Anal Bioanal Chem (2003) 376 : 715–720 DOI 10.1007/s00216-003-1962-6
O R I G I N A L PA P E R
B. Maichin · M. Zischka · G. Knapp
Pressurized wet digestion in open vessels
Received: 14 February 2003 / Revised: 2 April 2003 / Accepted: 4 April 2003 / Published online: 11 June 2003 © Springer-Verlag 2003
Abstract The High Pressure Asher (HPA-S) was adapted with a Teflon liner for pressurized wet digestion in open vessels. The autoclave was partly filled with water containing 5% (vol/vol) hydrogen peroxide. The digestion vessels dipped partly into the water or were arranged on top of the water by means of a special rack made of titanium or PTFEcoated stainless steel. The HPA-S was closed and pressurized with nitrogen up to 100 bars. The maximum digestion temperature was 250 °C for PFA vessels and 270 °C for quartz vessels. Digestion vessels made of quartz or PFATeflon with volumes between 1.5 mL (auto sampler cups) and 50 mL were tested. The maximum sample amount for quartz vessels was 0.5–1.5 g and for PFA vessels 0.2–0.5 g, depending on the material. Higher sample intake may lead to fast reactions with losses of digestion solution. The samples were digested with 5 mL HNO3 or with 2 mL HNO3+6 mL H2O+2 mL H2O2. The total digestion time was 90–120 min and 30 min for cooling down to room temperature. Auto sampler cups made of PFA were used as digestion vessels for GFAAS. Sample material (50 mg) was digested with 0.2 mL HNO3+0.5 mL H2O+0.2 mL H2O2. The analytical data of nine certified reference materials are also within the confidential intervals for volatile elements like mercury, selenium and arsenic. No cross contamination between the digestion vessels could be observed. Due to the high gas pressure, the diffusion rate of volatile species is low and losses of elements by volatilisation could be observed only with diluted nitric acid and vessels with large cross section. In addition, cocoa, walnuts, nicotinic acid, pumpkin seeds, lubrication oil, straw, polyethylene and coal were digested and the TOC values measured. The residual carbon content came to 0.2–10% depending on the sample matrix and amount. Keywords Wet digestion · Open vessel digestion · Pressurized digestion · Sample decomposition
B. Maichin · M. Zischka · G. Knapp (✉) Institute for Analytical Chemistry, Micro- and Radiochemistry, Graz University of Technology, Technikerstrasse 4, 8010 Graz, Austria e-mail:
[email protected]
Introduction Wet digestion in open vessels is wide spread in element analysis [1, 2]. The so called hot plate digestion is comparably cheap and allows high sample throughput as block digesters for simultaneous digestion of many samples are commercially available. Open vessel wet digestion however shows many disadvantages and systematic errors compared to closed vessel sample decomposition [3]. Low digestion temperature with nitric acid at ambient pressure and therefore inadequate oxidation capability are the reason for the application of sulphuric acid with all its problems in trace element analysis. The acid consumption is also comparably high causing elevated blank levels. Last but not least element losses by volatilisation and contamination by dust can be observed. Advantages are the digestion of high sample amounts and simple and cheap vessels made of quartz, PFA- or PTFE-Teflon. The goal of the investigation described in this paper is to combine the advantages of open vessel wet digestion and pressurized sample decomposition in closed vessels like low acid consumption, application of pure nitric acid without sulphuric acid, comparably high sample amount and simple vessel designs. To achieve this guideline the open digestion vessels are arranged in an autoclave and the autoclave is loaded with nitrogen at 80 bar. Afterwards the vessels are heated up to 270 °C without boiling of the digestion reagent consisting of nitric acid or mixtures of nitric acid with hydrochloric acid, hydrogen peroxide or hydrofluoric acid. The oxidation potential of nitric acid at temperatures ≥250 °C is high enough for fast and effective oxidation of organic compounds [4, 5]. For this new sample decomposition technique a High Pressure Asher (HPA-S) was used, which was equipped with a Teflon liner and special sample racks. Different vessel materials and vessel designs were investigated because of the fact that element losses with open vessel digestion may depend on the vessel shape [6].
