Anal. Chem. 1999, 71, 4050-4055
Microwave-Enhanced Flow System for High-Temperature Digestion of Resistant Organic Materials Ulrike Pichler, Anja Haase, and Gu 1 nter Knapp*
Institute of Analytical Chemistry, Micro- and Radiochemistry, Technical University Graz, Technikerstrasse 4, A-8010 Graz, Austria Markus Michaelis
Joanneum Research, Steyrergasse 17, A-8010 Graz, Austria
Microwave-assisted flow digestion systems open up new possibilities in fully automated sample preparation for element analysis. For an extensive and fast oxidation of organic materials with nitric acid, temperatures of more than 200 °C are necessary. To achieve the desired temperatures of ∼250 °C, it is essential that the pressure in the system can be increased up to 35 bar. The teflon tubes used, however, do not withstand the vapor pressure of the digestion mixture at these temperature levels. A high-pressure flow digestion device is described that enables the application of such high temperatures by means of a novel pressure equilibration system. PTFE or PFA tubes can be used up to 250 °C if no mechanical stress is applied to the tube wall. The pressure equilibration system keeps the pressure inside and outside the digestion tube equal even for extremely fast oxidation reactions. The digestion of easy, medium, and difficult oxidizable substances (glucose, glycine, and phenylalanine, respectively) shows the importance of digestion temperatures around 250 °C. The extremely violent digestion of glucose with concentrated nitric acid can be carried out as easy as the difficult oxidation of phenylalanine by means of this system. The SRM TORT 2 (defatted lobster hepatopancreas tissue) was digested under different conditions to indicate the high oxidation capabilities in comparison with a commercial mediumpressure flow digestion device. The analysis of the SRMs milk powder (BCR 063, BCR 151), bovine liver (BCR185), and pig kidney (BCR 186) after digestion at 245 °C and 5 min showed good agreement with the certified values. Wet chemical sample decomposition is an important analytical step in element analysis but usually still the most time-consuming part of combined analytical methods. Even when high-performance microwave-assisted closed-vessel digestion techniques are used, it takes a considerable amount of time for cleaning the vessels, dispensing of sample and reagent, and closing and reopening of the vessels after sample digestion. These steps can be omitted when flow digestion systems are used. Various microwave-assisted equipment for flow digestion at ambient pressure was described.1-7 4050 Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
Nitric acid is suitable as a digestion reagent due to its high degree of purity and few interferences with most analytical measurement techniques. Investigations of Wu¨rfels et al.8-11,23 showed that temperatures higher than 200 °C are necessary for rapid and effective oxidation of organic materials with nitric acid. This discovery led to the development of different digestion bombs.12-16 Modern microwave-assisted closed-vessel digestion systems operate at pressure levels up to 80 bar and temperatures up to 250 °C.17 Flow digestion systems respond to the same principle. Pressurized flow systems enable an elevated boiling point of the digestion mixture and make higher digestion temperatures available. To increase pressure in a flow digestion system, two (1) Hinkamp, S.; Schwedt, G. Anal. Chim. Acta 1990, 236, 345. (2) Benson, R. L.; McKelvie, I. D.; Hart, B. T. Anal. Chim. Acta 1994, 291, 233. (3) Carbonell, V.; De la Guardia, M.; Salvador, A.; Burguera, J. L.; Burguera, M. Anal. Chim. Acta 1990, 238, 417. (4) De la Guardia, M.; Carbonell, V.; Morales-Rubio, A.; Salvador, A.; Talanta 1993, 40, 1609. (5) Burguera, J. L.; Burguera, M.; Brunetto, M. R. At. Spectrosc. 