Comparison of ultrasound-assisted leaching with conventional and acid bomb digestion for determination of metals in sediment samples

Microchemical Journal 86 (2007) 65 – 70 www.elsevier.com/locate/microc

Comparison of ultrasound-assisted leaching with conventional and acid bomb digestion for determination of metals in sediment samples H. Güngör, A. Elik ⁎ Faculty of Art and Science, Department of Analytical Chemistry, University of Cumhuriyet, 58140-Sivas, Turkey Received 27 September 2006; received in revised form 18 October 2006; accepted 18 October 2006 Available online 28 November 2006

Abstract In this paper, a sample preparation method based on ultrasound assisted leaching of Pb, Cu, Zn, Ni and Mn from river and pond sediment samples under ultrasonic effect has been described. Parameters influencing leaching such as sonication time, sample amount, particle size and extractant were fully optimized. Leachatants obtained upon sonication were directly nebulised into an air-acetylene flame for fast metal determination by atomic absorption spectrometry. The best conditions for metal leaching were as follows: a 25 min sonication time, a 0.5 g sample amount (in 25 mL solvent), a particle size b 63 μm and a mixture of concentrated HNO3–HCIO4–HF (2:1:1, v/v/v). Analytical results for the five metals by ultrasound-assisted leaching, acid bomb and conventional digestion methods showed a good agreement, thus indicating the possibility of using mild conditions for sample preparation instead of intensive treatments inherent with the digestion methods. In addition, this method reduces the time required for all treatments (leaching or digestion, heating to dryness, cooling and separation) with acid bomb digestion method (from ∼ 8 h to ∼ 1.5 h) and conventional acid digestion method (from ∼ 14 h to ∼ 1.5 h). The accuracy of the method was tested either by comparing obtained results with those of acid bomb and conventional digestion methods or by application on a standard reference materials. The average relative standard deviation of ultrasound assisted leaching method varied between 0.7–1.9% for N = 6, depending on the analyte. © 2006 Elsevier B.V. All rights reserved. Keywords: Ultrasound; Sample preparation; Sediment; Leaching; Metals; AAS

1. Introduction Dissolution of solid samples such as soil, sediment, street dust and rock is one of the most crucial steps prior to trace element determination. Sample preparation techniques such as microwave, acid bomb digestion (ABDM) and conventional acid digestion (CADM) have widely been used for this purpose and over the years have become established standard methods for trace element dissolution from a large number of matrices [1–4]. This digestion techniques, however, require the use of concentrated mineral acids, high temperatures and high pressure, to effect the dissolution of elemental analyte from solid samples, and all sample preparation is basically the most time-consuming part of elemental analysis [5]. Therefore, considerable interest has been expressed for shortened and

⁎ Corresponding author. Fax: +90 346 2191186. E-mail address: [email protected] (A. Elik). 0026-265X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2006.10.006

simplified sample preparation procedures for trace element analysis [5–8]. Solid–liquid metal leaching can be enhanced with the use of ultrasound irradiation [8–11]. Ultrasonic energy, when imported to solutions, causes acoustic cavitations, that is, bubble formation and subsequent implosion [5,12]. The collapse of bubbles created by the sonication of solutions results in the generation of extremely high local temperature and pressure gradients, which may be regarded as localized “hot spots”. On a timescale of about 10− 10 s, effective local pressures and energies of about 105 atm and about 1 eV, respectively, are generated under sonochemical conditions [12]. Ultrasonic radiation can be considered another alternative for solid sample pre-treatment since ultrasound facilitates an auxiliary energy and accelerates some steps, such as dissolution, fusion and leaching, among others [13–16]. It has been reported that ultrasonic leaching method gives high recoveries of organics from sediment [17], and biological materials [18], and elements from atmospheric particulate [19], soils [20], plants [21–24] and