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Experimental Reagents Nitric acid (69.5%), hydrochloric acid (37.5%), hydrofluoric acid (40%) and hydrogen peroxide (30%) pro analysis reagent grade were obtained from Merck (Darmstadt, Germany). Nitric acid and hydrochloric acid were purified in-house by sub boiling distillation using a quartz still. Standard solutions for As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, Sr, V and Zn were prepared from 1000 mg/L (2% HNO3 ) standard solutions (SPEX Plasma Standard, SPEX, Metuchen, NJ). Solutions were made up with distilled, deionised (18 MΩ) water (Barnstead Nanopure, Dubuque, IA). The certified reference materials CRM (former BCR) 61 (aquatic plant) and CRM (former BCR) 62 (olive leaves), CRM 184 (bovine muscle), CRM 185 (bovine liver), CRM 186 (pig kidney), CRM 278 (mussel tissue) and CRM 422 (cod muscle) were obtained from the European Commission Joint Research Centre, Institute for Reference Materials and Measurements (IRMM) (B-2440 Geel, Belgium) and NIST 1515 (apple leaves) and NIST 1635 (coal) came from National Institute of Standards and Technology, Gaithersburg, USA. Instrumentation High Pressure Asher (HPA-S), a high temperature digestion device for digestion pressure up to 140 bar (Anton Paar, Graz, Austria and Perkin Elmer, Norwalk, CT, USA). 15-mL quartz vessels cat.no.
70865 and 50-mL quartz vessels cat.no. 70868 with PTFE-stoppers (Anton Paar, Graz, Austria). 22-mL PFA vessels cat.no. 0275 flat bottom (25 mm i.d., 52 mm length) with screw cap, 4-mL PFA vessels cat.no. 8004 (14 mm i.d., 42 mm length) and 1.5-mL PFA auto sampler cups cat.no. 7215 (all three from Savillex, Minnetonka, MN, USA). A Shimadzu TOC-5050 total organic carbon analyser (Shimadzu Europe, Duisburg, Germany) was used for the determination of the residual organic carbon content. Determination of elements was carried out by ICP-OES (Optima 3000 XL) and GFAAS (AAnalyst 800), all from Perkin Elmer, Norwalk, CT, USA. Liner and sample racks Figure 1 shows a cross section of the HPA-S with the Teflon liner and a rack for quartz vessels. The liner is 5 mm thick and consists of a cylindrical part and a flat disc at the top. The liner prevents the cooling of the water bath by the air cooled stainless steel wall of the HPA-S. The heating element in the centre of the HPA-S is covered with water containing 5% (vol/vol) hydrogen peroxide for reduction of nitrous oxides, which are generated by the decomposition process. The vessel rack is a disc made of titanium or PTFEcoated stainless steel with concentric wholes for the quartz vessels or for the 4-mL PFA vessels and the PFA auto sampler cups. The 22-mL PFA vessels are simply arranged on top of the titanium disc and do not dip into the water bath. In this case the heat transfer takes place via the gas phase. All vessels are covered with Teflon stoppers. These Teflon stoppers do not seal the vessels, but prevent the entrance of droplets.
Results and discussion Optimised digestion program The time temperature program of the HPA-S was optimised for all vessel types. The results of this investigation show, that the 50-mL quartz vessels allow higher sample amounts to digest than the 22-mL PFA vessels, though the inner diameter of this quartz vessels is smaller (about 23 mm). Different sizes and shapes of PFA vessels have been tested, but the maximum sample intake is about 30–40% of the sample intake with comparable quartz vessels. The reason for this behaviour might be the lower heat transfer through the PFA leading to an overheating of the reaction mixtures and loss of the digestion solution. Therefore in addition to the lower sample intake a time–temperature program with a soft temperature ramp from 60–90 °C is advantageous to slow down the primary reaction. Standard time–temperature program for 50-mL quartz vessels: – 90 °C 5 min. 90 °C – 90 °C 60 min. 270 °C – 270 °C 30 min. 270 °C Standard time–temperature program for 15-mL quartz vessels and the PFA vessels: Fig. 1 Cross section of the HPA autoclave for pressurized open vessel sample digestion. 1 autoclave, 2 lid of the autoclave, 3 ring retainer, 4 rupture disc, 5 PTFE liner, 6 heating element, 7 thermocouple, 8 water bath, 9 quartz vessel, 10 PFA vessel, 11 PTFE lid for digestion vessel, 12 titanium vessel rack
– 60 °C 30 min. 90 °C – 90 °C 60 min. 250 °C – 250 °C 30 min. 250°C
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Recovery and contamination
Sample decomposition in quartz vessels
For recovery studies 0.2 g glucose spiked with element standard solutions were digested with 5 mL HNO3+0.2 mL HCl in quartz vessels following the standard time– temperature program for quartz vessels. The digestion residue was made up to 20 mL with distilled water. The element concentration in the final solution was 100 µg/L for Hg and 1 mg/L for As, Be, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Mo, Ni, Pb, Sb, Se, Sr, Ti, Tl, V and Zn. The recovery of these elements came to 96–100%. Se showed recoveries of 94±1%. For the determination of blank levels, two vessels loaded with the same material as for the determination of the recoveries and four vessels just filled with acid were digested. The results show slightly elevated blank values for Ca, Fe and Mg after sample digestion with quartz and PFA vessels. All other elements are below the detection limits of the axial ICP-OES in the low µg/L range. No cross contamination of volatile elements like Hg, As, or Se could be observed. The digestion procedures were carried out in a normal analytical lab without clean air facilities.