1993, 14, 90. (6) Burguera, J. L.; Burguera, M.; Carrero, P.; Rivas, C.; Galignani, M.; Brunetto, M. R. Anal. Chim. Acta 1994, 308, 349. (7) Burguera, J. L.; Burguera, M. J. Anal. At. Spectrom. 1993, 8, 235. (8) Wu ¨ rfels, M.; Jackwerth, E. Fresenius Z. Anal. Chem. 1985, 322, 354. (9) Wu ¨ rfels, M.; Jackwerth, E.; Stoeppler M. Fresenius Z. Anal. Chem. 1987, 329, 459. (10) Wu ¨ rfels, M.; Jackwerth, E.; Stoeppler, M. Fresenius Z. Anal. Chem. 1988, 330, 159. (11) Wu ¨ rfels, M. LABO 1989, 3, 7. (12) Bernas, B.; Anal. Chem. 1968, 40, 1682. (13) Kotz, L.; Kaiser, G.; Tscho¨pel, P.; To¨lg, G. Fresenius Z. Anal. Chem. 1972, 260, 207. (14) Stoeppler, M.; Backhaus, F. Fresenius Z. Anal. Chem. 1978, 291, 116. (15) Schramel, P.; Wolf, A.; Seif, R.; Klose, B. J. Fresenius Z. Anal. Chem. 1980, 302, 62. (16) Knapp, G. Fresenius Z. Anal. Chem. 1984, 317, 213. (17) Kingston, H. M.; Haswell S. Microwave Enhanced Chemistry; American Chemical Society Professional Book Series; American Chemical Society: Washington, DC, 1997. (18) Haswell, S. J.; Barclay, D. Analyst, 1992, 117, 117. (19) Sturgeon, R. E.; Willie, S. N.; Methven, B. A.; Lam, J. W.; Matusiewicz, H. J. Anal. At. Spectrom. 1995, 10, 981. (20) Karanassios, V.; Li, F. H.; Liu, B.; Salin, E. D. J. Anal. At. Spectrom. 1991, 6, 457. (21) Gluodenis, T. J.; Tyson, J. F. J. Anal. At. Spectrom. 1993, 8, 697. (22) Knapp G. U.S. Patent 5,672,316, 1997. (23) Wu ¨ rfels, M.; Jackwerth, E. Anal. Chim. Acta 1989, 1, 226. 10.1021/ac9903048 CCC: $18.00
© 1999 American Chemical Society Published on Web 08/17/1999
Figure 1. Scheme of the flow digestion system and the principle of the pressure equilibration. Key: A pressure reactor; B heating zone; C cooling zone; D digestion coil; E cooling device; F connection for gas supply; G restrictor tube; H, collector vial; I temperature sensor; J highpressure pump; K injection valve; L sample loop; M sample; N and O peristaltic pumps.
principles are described. One possibility is the application of a pressure-controlled restrictor valve at the outlet of the system.18,19 The so-called stopped-flow system offers the other possibility for pressurized wet digestion in a tube system.20,21 In this case, the Teflon tube is closed on both ends with valves after loading of the digestion mixture and heated in a microwave field. This technique is comparable to wet digestion in a digestion bomb. Both systems are limited by the mechanical strength of the reaction tube, which is made of polytetrafluorethylene (PTFE)or perfluoralkoxy (PFA)-Teflon. With these materials only pressure levels of a maximum 20 bar and corresponding temperatures up to 180 °C are possible. Quartz tubes exclude the application of hydrofluoric acid, which is necessary for the decomposition of soil, sediments and plant materials containing silica and therefore cannot be recommended. The goal of the development shown in this paper was a microwave-assisted high-pressure flow digestion system with PTFE or PFA tubes for digestion temperatures up to 250 °C. EXPERIMENTAL SECTION A Novel Microwave-Assisted Flow Digestion System for High Temperatures. Teflon tubes used for microwave-assisted flow digestion systems do not resist the vapor pressure of nitric acid at temperatures higher than 180 °C (>20 bar). To implement a flow digestion system for 250 °C (∼35 bar) a novel pressure equilibration system has been developed.22 PTFE or PFA tubes can be used at this temperature level if no mechanical stress is applied to the tube wall. Figure 1 shows the schematic of the flow digestion system and the principle of pressure equilibration. The equipment consists of a pressurized reactor vessel (A) which is made of two different materials. The heating zone (B) located in the microwave field of a waveguide is made of alumina ceramics, and the cooling zone (C) is made of polyetheretherketone (PEEK). Both parts are connected gastight.
Figure 2. Boiling diagram of water and 10% nitric acid.