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H. Güngör, A. Elik / Microchemical Journal 86 (2007) 65–70

street dust [25] in a much shorter time than is required for other extraction procedures. Ultrasonic leaching of metals from sediments, although not yet sufficiently exploited, could be an attractive alternative to conventional, acid bomb and microwave digestion since apart from the time required for digestion, cooling of the reactors needs to be accomplished before opening [26–28]. The aim of this work was to improve sample preparation performance, proposing the development of an ultrasoundassisted leaching method (UALM) for fast and reproducible recovery of some heavy metals in sediment samples, for total heavy metal determination. Parameters influencing ultrasound assisted leaching such as sonication time, sample amount, particle size and extractant are fully investigated. Metal determination in the leachatants is carried out by flame AAS, the results being compared with those obtained by ABDM and CADM.

were homogenized and stored in polyethylene vessels at room temperature in desiccators until digestion or leaching. Accuracy was also evaluated using a standard reference materials: pond sediment CRM-2 from NIES (National Institute for Environmental Studies, Japan). These reference materials were prepared according to the instructions provided by producer. The all materials were dried in air-oven at 80 °C for 4 h before use. All reagents used were of analytical reagent-grade. Nitric, hydrochloric, perchloric and hydrofluoric acids were spectroscopic grades (Merck, Darmstadt, Germany) and de-ionized distilled water was used throughout the work. All glassware and plastic ware used were washed with 5% v/v nitric acid and rinsed with de-ionized distilled water prior to use. Stock standard solutions for Pb, Cu, Zn, Mn and Ni (1000 μg mL− 1) were made by dissolving the nitrate salts (Merck) in 2% v/v nitric acid. Calibration standards of each metal were obtained by suitable dilution of the stock solutions.

2. Experimental 2.3. Procedures 2.1. Instrumentation The determination of Pb, Cu, Zn, Mn and Ni was carried out using an Atomic Absorption Spectrophotometer (UNICAM Model 929) with an air/acetylene burner equipped with a deuterium lamp background-correction system (Cambridge, UK). Hallow cathode lamps (Unicam, CT, UK) of the different metals were used as the radiation sources and the analytical measurements based on time-averaged absorbance. Resonance lines at 324.8, 217.0, 232.0, 279.5 and 213.9 nm were employed for Cu, Pb, Ni, Mn and Zn, respectively. Lamp intensity (4– 6 mA) and band pass (0.2–0.5 nm) were used according to the manufacturer's recommendations. Air/acetylene flow rates were between 0.9–1.1 L/min− 1 for all metals. Ultrasound-assisted leaching experiments were carried out with an ultrasonic bath (Ney 300, USA), which produced a nominal frequency of 50–60 kHz. An acid digestion bomb (Parr-4749, USA), which consists of a Teflon PTFE (polytetrafluoroethylene) vessel of 23 mL capacity and a stainless steel jacket, and a hot-plate were used for a complete dissolution of the sample. The sieving of dried sediment samples was performed with an Endecotts (Octagon-200, London, UK) shaker including suitable sieves. The separation of the final solution from the solid residue at end of each leaching was accomplished by centrifugation at 5000 rpm for 10 min with a laboratory-built centrifuge (Mistrial 2000, UK). 2.2. Materials and reagents Sediments collected from the Kizil river in water depths of 2 m and the Hafik pond in water depths of 3 m in Sivas city, Turkey (Sivas district, coordinates 40°N, 37°E) were used to test the leaching technique. The sediment samples were transferred to a polyethylene container. Once in the laboratory, the samples were dried in an air-oven at 80 °C for 12 h, then ground with an agate mortar. Then, the samples were sieved in order to separate the material into different fractions: b 63, 63– 151, 151–212 and N212 μm. The selected sediment samples