Certified reference materials were digested in 15-mL and 50-mL quartz vessels. Within the same digestion run vessels with pure digestion reagent (blank digestion) were arranged in the autoclave. Also in this case no cross contamination could be observed. Certified reference materials (0.2 g each) were digested with 2 mL HNO3+0.1 mL HCl in 15-mL quartz vessels. Certified reference materials (1 g each) were digested in 50-mL quartz vessels with 5 mL HNO3+0.2 mL HCl (a) and with 2 mL HNO3+6 mL H2O+2 mL H2O2+0.2 mL HCl (b). The analytical data given in Tables 1 and 2 agree with the certified values. The digestion of organic materials with diluted nitric acid has the advantage, that the digestion residue can be measured without further dilution. Clear and colourless digestion solutions are obtained, but the remaining carbon content is significantly higher. The remaining carbon content of all CRMs was <1% after digestion with concentrated nitric acid and 5–10% after digestion with diluted nitric acid.
Table 1 Digestion of certified reference materials in 15-mL quartz vessels (3 replicates). Sample weight 0.2 g; digestion reagents 2 mL HNO3+0.1 mL HCl. Digestion solution was diluted to 10 mL with distilled water Element
Certified value
Measured
NIST 1515 apple leaves Al 286±9 µg/g Ba 49±2 µg/g Ca 1.526±0.015% Cu 5.64±0.24 µg/g Fe 83±5 µg/g K 1.61±0.02% Mg 0.271±0.008% P 0.159±0.011% S 0.18% Zn 12.5±0.3 µg/g
283±2.4 µg/g 47±0.5 µg/g 1.54±0.04% 5.53±0.06 µg/g 78±2 µg/g 1.57±0.04 % 0.27±0.001% 0.16±0.005% 0.20±0.005 12.2±0.08 µg/g
CRM 422 cod muscle As 21.1±0.5 µg/g Cu 1.05±0.07 µg/g Fe 5.46±0.30 µg/g Hg 0.559±0.016 µg/g Mn 0.543±0.028 µg/g Se 1.63±0.07 µg/g Zn 19.6±0.5 µg/g
20.70±0.25 µg/g 1.07±0.08 µg/g 5.4±0.2 µg/g 0.548±0.004 µg/g 0.544±0.02 µg/g 1.65±0.05 µg/g 19.2±0.3 µg/g
CRM 186 pig kidney Cd 2.71±0.15 µg/g Cu 31.9±0.4 µg/g Fe 299±10 µg/g Hg 1.97±0.04 µg/g Mn 8.5±0.3 µg/g Se 10.3±0.5 µg/g Zn 128±3 µg/g
2.67±0.01 µg/g 31.8±0.6 µg/g 295±7 µg/g 1.95±0.015 µg/g 8.3±0.2 µg/g 9.9±0.2 µg/g 127±2.6 µg/g
Table 2 Digestion of certified reference materials in 50 mL quartz vessels (4 replicates). Sample weight 1.0 g; digestion reagents (a) 5 mL HNO3+0.2 mL HCl, (b) 2 mL HNO3+6 mL H2O+2 mL H2O2+0.2 mL HCl. Digestion solution was diluted to 20 mL with distilled water Element
