Article pubs.acs.org/EF
Combination of Dispersive Liquid−Liquid Microextraction and Emulsion Breaking for the Determination of Cu(II) and Pb(II) in Biodiesel and Oil Samples Lucas C. Lima,† Thiago R. L. C. Paixaõ ,‡ Cassiana S. Nomura,‡ and Ivanise Gaubeur*,† †
Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Av. dos Estados 5001, 09210-971 Santo André, São Paulo, Brazil ‡ Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000 São Paulo, São Paulo, Brazil ABSTRACT: Trace element determination in oil and biodiesel samples is not a simple task. To improve detectability and minimize the matrix effect, a preconcentration procedure before analysis is often required. In this work, we apply dispersive liquid−liquid microextraction to the determination of copper and lead in biodiesel and oil samples. The complexes formed between Cu(II), Pb(II), and di-2-pyridyl ketone salicyloylhydrazone were extracted using 1-undecanol as an extractant solvent and ethanol as a disperser solvent. The concentration of analytes was determined using graphite furnace atomic absorption spectrometry after matrix modification and Zeeman background correction. The complexant concentration, pH value, and extractant solvent volume were optimized using a multivariate consisting of a central composite design and a univariate analysis. Under optimum complexation and microextraction conditions (pH 9.2, 50 μL of extractant, and 150 μL of disperser solvent), calibration curves were obtained with linear ranges of 1.0−25 μg L−1 (Cu(II)) and 1.0−40 μg L−1 (Pb(II)) and limits of detection of 0.23 and 0.24 μg L−1 for Cu(II) and Pb(II), respectively. Before microextraction, an emulsion-breaking procedure was applied in biodiesel and oil samples by mixing 100 mg of sample with 100 μL of hexane, 8% (w/v) Triton X-114, and 3.3 mol L −1 HNO3. The accuracy of the proposed method was confirmed through the analysis of two standard reference materials (B100 biodiesel soy-based and Conostan oil analysis standards) and mineral oil and corn biodiesel samples.
1. INTRODUCTION In recent decades, as fossil fuels have become more scarce over time, attention has turned to green and renewable fuels. Among the available renewable fuels, biodiesel can be used for existing diesel engines without the need for significant engine modifications. This fuel can be produced from various feedstocks, such as vegetable oils (edible or inedible), animal fats, and used cooking oil, and a transesterification process is required to reduce the viscosity of the oil to be used as the fuel source. Although biodiesel is advantageous over petroleumbased diesels in many regards, it has also been proven to be disadvantageous due to the feedstock’s high cost as a result of its limited availability and low stability, as a result of its high susceptibility to oxidation and/or autoxidation in long-term storage.1−6 Automotive lubricant oil and fuel quality control testing includes metal determination due to its influence on the fuel quality and motor performance. Standards ASTM D 6751 (biodiesel standard) and EN 14214 (European biodiesel standard) have some specifications for biodiesel which include, among others, the metals of group I (Na + K) and group II (Ca + Mg). Trace metals such as copper and lead, for example, are introduced into biodiesel and other fuels during the production process (from raw material, for instance), handling, and storage. When some components found in biodiesel and other fuels are exposed to air at elevated temperatures in the presence of metal ions, degradation products can be generated, resulting in operational and environmental problems as, for example, © XXXX American Chemical Society
organic lead compounds, such as tetraethyllead, are highly toxic.6−9 On this basis, metal determination in such matrixes is required; however, this kind of analysis is not common, and a sample preparation step is required.10 This step can be carried out by either a simple dilution in an adequate organic solvent with the aim of reducing the viscosity or a complete sample digestion in an acid medium using a microwave oven.11 Despite the sample dilution in organic solvent being quick and easy, this procedure has some drawbacks, such as time consumption, incompatibility with the detection method, and a lack of standards, with similar chemical characteristics, for obtaining a calibration curve. The second drawback can be overcome when the sample is digested within an acid medium, as standards in aqueous media can be used to obtain calibration curves. As the sample preparation for oils and biodiesel presents a challenge in analytical chemistry, some procedures have been proposed. The formation of emulsions and microemulsions among these samples and surfactant (in an acid medium) has become an attractive alternative for metal determination.12,13 Emulsion breaking is another alternative procedure, and it is based on heating or centrifugation of this emulsion.14,15 In spite of some advancements in analytical instrumentation, metals in low concentrations, such as Cu(II) and Pb(II), for example, in complex matrixes, such as oil and biodiesel, require Received: May 17, 2017 Revised: August 10, 2017