2.3.1. Ultrasound assisted leaching method, UALM For ultrasonic leaching optimization, sonication times (10, 15, 25, 45 and 65 min), different extractants [concentrated HNO3, HCl, a mixture of HNO3–HCl (4:1, v/v) and HNO3– HCIO4–HF (2:1:1, v/v/v)], sample amounts (0.10, 0.25, 0.50, 1.0 and 2.0 g, for 25 mL solvent) were tested. A study related to particle size was also performed by varying the particles from b63 to N 212 μm. To evaluate the efficiency of the process, the results obtained with the UALM were compared with those from ABDM and CADM. A portion (0.50 g ± 0.1 mg) of sediment sample was weighed in to polypropylene beakers (50 mL capacity) and 25 mL of concentrated acid or a mixture of acids were added. Then, the sample was sonicated for 20 min. After sonication, the supernatant liquid was evaporated (∼1 h) to approximately 0.5 mL final volume on a heating plate. Final solution was made up to 10 mL with 2% HNO3 and subjected to sonication for another 5 min. Then, the solutions were centrifuged at 5000 rpm for 10 min, and the final volume was made up to 25 mL with 2% HNO3. The final solutions were collected in polyethylene flask for AAS determinations of metals. Blanks were also treated in the same way. 2.3.2. Conventional acid digestion method, CADM The 0.50 g ± 0.1 mg of each dried sediment sample was placed into a clean 100 mL PTFE beaker, 5 mL of HNO3, 10 mL of HF then 10 mL of HClO4 and 5 mL of HNO3 were added sequentially after, each addition, complete dryness of the sample was achieved at 150 °C. The digestion process took 14 h [29]. The residue was made up to 10 mL with 2% HNO3 and centrifuged at 5000 rpm for 10 min. The final volume was made up to 25 mL with 2% HNO3, for AAS determinations of metals. Blank digestions were also carried out. 2.3.3. Acid digestion bomb method, ADBM The 0.50 g ± 0.1 mg sample and 5 mL each of concentrated HNO3 and HF were placed into PTFE beaker. The bomb was

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Table 1 Operating conditions for ultrasound-assisted leaching of Pb, Cu, Zn, Mn and Ni from sediment samples Variable

Studied interval

Optimum leaching conditions

Sonication time (min) Sample amount (g) Particle size (μm) Solvent systems (concentrated)

10–65 0.1–2.0 b63–N212 HNO3, HCl, HNO3–HCl, HNO3–HClO4–HF

25 0.50 b63 HNO3–HClO4–HF

closed and heated in a hot-plate at 90 °C for 2 h and then increase the temperature to 140 °C and holds it for 4 h at temperature [30]. After cooling the bomb overnight, open the stainless steel cap carefully and residual solutions were evaporated to 0.5 mL final volume on a heating plate. Final solutions were made up to 10 mL with 2% HNO3 and centrifuged. The final volume was made up to 25 mL with 2% HNO3, for AAS determinations of metals. Blanks were treated in the same way. 2.4. Analytical determinations Three sub-samples of each sediment sample were used for analytical determinations with the digestion and leaching procedures. With each series of digestions and leaching a blank was measured. Each result is the average four readings for sample solutions and standard solutions. The concentrations were obtained directly from calibration graphs after correction of the absorbance for the signal from an appropriate reagent blank. In most cases, the blanks constituted only a small fraction (≤ 0.5%) of the metal concentration in the samples. 3. Results and discussion 3.1. Optimization of the UALM Each result was the average value of three determination performed in separate batches. The Kizil river sediment samples were used for optimization purposes. Variables influencing the leaching process were optimized within the intervals shown in Table 1.

Fig. 1. Comparison of UALM rate curves and recoveries at recommended times with CADM for metals in sediment samples, #: CADM, (Ni and Mn = ×10).

Fig. 2. Effect of the sample amount on the metal recovery from acid solvent with UALM, (Ni and Mn = ×10).

3.1.1. Influence of sonication time The ultrasonic-assisted leaching rate curves with UALM and the recoveries at recommended times with CADM for metals in sediment samples were compared in Fig. 1. While there were some variations in leaching time from metal to metal, the UALM required maximum 25 min to reach the same recoveries given for each metal by CADM. For the metals, leaching efficiency increased with increasing sonication time from 10 to 25 min. There was no significant difference between 25 and 65 min sonication periods for all metals at 0.05 probability but was 15 and 25 min. The results show that 25 min exposure time is enough for metals from the studied samples. 3.1.2. Sample amount optimization In this work, the 0.10–2.0 g amount interval was investigated for 25 mL solvent volume. Sample amount used largely depends on the procedure followed. A sample amount of up to 0.50 g has been reported in the work with an ultrasonic bath for leaching [7,20,25,31–33]. As can be seen in Fig. 2, a significant decrease in metal recovery from sediment samples is obtained when the sample amount is larger than 0.50 g. There was a significant difference between 0.50 and 2.0 g sample amount for all metals at 0.05 probability. The sediment amount/solvent volume ratio appears to be an important parameter for metals which leaching efficiency is affected by the solvent volume. In

Fig. 3. Effect of the particle size on the metal recovery from acid solvent with UALM, (Ni and Mn = ×10).