Certified value
Measured (a)
Measured (b)
CRM 184 bovine muscle Cd 13±2 ng/g Pb 239±11 ng/g Hg 2.6±0.6 ng/g Cu 2.36±0.06 µg/g Zn 166±3 µg/g Fe 79±2 µg/g Mn 334±28 ng/g
14±2 ng/g 236±23 ng/g 2.3±0.2 ng/g 2.39±0.3 µg/g 164±1 µg/g 78±1 µg/g 326±6 ng/g
14.7±3 ng/g 228±14 ng/g 2.6±0.2 ng/g 2.41±0.3 µg/g 168±4 µg/g 82±2 µg/g 323±13 ng/g
CRM 185 bovine liver Cd 298±25 ng/g Pb 501±27 ng/g Hg 44±3 ng/g Se 446±13 ng/g Cu 189±4 µg/g Zn 142±3 µg/g Fe 214±5 µg/g Mn 9.3±0.3 µg/g
303±5 ng/g 497±41 ng/g 42±3 ng/g 438±33 ng/g 190±1 µg/g 141±2 µg/g 208±10 µg/g 8.8±0.2 µg/g
295±6 ng/g 501±37 ng/g 47±3 ng/g 445±34 ng/g 193±2 µg/g 143±3 µg/g 218±2 µg/g 8.8±0.2 µg/g
CRM 278 mussel tissue As 5.9±0.2 µg/g Cd 340±20 ng/g Cr 800±80 ng/g Cu 9.6±0.6 µg/g Fe 133±4 µg/g Hg 188±7 ng/g Mn 7.3±0.2 µg/g Pb 1.91±0.04 µg/g Se 1.66±0.04 µg/g Zn 76±2 µg/g
6.0±0.2 µg/g 320±10 ng/g 750±30 ng/g 9.6±0.1 µg/g 137±2 µg/g 191±10 ng/g 7.0±0.2 µg/g 1.88±0.05 µg/g 1.62±0.1 µg/g 77±1 ng/g
5.7±0.1 µg/g 320±5 ng/g 800±20 ng/g 9.3±0.1 µg/g 135±2 µg/g 183±7 ng/g 7.1±0.1 µg/g 1.83±0.02 µg/g 1.63±0.05 µg/g 77±1 ng/g
718 Table 3 Digestion of certified reference materials in 25-mL PFA vessels (4 replicates). Digestion reagents (a) 5 mL HNO3+0.2 mL HCl, (b) 1 mL HNO3+3 mL H2O+1 mL H2O2+0.2 mL HCl, and (c) 5 mL HNO3+0.5 mL HF+ 0.5 mL HCl,. The digestion residue was diluted to 20 mL with distilled water
Element
Certified value
Measured (a)
Measured (b)
CRM 62 olive leaves Cd 0.10±0.02 µg/g Cu 46.6±1.8 µg/g Hg 0.28±0.02 µg/g Mn 57±2.4 µg/g Pb 25±1.5 µg/g Zn 16±0.7 µg/g
0.10±0.02 µg/g 46.6±0.4 µg/g 0.26±0.02 µg/g 55.6±0.6 µg/g 24.3±0.4 µg/g 17±2 ng/g
0.08±0.01 µg/g 45.8±0.5 µg/g 0.16±0.01 µg/g 51.3±0.5 µg/g 22.0±0.2 µg/g 15±1 ng/g
CRM 422 cod muscle As 21.1±0.5 µ/g Cd 17±2 ng/g Cu 1.05±0.07 µ/g Fe 5.46±0.30 µg/g Hg 0.559±0.016 µg/g Mn 5.43±0.028 µg/g Se 1.63±0.07 µg/g Zn 19.6±0.5 µg/g
20.8±0.5 µg/g 19±3 ng/g 1.06±0.03 µg/g 5.85±0.6 µg/g 0.557±0.02 µg/g 5.72±0.08 µg/g 1.68±0.07 µg/g 18.8±0.2 µg/g
18.0±0.4 µg/g 14±4 ng/g 1.07±0.08 µg/g 3.65±0.6 µg/g 0.492±0.02 µg/g 4.92±0.15 µg/g 1.60±0.13 µg/g 137.9±0.5 µg/g
Measured (c)