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DOI: 10.1021/acs.energyfuels.7b01430 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
2.3. Samples. For accuracy evaluation of the proposed method, a standard reference material, B100 biodiesel soy-based NIST 2772, obtained from the National Institute of Standards and Technology (Gaithersburg, MD), and a multielement oil analysis standard, S-21 Conostan (SCP SCIENCE, Quebec, Canada), were used. Mineral oil sample was purchased from a local market and cornbased biodiesel sample was prepared by a transesterification reaction with corn oil and methanol with potassium hydroxide as the catalyst. Samples were maintained in tightly closed vessels in the absence of light.24 2.4. Emulsion-Breaking Procedure. The emulsion was obtained from a mixture of 100 mg of sample, 100 μL of hexane, 800 μL of Triton X-114 (10%, w/v), and 200 μL of HNO3 (65%) containing different quantities of reference solutions (1 mg L−1 Cu(II) and Pb(II)), except for Conostan, in a 15 mL graduated tube. The emulsion was formed through mixture agitation for 15 min, and the phase separation (emulsion breaking) was performed by centrifugation at 3600 rpm (centrifugal force estimated as 840g) for 15 min. The aqueous acid phase (approximately 1 mL) was removed with a micropipet and transferred to a 10 mL graduated glass tube. 2.5. Dispersive Liquid−Liquid Microextraction Procedure. To a reference solution (Cu(II) + Pb(II)) or to the sample obtained after the emulsion-breaking procedure (section 2.4) were added 68 μL of 1.0 × 10−2 mol L−1 DPKSH, 450 μL of 1 mol L−1 NH4OH/NH4Cl buffer solution (pH 9.2), and enough water for a total volume of 9 mL. Using a 1 mL syringe, a mixture of 50 μL of 1-undecanol and 150 μL of ethanol was injected rapidly into the mixture. A cloudy solution was formed in the glass tube, and the formed Cu(II)−DPKSH and Pb(II)−DPKSH complexes were extracted into fine droplets of 1undecanol. Then the solution was centrifuged at 3600 rpm (centrifugal force estimated as 840g) for 5 min, and the droplets of 1-undecanol floating in the glass tube (about 45 μL) were removed and diluted to 150 μL using anhydride ethanol. A blank solution was obtained by the same procedure described above without the addition of samples or reference solutions. 2.6. Optimization of the GF AAS Temperature Program. The thermal behaviors of the analytes were evaluated by pyrolysis and atomization temperature curves after the liquid−liquid extraction procedure. For this, 3 mL of solution containing 20 and 40 μg L−1 concentrations of Cu(II) and Pb(II), respectively, was mixed with 1 mL of 2.0 × 10−2 mol L−1 DPKSH in 1-undecanol, and the mixture was stirred for 4 min. After phase separation, aliquots of 20 μL of the mixture were injected for GF AAS using a microsyringe. A volume of 5 μL of chemical modifier containing 1 g L−1 Pd and 0.5 g L−1 Mg was added for Cu(II) and Pb(II) measurements. The heating program for metal determinations after DLLME is presented in Table 1.
separation and preconcentration steps before detection. In this paper we report, for the first time, the use of dispersive liquid− liquid microextraction (DLLME)16−19 combined with emulsion breaking for determining Cu(II) and Pb(II) in oil and biodiesel samples. Reverse dispersive liquid−liquid microextraction was used by López-Garciá et al. for the determination of Cd and Pb20 and arsenic species21 in edible oils. The di-2-pyridyl ketone salicyloylhydrazone (DPKSH) is a complexing reagent that has been used in the spectrophotometric determination of metal ions, including Fe(II), Fe(III), Zn(II), and Cu(II). In view of its hydrophobic nature and reactivity, DPKSH was used in cloud point extraction aimed to determine Ni, Cd, and Pb in different samples.22,23 DPKSH was used in this study as a complexing reagent of Cu(II) and Pb(II), and it has been combined with DLLME for the first time. On this basis, this work proposes a DLLME procedure based on the use of 1-undecanol and ethanol as extractant and disperser solvents, respectively, for the extraction of Cu(II) and Pb(II) ions as DPKSH complexes combined with graphite furnace atomic absorption spectrometry (GF AAS) for trace elemental analysis of mineral oil and biodiesel samples. The main experimental factors affecting complexation and microextraction were optimized using multivariate and univariate analyses. To apply the proposed method to mineral oil and biodiesel samples, DLLME was combined with an emulsionbreaking procedure. In addition, the main experimental factors affecting the emulsion breaking (surfactant and acid concentrations and solvent volume) were also evaluated and optimized. To show the applicability of the proposed method, it was applied to standard reference materials (B100 biodiesel soy-based SRM NIST 2772 and multielement oil analysis standard S-21 Conostan) and mineral oil and biodiesel samples.