The reaction coil (D) consisting of a ∼6-m PTFE- or PFATeflon tube (1.0 × 1.6 mm) is placed within the heating zone of the reactor. The length of the digestion tube is restricted by the dimension of the waveguide. The longer the digestion tube, the higher the reactive volume within the microwave field and therefore the higher the sample throughput. Investigations with bigger reaction coils are planned applying a microwave oven. Directly outside the microwave field, the temperature of the reaction mixture is measured by means of a Pt-100 sensor (I). For calibration of this sensor, the boiling diagram of water at pressure levels between 15 and 35 bar can be used (Figure 2). The cooling device (E) consists of a water-cooled titanium rod wrapped with a 1-m Teflon tube. The reaction tube ends open at the funnel-shaped bottom of the reactor vessel. The reactor is connected to a nitrogen bomb (F) and supplied with N2 of a constant pressure up to 35 bar. A restrictor tube (0.5 × 1.6 mm and 25 m length) is connected gastight to the bottom of the reactor vessel. Thus, there is a small continuous gas stream through the Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
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reactor vessel. The elevated pressure within the reactor causes the boiling temperature of the reaction mixture flowing through the digestion coil (D) to rise without applying mechanical strains on the tube wall. The cause is the pressure balance inside and outside the reaction tube. It is necessary to supply enough microwave energy to reach the boiling point inside the reaction tube. At the boiling stage, vapor bubbles are formed that can no longer absorb microwave energy; the vapor condenses and microwave energy is absorbed againsa boiling equilibrium is maintained. The surplus microwave energy is reflected to the magnetron. That way the temperature is controlled only by the pressure applied. The outstanding advantage of this pressure equilibrium system is that even when extremely fast oxidation reactions take place (e.g., digestion of glucose) the pressure inside and outside of the reaction tube remains equal and the PTFE tube is not blown up. The digestion solution is cooled to room temperature passing the cooling zone (C), drops on the funnel-shaped bottom of the reactor vessel, and is conveyed rapidly through the restrictor tube (G) by the gas stream. Passing the restrictor tube, the gas segmented stream of digest and carrier liquid is expanded to ambient pressure and collected in a vial (H) of an autosampler for off-line analysis. The other part of the system is a flow injection device consisting of a high-pressure piston pump (J) and an acid-resistant sample introduction valve (K) with a sample loop (L). A highpressure pump (>35 bar) is necessary to convey the carrier stream with the sample into the pressurized reactor. The volume of the sample loop is variable between 1 and 5 mL. The sample (M) is drawn into the sample loop (L) by means of the peristaltic pump (N). At the same time, the sample stream is mixed with 65% nitric acid by means of the peristaltic pump (O). The flow rate of pump N is twice the pump rate of pump O. In that way, the sample is mixed with concentrated nitric acid 1:1. Another possibility is to mix sample and digestion reagent off-line in a sample vessel without addition of nitric acid by means of pump O. When the sample loop is filled, the peristaltic pump (N) is switched off before the sample-acid mixture reaches pump N. Then valve K is switched in the other position and the digestion mixture is transported into the reactor. The digestion time (residence time in the microwave zone) can be adjusted by the flow rate of the carrier stream. Before loading the sample loop (L) with the next sample, the sampling tube system is rinsed by operating pump N in the reverse direction. Instrumentation. Total organic carbon (TOC) analyzer Shimadzu TOC 5050 was used for the determination of the residual carbon content; Other equipment used was AAS the Perkin-Elmer 5000 with graphite furnace HGA 500, ICP-OES Perkin-Elmer Optima 3000 XL, and the Hg-Mat, Anton Paar Co., Graz, Austria. Reagents and Samples. These were as follows: concentrated nitric acid purified by subboiling distillation; hydrogen peroxide, 30% v/v, (Merck); distilled water; glucose (Merck); glycine (Merck); phenylalanine (Merck); standards for TOC calibration prepared by dissolving potassium hydrogen phthalate Cica reagent in distilled water; element standards prepared by dilution of standard solutions from Merck; standard reference materials milk powder BCR 063 and BCR 151; bovine liver BCR 185; pig kidney 4052 Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
