acides & solvants Ultraclean Chemistry Line «Dans la préparation de réactifs ultrapurs, une seule méthode de choix: la sub-boiling distillation.» dixit US EPA SW-846 chap. 3, update IVB. - Appareil permettant la préparation de 1 ou 2 réactifs - Économie significative pour des utilisateurs d acides ultrapurs, solvants, - Facile d usage et d entretien - Production de 500 ml environ par canal en 6h d acide ultrapur à partir d acide de qualité ordinaire - subpur: 1 canal, duopur: 2 canaux How the DuoPUR and SubPUR work: purification