2. MATERIALS AND METHODS 2.1. Instrumentation. A Metrohm pH meter (model 713) with a combined glass electrode was used for pH measurements. A shaker (Marconi model MA 140) and a centrifuge (Quimis model Q222TM) were used in the emulsion-breaking and DLLME procedures. A graphite furnace atomic absorption spectrometer (Zeenit 600 model, Analytik Jena AG, Jena, Germany) equipped with hollow copper and lead cathode lamps (operated at 324.8 and 283.3 nm and 5 and 10 mA with spectral bandpasses of 0.2 and 0.8 nm, respectively), a Zeeman effect background corrector, and a pyrolytically coated transverse-heated graphite tube were used throughout the process. Argon (99.998%, v/v) was used as the purge gas. All measurements were performed using the integrated absorbance (peak area) and carried out in triplicate. 2.2. Reagents and Solutions. Analytical-grade chemicals were used, and solutions were prepared with deionized water or ethanol as a solvent. A DPKSH 1.0 × 10−2 mol L−1 stock solution was prepared by dissolving appropriate amounts of the reagent in anhydrous ethanol (Carlo Erba, Milan, Italy). 1-Undecanol (Sigma-Aldrich, St. Louis, MO) and ethanol (Carlo Erba) were used to perform the DLLME procedure. Buffer solutions (Merck, Darmstadt, Germany, or SigmaAldrich) were prepared from CH2ClCOONa−CH2ClCOOH at pH 2.7, CH3COONa−CH3COOH at pH 3.8, Na2HPO4−NaH2PO4 at pH 6.5 and 7.2, NH4OH−NH4Cl at pH 8.5, 9.2 and 9.7, and NaHCO3− Na2CO3 at pH 10.3. Copper and lead reference solutions were prepared by suitable dilutions of a 1000 mg L−1 stock solution (Ultra Scientific, United States). For determination of lead by GF AAS, 5 μL of chemical modifier solution (1 g L−1 Pd + 0.5 g L−1 Mg) was prepared by dilution of the appropriate stocks (Suprapur, Merck). For the emulsion-breaking procedure, solutions of the nonionic surfactant Triton X-114 (Sigma-Aldrich) prepared at a 10% (w/v) concentration, nitric acid (65%, Sigma-Aldrich), and hexane (Carlo Erba) were used.
3. RESULTS AND DISCUSSION Before evaluation of the main experimental factors affecting complexation and microextraction, an optimization of the GF AAS temperature program was performed. Using the selected GF AAS program temperature, the extractant solvent volume, Table 1. Graphite Furnace Program for Cu(II) and Pb(II) Determination after DLLME temp (°C) step
Cu
Pb
temp ramp (°C/s)
hold time (s)
Ar flow rate (L min−1)
predrying drying pyrolysisa atomizationb cleaning
100 270 800 2100 2300
100 270 1200 2100 2300
5 10 50 1500 1300
15 15 20 3 3
1.0 1.0 1.0 0 1.0
a Pyrolysis temperature range: 500−1300 °C (Cu) and 500−1500 °C (Pb). bAtomization temperature range 1800−2500 °C (Cu) and 1800−2200 °C (Pb).