BCR 186; NRC of Canada TORT 2 defatted lobster hepatopancreas tissue; Triton X-100 (Merck). Sample Preparation and Digestion. For parameter studies, 2.50 g of glucose, 2.80 g of glycine, and 1.38 g of phenylalanine were dissolved in 100 mL 10, 15, 32, and 65% of nitric acid, respectively. Each of these solutions has a carbon content of 9.1, 9.0, and 8.7 g/L, respectively. A 2-mL aliquot of each these solutions was digested at different parameter settings (microwave power, digestion time, and pressure/temperature). The digest was collected in a volume of 10 mL together with the carrier liquid. For recovery and accuracy studies, standard reference materials were digested by the following procedure. Milk powder (100 mg) was dissolved in 10 mL of distilled water containing 0.1 mL of Triton X-100 by use of sonication. This solution was mixed online 1:1 with 65% nitric acid and digested at 30 bar (235 °C), 120-W microwave power, and 3-min digestion time (residence time in the microwave field, corresponding to a flow rate of 1.6 mL/min). One percent slurries of bovine liver and pig kidney, respectively, in 32% nitric acid were digested at 35 bar (245 °C), 110-W microwave power, and 5-min digestion time (flow rate 0.9 mL/ min). Safety Considerations. Owing to the use of strong acids at elevated pressure and temperature, the system is shielded by a Plexiglas cover to prevent an accident in case of a broken tube. RESULTS AND DISCUSSION Dispersion. In every flow system, the injected sample will be diluted during transportation. It is important to know the dispersion pattern to collect the whole sample at the outlet of the system. The dispersion in a flow digestion system depends on the formation of gas bubbles during the reaction as well as the violence of the digestion reaction. Thus, for studying the dispersion behavior, two extreme samples were investigated. In the first case, a mixture of pure water with nitric acid was used. The mixture is boiling in the digestion zone but no chemical reaction with formation of CO2 occurs. Additionally, a mixture of glucose and 32% nitric acid was digested. In this case, a violent reaction with a large amount of CO2 formation takes place. Be (2 mg/L) was added to each sample solution and measured by means of flame-AAS after digestion. The distribution of Be indicates that the sample solution appears after the dead volume and is completely washed out by carrier solution with a 3-fold sampling volume independent of the sample matrix. The appearance time of the sample at the outlet depends strongly on the sample matrix. Due to the CO2 development, organic samples appear before inorganic ones because of the volume expansion inside the reactor. Therefore, digestion time cannot be measured precisely. The theoretical digestion time is calculated from the flow rate of the carrier stream and the tube volume inside the microwave field. Influence of Different Digestion Parameters on the Oxidation of Organic Substances. The TOC value of a digest is used to determine the oxidation efficiency. Usually 1-30% of the organic carbon remains as soluble organic compounds after microwaveassisted wet digestion at moderate-pressure levels. For the investigation of the influence of digestion parameters on the oxidation efficiency, glucose, glycine, and phenylalanine have been used as representatives for substances that are easy, moderate,
Figure 3. Influence of nitric acid concentration on digestion efficiency. Digestion parameters: 110 W, 20 bar (212 °C), 3 min (flow rate 1.6 mL/min).
and difficult to digest, respectively. Actually, these parameter studies are not an optimization of the digestion procedure, because the decisive parameters temperature, concentration of the oxidizing reagent, and digestion time, are effective in one directions the higher the temperature and the oxidant concentration, and the longer the digestion time, the better is the oxidation efficiency. But there are practical limitations; the longer the digestion time, the worse the sample throughput. The maximum concentration of ultrapure nitric acid is ∼65% and the maximum temperature for Teflon tubes is 250 °C. Many sample materials can be sufficiently oxidized at lower temperatures and this takes care of the digestion device. Microwave Power. As mentioned above, the boiling temperature in the digestion system discussed is only affected by the pressure applied as long as it is ensured that enough microwave power is available to reach the boiling temperature of the mixture. Therefore, the digestion efficiency is independent of the microwave energy. To demonstrate this fact, glucose solution prepared in 32% nitric acid was digested at 20 bar (212 °C) and 3-min digestion time (flow rate 1.6 mL/min) at microwave power levels of 90, 100, and 110 W, respectively. Temperature and thus the oxidation efficiency was equal in all cases. The TOC values were 30% independent of the microwave power applied. The minimum required microwave power is 70 W in this experimental design. An excess of microwave power has the advantage that the steady state after switching on the instrument is reached earlier. The steady state is reached after 3 and 10 min, respectively, dependent on the flow rate of the carrier stream, the pressure, and the microwave power applied. Acid Concentration. The influence of nitric acid concentration on the oxidation efficiency is shown in Figure 3. Glucose solutions in 10, 15, 32, and 65% nitric acid were digested at 20 bar (212 °C) and 3-min digestion time (flow rate 1.6 mL/min). As expected, the best results are achieved when the sample is dissolved or suspended in 65% nitric acid and then digested. The violence of the reaction does not disturb the pressure equilibrium system. To increase the oxidation potential of diluted nitric acid, hydrogen peroxide was used as diluent instead of water. The measurement of the remaining carbon content shows that hydrogen peroxide has no significant influence on the oxidation capability of nitric acid.