CRM 61 aquatic plant Cd 1.07±0.08 µg/g Cu 720±31 µg/g Hg 0.23±0.02 µg/g Mn 3771±78 µg/g Pb 64±3.5 µg/g Zn 566±13 µg/g
0.98±0.04 µg/g 683±23 µg/g 0.22±0.01 µg/g 3630±108 µg/g 59±1.3 µg/g 564±19 µg/g
NIST 1635 coal Al 0.32% Cr 2.5±0.3 mg/kg Cu 3.6±0.3 mg/kg Fe 0.239±0.005% Mn 21.4±1.5 mg/kg Na 0.24% Ni 1.74±0.3 mg/kg S 0.362±0.002% Ti 0.02% V 5.2±0.5 mg/kg Zn 4.7±0.5 mg/kg
0.31±0.01% 2.4±0.1 mg/kg 3.8±0.3 mg/kg 0.236±0.014% 20.8±1.2 mg/kg 0.23±0.004% 1.80±0.1 mg/kg 0.340±0.015% 0.019±0.001% 5.4±0.3 mg/kg 4.6±0.2 mg/kg
Sample decomposition in PFA vessels Two sizes of PFA vessels with screw caps were tested, 25-mL vessels for a sample intake up to 0.5 g and 4-mL vessels for the digestion of 0.1 g organic material with 2 mL HNO3. Up to 15 of the smaller PFA vessels can be loaded in the HPA-S. With the 25-mL PFA vessels 0.5 g each of CRM 62 (olive leaves) and CRM 422 (cod muscle) were digested with 5 mL HNO3+0.2 mL HCl (a) and 1 mL HNO3+3 mL H2O+1 mL H2O2+0.2 mL HCl (b). 0.5 g CRM 61 (aquatic plant) and 0.4 g NIST 1635 (coal) were digested with 5 mL HNO3+0.5 mL HF+0.5 mL HCl (c). These analytical data are presented in Table 3. The digestion with diluted nitric acid leads to element losses. The reason for this might be the larger cross section of the PFA vessels in comparison to the quartz vessels. In combination with the higher vapour pressure of the diluted acid significant losses of elements by volatilisation is the consequence.
The residual carbon content of all CRMs digested with 5 mL HNO3 was <1% and 5–10% after digestion with diluted acid. Sample decomposition in PFA auto sampler cups The 1.5-mL auto sampler cups are useful for decomposition of small sample sizes. Twenty sample cups can be loaded into the HPA-S, which enables a high sample throughput. CRM 62 (olive leaves), CRM 185 (bovine liver), CRM 278 (Mussel tissue) and CRM 422 (cod muscle) were digested with diluted acid and can be directly analysed afterwards by means of GFAAS. For measurement with ICP-OES, ICP-MS or flame AAS the digest has to be diluted. Table 4 shows the analytical results of 50 mg CRM digested with 0.2 mL HNO3+0.5 mL H2O+0.2 mL H2O2 and analysed with GFAAS, ICP-OES and CVAAS. The TOC values of all digested samples were ≤1.0%.