B
DOI: 10.1021/acs.energyfuels.7b01430 Energy Fuels XXXX, XXX, XXX−XXX
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2100 and 2500 °C atomization temperatures in the presence or absence of the chemical modifier. On this basis, it was decided that the temperature of atomization would be maintained at 2100 °C. As observed in Figure 1a, the pyrolysis and atomization curve behavior did not change in the presence or absence of the chemical modifier. It was decided that the heating program without chemical modifier addition would be adopted for further experiments. Due to the fact that the pyrolysis and atomization curves have different behaviors and a significant decrease in integrated absorbance values without the chemical modifier, it was mandatory to include the addition of the Pd + Mg for lead detection, as shown in Figure 1b. The higher absorbance values for lead in the pyrolysis curves were achieved between 1000 and 1200 °C, so due to the standard derivation values, peak shapes, and background signal observed, a pyrolysis temperature of 1200 °C was chosen. For the atomization curve (Figure 1b), a lower standard deviation was achieved at 2100 °C, so this temperature was fixed. 3.2. Selection of the Variables of the DLLME Procedure by Factorial Design. To achieve the best analytical figures of merit for the proposed method and experimental conditions, a central composite design with three replicates of the center points and three factors, improved with a group of axial points (star points), was used. The order of the design points was randomized, and the total number of experiments was 17. In previous factorial designs, we found no significant values for the disperser solvent, so the ethanol volume was fixed at 150 μL. Table 2 shows the factors studied and their respective levels, and Table 3 gives the full factorial design matrix and integrated absorbance values.
pH, and complexant (DPKSH) concentration were assessed. Under optimum complexation and DLLME conditions, the emulsion-breaking variables were also evaluated: hexane volume, concentration of the emulsifier agent (Triton X-114), and nitric acid concentration. 3.1. Heating Program Temperature. The influence of the pyrolysis and atomization temperatures on the absorbance in GF AAS was the first parameter studied in this procedure. The study was carried out with an aliquot of 20 μL of 1undecanol containing Pb(II)−DPKSH and Cu(II)−DPKSH (section 2.6). Both studies were carried out in the presence and absence of the chemical modifier (Pd + Mg). Peak shapes, background signals, and standard deviations were used to choose the heating program conditions. Figure 1 shows the pyrolysis and atomization absorbance curves for Cu(II) (Figure 1a) and Pb(II) (Figure 1b).
Table 2. Amounts and Levels of Parameters Used in the Central Composite Design parameter 1-undecanol vol (μL) [DPKSH] (μmol L−1) pH
level −1
level 0 level +1
star point low
star point high
50
75
100
33
117
10
36
66
0
78
4.7
7.6
9.2
3.8
11.4
Using the results displayed in Table 3, a Pareto chart (Figure 2) and a second-order polynomial regression equation for Cu(II) and Pb(II) were obtained to fit the experimental values from the design using factors with a p value higher than 0.05. The R2 values of the predicted model for Cu(II) and Pb(II), respectively, were 0.7192 and 0.5238, showing a lack of fit for a second-order polynomial regression to postulate a prediction model for the experimental values. Hence, and on the basis of the Pareto chart in Figure 2, the most significant variables were selected by the central composite design, where the pH value and DPKSH concentration were selected for Cu(II) complexation and microextraction, and no significant variables were selected for Pb(II) complexation and microextraction. On the basis of these observations and the previous work from our research group using DPKSH as the reagent for Pb(II) and Cd(II) complexation,23 we decided to evaluate the DPKSH concentration and pH value for Cu(II) and Pb(II) using a univariate approach maintaining 50 μL of 1-undecanol and 150 μL of ethanol as the extractant and disperser solvents, respectively.
Figure 1. Pyrolysis and atomization temperature curves for (a) 20 μg L−1 Cu (II) in 1-undecanol phase with (●) and without (■) chemical modifier. (b) 40 μg L−1 Pb(II) in the 1-undecanol phase with (●) and without (■) chemical modifier.