Figure 4. Digestion efficiency dependence of glucose on digestion time and temperature. Digestion parameters: 110 W, nitric acid 32%.
Figure 5. Digestion efficiency dependence of glycine on digestion time and temperature. Digestion parameters: 110 W, nitric acid 32%.
Figure 6. Digestion efficiency dependence of phenylalanine on digestion time and temperature. Digestion parameters: 110 W, nitric acid 32%.
Temperature and Digestion Time. Solutions of glucose, glycine, and phenylalanine were digested at 20, 25, 30, and 35 bar corresponding to 212, 228, 235, and 245 °C and 1-, 2-, 3-, 4-, 5-, and 8-min digestion time (flow rates 4.7, 2.4, 1.6, 1.2, 0.9, and 0.6 mL/min). The corresponding TOC values are shown in Figures 4-6. The relative standard deviation of the TOC measurement is (3%. As expected, the oxidation efficiency rises with increasing temperature and digestion time. The results indicate that in flow systems the digestion temperature is even more Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
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Table 1. Digestion of TORT 2a and Comparison with the Results Obtained with a Commercial Medium-Pressure Flow Digestion System19 TOC (%) HNO3 (%)
H2O2 (%)
medium-pressure system
30 bar, 235 °C
35 bar, 245 °C
10 20 30 60 5 10 20 30 60
0 0 0 0 3 3 3 3 3
73 53 42
22.4 18.2 15.5 14.0 32.5 24.7 21.8 15.1 8.5
22.4 13.3 11.8 10.8 30.4 21.3 18.1 13.0 6.2
a
46 36
Table 2. Recovery Studies of Elements in SRM Milk Powder BCR 063 and BCR 151a SRM milk powder BCR 063
BCR 151
elements
certified
measured (n ) 5)
Mg (mg/g) Ca (mg/g) K (mg/g) Na (mg/g) Cu (µg/g) Fe (µg/g) Zn (µg/g) Pb (µg/g)
1.263 ( 0.024 13.49 ( 0.10 17.68 ( 0.19 4.37 ( 0.031 5.23 ( 0.08 50.1 ( 1.3 50 2002 ( 26
1.25 ( 0.04 13.2 ( 0.4 17.4 ( 0.3 4.3 ( 0.1 5.1 ( 0.4 52 ( 3 54 ( 3 2340 ( 430
a Digestion parameters: 30 bar (235 °C), 110 W, and 3 min (flow rate 1.6 mL/min).
Digestion parameters: 110 W and 4 min (flow rate 1.2 mL/min). Table 3. Recovery Studies of Trace Elements in SRM Bovine Liver BCR 185 and SRM Pig Kidney BCR 186a
important than in closed pressurized digestion devices because of the short digestion times. A special effect was observed during the digestion of phenylalanine. The formation of CO2 appears only at temperatures higher than 240 °C. Below these temperatures, a polymerization reaction seems to take place. A significant amount of white flaky precipitate remained in the resulting digest. To investigate the oxidation capability of the equipment for real samples, the standard reference material TORT 2 was digested at different time and temperature parameters and the remaining organic carbon content was measured as TOC (Table 1). Five milliliters each of 0.5% slurries of TORT 2 with 10, 20, 30, and 60% nitric acid were digested at 30 and 35 bar pressure corresponding to temperatures of 235 and 245 °C. In a second run, 3% H2O2 was added to each of the slurries in nitric acid. The microwave power was 110 W and the calculated digestion time 4 min (flow rate 2.4 mL/min). The digest was collected in a volume of 25 mL. The results given in Table 1 show the much better oxidation capability of the high-pressure/high-temperature flow digestion device in comparison with a commercial mediumpressure flow digestion apparatus (CEM SpectroPrep).19 Recovery Studies. Dispersion of the analyte in a flow digestion system increases with samples containing undissolved particles, e.g., slurries or precipitated components after mixing sample and nitric acid. An example for a sample that precipitates in contact with acid is milk. When the sample is mixed on-line with 65% nitric acid, white flakes of undissolved proteins are formed. To investigate the tailing of such a sample by particle size distribution during conveyance through the tube system, the sample was collected together with a 3-fold excess of carrier solution as described before and the following carrier solution was collected in another vessel to measure the blank values. Five milliliters of each milk powder solution (BCR 063, BCR 151) was mixed on-line with 65% nitric acid and digested at 30 bar corresponding to 235 °C, 3-min digestion time (flow rate 1.6 mL/min) and 110-W microwave power. The digest was collected in a total volume of 20 mL. The following 10 mL of carrier solution was collected for the determination of possible memory effects. Table 2 shows the recoveries of elements in BCR 063 and BCR 151. The results agree with the certified values. No memory effects between samples and blanks could be observed. The high amount of lead in BCR 151 can only be explained by contamination. 4054 Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