719 Table 4 Digestion of certified reference materials in 1.5-mL PFA auto sampler cups (3 replicates). Sample weight 50 mg; digestion reagent 0.2 mL HNO3+0.5 mL H2O+0.2 mL H2O2 Element
Certified value
Measured
CRM 62 olive leaves Cd 0.1±0.02 µg/g Cu 46.6±1.8 µg/g Hg 0.28±0.02 µg/g Mn 57±2.4 µg/g Pb 25±1.5 µg/g Zn 17±0.7 µg/g
0.08±0.02 µg/g 46.6±1.2 µg/g 0.26±0.01 µg/g 59±1.9 µg/g 25±0.3 µg/g 16.6±0.6 µg/g
CRM 185 bovine liver Cd 298±25 ng/g Cu 189±4 µg/g Fe 214±5 µg/g Mn 9.3±0.3 µg/g Se 446±13 ng/g Zn 142±3 µg/g
278±25 ng/g 191±7 µg/g 201±9 µg/g 9.2±0.4 µg/g 429±22 ng/g 141±6 µg/g
CRM 278 mussel tissue As 5.9±0.2 µg/g Cd 0.34±0.02 µg/g Cu 9.60±0.6 µg/g Fe 133±4 µg/g Hg 0.188±0.007 µg/g Mn 7.3±0.2 µg/g Pb 1.91±0.04 µg/g Se 1.66±0.04 µg/g Zn 76±2 µg/g
5.8±0.2 µg/g 0.32±0.02 µg/g 9.6±0.5 µg/g 133±5 µg/g 0.168±0.01 µg/g 7.1±0.2 µg/g 1.89±0.015 µg/g 1.57±0.08 µg/g 74±2 µg/g
CRM 422 cod muscle As 21.1±0.5 µg/g Cd 0.017±0.002 µg/g Hg 0.559±0.016 µg/g Se 1.63±0.07 µg/g Zn 19.6±0.5 µg/g
21.5±0.9 µg/g 0.011±0.001 µg/g 0.508±0.018 µg/g 1.53±0.05 µg/g 20.3±0.9 µg/g
Conclusion
Residual carbon content It is important to know about the residual carbon content (TOC value) of the digestion solution because of the interfering effects with some measurement methods. Inverse Table 5 Residual carbon content after digestion of various organic materials with (a) 15-mL quartz vessels and 2 mL HNO3, (b) 50-mL quartz vessels and 5 mL HNO3 and with 25-mL PFA vessels with 5 mL HNO3 and a mixture of 5 mL HNO3+0.5 mL HCl+ 0.5 mL HF for coal. The residual carbon content (TOC) is given in % of the original carbon
Sample material
Cocoa Pumpkin seeds Lubrication oil Polyethylene Polyethylene Polypropylene Nicotinic acid Bovine liver Bovine liver Sunflower oil Human blood
voltammetry is very sensitive but also ICP-MS show some interferences with carbon containing cluster ions. The residual carbon content of the digestion solution depends on the sample matrix, the digestion temperature, the digestion reagents and the ratio of reagent volume and sample mass. The residual carbon contents after the predominantly used microwave assisted wet digestion in medium pressure vessels at about 200 °C are between 5–20%. Such TOC values may interfere with the measurement. To get an idea about the residual carbon content after sample digestion with the method described, Table 5 show the results for different sample materials, acid mixtures and vessel types. The 4-mL PFA digestion vessels enables the digestion of 0.1 g organic material with 2 mL HNO3. The residual carbon content for all the organic materials tested was ≤3%. This vessel size is useful for small biological samples. Up to 15 samples can be digested simultaneously. Comparably to the other vessels the analytical results of CRMs agree with the certified values.
The pressurized open vessel digestion technique is powerful and easy to handle. In comparison to closed vessel microwave digestion systems the vessels made of quartz or PFA are simple and inexpensive. The analytical data obtained with pressurized open vessel digestion are as good as the results obtained with microwave assisted closed vessel digestion. No cross contamination between the vessels and losses of volatile elements could be observed. The comparison of quartz and PFA vessels showed, that quartz vessels enables the digestion of higher sample amounts. The number of samples digested simultaneously depends on the vessel size and the dimension of the autoclave used. Actually the HPA-S was applied for pressurized open vessel digestion, but other high pressure autoclaves can also be used like autoclaves with conductive heating from Parr Instrument Company, Moline, Illinois 61265 USA, or the ultraClave 2, a high pressure autoclave with microwave heating from MLS, Leutkirch, Germany.
(a)
(b)
(c)
Sample weight (g)
TOC (%)
Sample weight (g)
TOC (%)
Sample weight (g)
TOC (%)
0.2 0.2 0.05 0.1
0.35±0.05 0.26±0.01 0.87±0.1 0.06±0.01
1.0 1.0 0.5 0.5 0.75
1.1±0.1 2.2±0.2 3.6±0.4 0.4±0.2 2.8±1.8
0.5 0.4 0.2 0.2
0.71±0.04 1.7±0.06 1.9±0.36 0.06±0.005
0.1 0.2 0.1
0.36±0.03 0.9±0.3 0.26±0.04
1.0
8.2±0.2
0.2 0.5 0.5
0.35±0.11 0.19±0.08 2.0±0.3
0.3
0.45±0.13
1.0
0.5±0.05
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