For the pyrolysis temperature curves, higher integrated absorbance values were reached at 500 and 600 °C without and with chemical modification, respectively (Figure 1a). Due to the background signal and soot formation at pyrolysis temperatures below or equal to 700 °C, a temperature of 800 °C was fixed to obtain the maximum pyrolysis efficiency. The higher integrated absorbance values were achieved between C
DOI: 10.1021/acs.energyfuels.7b01430 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
The 1-undecanol volume was fixed at 50 μL since, as per the Pareto chart, Figure 2, this microextraction variable is not significant to Cu(II) and Pb(II). An increase in the extractant solvent volume could lead to a dilution effect, thus decreasing the preconcentration factor.18 3.3. Optimization of the DLLME Procedure by the Univariate Approach. The pH value was varied from 2.7 to 10.3, and the DPKSH concentration was varied between 5 and 85 μmol L−1. The results for pH variation are shown in parts a and b of Figure 3 and the results for DPKSH concentration are shown in parts c and d of Figure 3 for Cu(II) and Pb(II), respectively. As noted in Figure 3a, for Cu(II), the value of the integrated absorbance is increased to pH 7.2, and after this value, a slight decrease can be seen. For Pb(II), the integrated absorbance value was increased significantly at pH 7.2, leading to a maximum value at pH 9.2. The complexation of Cu(II) and Pb(II) by DPKSH is unfavorable in the acid medium, most probably due to the protonation of one nitrogen from pyridine, a possible ligand coordination site (pK1 = 3.5), becoming much more favorable at pH > 6.85, where the DPKSH molecule is deprotonated (pK2 = 6.85).25 The significant reduction in the integrated absorbance value for Pb over pH 9.2 can be attributed to metal hydrolysis, which can also occur for Cu(II) at such pH values. On the basis of the results shown in Figure 3b, the pH value is critical and must be carefully checked. With the objective of carrying out complexation and microextraction simultaneously for both metals, a pH of 9.2 was chosen for further experiments. The amount of DPKSH may be enough to ensure the complexation of the analytes of interest in the presence of other metals, which also form complexes with DPKSH. Figure 3c shows that the values of the integrated absorbance did not have any significant variations within the range from 5 to 75 μmol L−1 for Cu(II). The variation was more significant for Pb(II), as shown in Figure 3d, up to 75 μmol L−1. On the basis of the results and with the aim of achieving a commitment condition for complexing both metals, a DPKSH concentration equivalent to 75 μmol L −1 was selected, which represents an approximately 150-fold DPKSH excess. On the basis of the results and simultaneous complexation and microextraction, the following conditions were considered: pH 9.2, 75 μmol L−1 DPKSH, 50 μL of 1-undecanol, and 150 μL of ethanol. 3.4. Optimization of the Emulsion-Breaking Procedure. The emulsion breaking can be performed either by heating or by centrifugation,14,15 and for this study, we opted for emulsion breaking by centrifuging the surfactant mixture (in an acid medium) and samples for 15 min. With the aim of reducing the sample viscosity and making the formation of the emulsion easier, the addition of different volumes of hexane (0−600 μL) was assessed, and after observation of the emulsion formation and breaking, it was decided that the volume of hexane would be maintained at 100 μL. The concentration of the emulsifier agent (Triton X-114) was also evaluated with a variation of the Triton X-114 concentration in the range from 2% to 10% (w/v). We noticed that the best conditions for emulsion formation and breaking, under a fixed time (30 min in total), were obtained by maintaining the concentration of Triton X-114 at 8% for both metals. The concentration of nitric acid, responsible for metal extraction for the aqueous solution, was assessed by varying the same between 1.6 and 3.3 mol L−1. As there was no observation
Table 3. Results for the Central Composite Design Fixing 150 μL of Disperser Solvent integrated absorbance (s)
1 2 3 4 5 (C) 6 7 8 9 10 (C) 11 12 13 14 15 16 17 (C)
pH
[DPKSH] (μmol L−1)
1-undecanol vol (μL)
Cu
Pb
4.70 4.70 9.20 9.20 7.60 4.70 4.70 9.20 9.20 7.60 3.80 11.40 7.60 7.60 7.60 7.60 7.60
10 66 10 66 36 10 66 10 66 36 36 36 0 78 36 36 36
50 100 100 50 75 100 50 50 100 75 75 75 75 75 33 117 75
0.1192 0.7551 0.1263 0.3236 0.9635 0.1895 0.5452 0.1876 0.1571 0.8002 0.1983 0.2524 0.1268 1.003 0.7927 1.153 1.012
0.0093 0.0097 0.06510 0.2142 0.0196 0.01093 0.01200 0.004112 0.06022 0.00787 0.01144 0.02192 0.004503 0.01067 0.2590 0.01049 0.01491
Figure 2. Pareto chart for estimating the effects of the pH, DPKSH concentration, and extractant volume. The fixed pyrolysis and atomization temperatures were 800 and 2100 °C (Cu(II)) and 1200 and 2100 °C (Pb(II)), respectively.