bovine liver (BCR 185) elements
certified
Zn (µg/g) Cd (µg/g) Cu (µg/g) Fe (µg/g) Mn (µg/g) Mg (µg/g) Ca (µg/g) TOC (%)
142 ( 3 0.298 ( 0.025 189 ( 4 214 ( 5 9.3 ( 0.3 (634) (131)
measured (n ) 7) 142 ( 10 0.24 ( 0.1 186 ( 13 219 ( 8 10 ( 1 602 ( 41 140 ( 19 26 ( 3
pig kidney (BCR 186) certified 128 ( 3 2.71 ( 0.15 31.9 ( 0.4 299 ( 10 8.5 ( 0.3 (829) (295)
measured (n ) 10) 131 ( 2 2.5 ( 0.5 32 ( 4 303 ( 7 8.7 ( 0.4 889 ( 40 340 ( 10 34 ( 4
a Digestion parameters: 35 bar (245 °C), 110 W, and 5 min (flow rate 0.9 mL/min).
Additionally, slurries of two other standard reference materials (bovine liver BCR 185, pig kidney BCR 186) were digested at 35 bar corresponding to 245 °C, 5-min digestion time (flow rate 0.9 mL/min), and 110-W microwave power. The higher temperature and longer digestion time in comparison with milk powder depends on the matrixes liver and kidney, which are more difficult to oxidize than milk powder. The digest of a 5-mL slurry was collected in a total volume of 20 mL. Cu, Ca, Mg, Mn, Fe, Zn, and Cd were measured by means of ICP-OES. This results, shown in Table 3, also agree well with the certified values. Since Hg is a very volatile element, its behavior in the flow digestion system was also investigated. A solution consisting of 10% orange juice, 32% nitric acid, and 100 µg/L Hg was prepared and digested at pressure levels of 20 and 35 bar and digestion times of 1, 3, and 5 min (flow rate 4.7, 1.6, and 0.9 mL/min). The mercury was measured by means of the Hg-Mat. As shown in Table 4, no losses of Hg could be observed. Sample Throughput. The sample throughput is dependent on the sampling volume and the digestion time. The total volume per sample that has to be pumped through the reactor is 4 times the sampling volume. The dead volume of the microwave heated zone is ∼5 mL. Therefore the throughput with a 1-mL sampling volume and 2-min digestion time (flow rate 2.4 mL/min) is 37 samples/h. With a 5-mL sampling volume and 5-min digestion time (flow rate 0.9 mL/min), the throughput is only 3 samples/ h. The precondition for this calculation is the application of an eight-port sampling valve with two sample loops. Sample Dilution. The maximum amount of solid sample material digested in the flow system is limited by the possibility
Table 4. Recovery Studies of Hg at Various Pressures and Digestion Timesa pressure/temp (bar)/(°C)
digestion time/flow rate (min)/(mL/min)
recovery of Hg (%)
20/212 20/212 20/212 35/245 35/245 35/245
1/4.7 3/1.6 5/0.9 1/4.7 3/1.6 5/0.9
98 101 100 100 97 100
a
Microwave power 100 W; SD ) (4% (n ) 12).
of loading the slurry into the tube system (0.2 g in 5 mL). Assuming that the sample loop has a volume of 3 mL, the overall dilution of the sample is ∼0.12 g of sample in 15 mL of digest, leading to a concentration of 8 mg/mL.
CONCLUSION The described flow digestion system has two main advantages. The remarkable increase of the digestion temperature up to 245 °C by means of the novel pressure equilibration system raises the oxidation potential of nitric acid significantly. This enables the digestion of tough organic compounds and increases the speed of the oxidation reaction in general. This is of significant importance with flow digestion systems because of the short digestion time applied. The second remarkable advantage of this digestion device is the extremely short reaction time of the pressure equilibration system. In this way, violent oxidation reactions such as decomposition of glucose with concentrated nitric acid will not destroy Teflon digestion tubes and do not affect the digestion sequence of the equipment. Received for review March 22, 1999. Accepted June 11, 1999. AC9903048
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