D
DOI: 10.1021/acs.energyfuels.7b01430 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Effect of the pH value on the analytical signal for (a) 20 μg L−1 Cu(II) and (b) 40 μg L−1 Pb(II) after the DLLME procedure using 50 μL of 1-undecanol, 150 μL of ethanol, and 85 μmol L−1 DPKSH (DLLME procedure as previously described) and effect of the DPKSH concentration on the analytical signal for (c) 20 μg L−1 Cu(II) and (d) 40 μg L−1 Pb(II) after the DLLME procedure using 50 μL of 1-undecanol, 150 μL of ethanol, and pH 9.2 (DLLME procedure as previously described). The fixed pyrolysis and atomization temperatures were 800 and 2100 °C (Cu(II)) and 1200 and 2100 °C (Pb(II)), respectively.
Table 4. Figures of Merit for the Proposed Method Cu(II)
Pb(II)
parameter
GF AAS−DLLME
GF AAS
GF AAS−DLLME
GF AAS
linear concn range (μg L−1) coefficient of determinationa slopea,b LOD (μg L−1) LOQ (μg L−1) RSDc (%) (n = 10) relative sensitivityd
1.0−25 0.9970 0.105 ± 0.009 0.23 0.76 1.5
3.0−30 0.9998 0.006 ± 0.001 0.70 2.3 0.9
1.0−40 0.9957 0.0063 ± 0.0008 0.24 0.81 5.2
5.0−40 0.9994 0.0025 ± 0.014 0.81 2.7 2.7
18
2.5
Number of calibration points n = 5. Value ± standard deviation. Relative standard deviation: n = 10, [Cu(II)] = 20 μg L−1, and [Pb(II)] = 30 μg L−1. dSlope for GF AAS−DLLME/slope for GF AAS. The fixed pyrolysis and atomization temperatures were 800 and 2100 °C (Cu(II)) and 1200 and 2100 °C (Pb(II)), respectively. a
b
c
of a significant variation of the integrated absorbance of 0.9892 ± 0.0023 and 1.1097 ± 0.0049 (Cu(II)) and 0.2619 ± 0.0149 and 0.2394 ± 0.0237 (Pb(II)), respectively, due to the concentration of the acid, the latter was kept at 3.3 mol L−1. The concentrations of Triton X-114 and nitric acid selected as being ideal for the emulsion formation procedure in this study are similar to those in other studies, which present
extensive details about these variables for metal determination after emulsion breaking in diesel,14 lubricating oil,15 biodiesel,26 and edible oil27 samples. 3.5. Figures of Merit and Application to the Samples. After the DLLME and emulsion-breaking optimization, calibration curves were obtained by preconcentration of the reference solutions containing Cu(II) and Pb(II) with E
DOI: 10.1021/acs.energyfuels.7b01430 Energy Fuels XXXX, XXX, XXX−XXX
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Table 5. Determination and Recovery of Cu(II) and Pb(II) at Different Levels of Intentional Contamination in Real Samples amt added (μg g−1) sample mineral oil
corn biodiesel
B100 biodiesel soy-based SRM NIST 2772
Cu
Pb
0 13 15 20 0 13 15 20 0 13 15 20
0 20 25 40 0 20 25 40 0 20 25 40
amt founda (μg g−1) Cu 1.1 13.3 15.3 20.5 1.0 12.6 16.0 22.3 2.4 14.1 13.3 19.9 101.3
Conostan
± ± ± ± ± ± ± ± ± ± ± ± ±
recovery (%) Pb
0.1 0.5 0.7 0.2 0.2 0.9 0.2 0.1 0.1b 0.8 0.2 0.3 1.0c
0.9 22.0 27.9 38.9 1.0 23.1 25.7 39.3 0.8 21.5 28.4 40.4 100.3
± ± ± ± ± ± ± ± ± ± ± ± ±
0.2 0.2 0.7 1.0 0.1 0.9 2.0 1.1 0.1 0.4 1.2 0.8 1.1c
Cu
Pb
94 ± 4 95 ± 5 97 ± 1
106 ± 1 108 ± 3 95 ± 3
88 ± 9 115 ± 2 98 ± 1
110 ± 4 113 ± 2 96 ± 3
90 ± 6 113 ± 2 87 ± 2
104 ± 2 107 ± 3 99 ± 2
Value ± experimental standard deviation, n = 3. bInformed value