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H. Güngör, A. Elik / Microchemical Journal 86 (2007) 65–70 Table 3 Analytical results for metals as determined by UALM, ABDM and CADM Metal

Pb

Cu Fig. 4. The effect of extractants on the ultrasonic-assisted leaching of metals from sediment samples, (Ni and Mn = ×10). Ni

this work, sample amount/solvent volume ratio was chosen as 0.50 g/25 mL. 3.1.3. Particle size optimization Particle size was among the more critical parameters influencing ultrasound assisted leaching. As was to be expected, reactions were enhanced on increasing the contact surface. The particle size attempted for metal leaching in this work with the use of ultrasonic bath ranged from less than 63 up to N 212 μm. The results obtained in this study are shown in Fig. 3. As can be observed, leaching efficiency decreased when the particle size was larger than 63 μm for the metals. There was no significant difference between b63 and 63–151 μm particle sizes for all metals at 0.05 probability but was b 63 and 151–212 μm. In this way, the b 63 μm particle size was chosen for evaluation of the accuracy. 3.1.4. Influence of extractants Acid type and concentration in the liquid extractant was seen to be the most critical parameter affecting ultrasound leaching. Different acid mixtures such as 15% HNO3–1% HCl (1:1, m/m) [34], 25% HNO3–HCl (1:1, v/v) [4] concentrated HNO3–HCl (1:1, v/v) [4,32] and concentrated HNO3–HClO4–HF (2:1:1, v/ v/v) [24] were reported to be used as solvents for leaching of metals. HF is used in extractions the dissolution of metal species that are bound up in silicate materials and that would otherwise be insoluble, even in other acid solutions [4,35]. The effectiveness of mixtures of concentrated HF and other acids such as HNO3, HClO4 and HCl for the ultrasonic leaching of target metals, followed by atomic spectrometric analysis, has been demonstrated previously for bulk certified reference materials such as sediments, soils and fly ash [35].

Table 2 Validation of the UALM against the certified reference materials Metal

NIES

CRM-2 −1

Pb Cu Ni Zn Mn b a b

Certified value μg g

ULM a value μg g− 1

Relative error (%)

105 ± 6 210 ± 12 40 ± 3 343 ± 17 770

102 ± 3 213 ± 7 38 ± 1 332 ± 13 758 ± 14

− 2.9 +1.4 − 5.0 − 3.2 − 1.6

Average value ± standard deviation (N = 3). Reference value.

Zn

Mn

a

Method

UALM CADM ABDM UALM/CADM×100 UALM CADM ABDM UALM/CADM×100 UALM CADM ABDM UALM/CADM×100 UALM CADM ABDM UALM/CADM×100 UALM CADM ABDM UALM/CADM×100

Concentration, (μg g− 1) a River sediment

Pond sediment

21.1 ± 0.2 21.2 ± 0.3 21.5 ± 0.7 99.5 28.8 ± 0.4 29.2 ± 0.6 28.6 ± 0.7 98.6 157.6 ± 1.7 158.5 ± 2.2 158.3 ± 1.8 99.4 79.5 ± 0.4 79.1 ± 0.7 79.1 ± 0.9 100.6 544.2 ± 7.9 543.0 ± 5.8 543.7 ± 4.6 100.2

30.7 ± 0.5 30.1 ± 0.7 31.1 ± 0.8 102 29.7 ± 0.7 29.5 ± 0.8 29.9 ± 1.3 100.6 35.8 ± 0.6 36.6 ± 0.8 36.1 ± 0.6 98 120.1 ± 1.1 120.0 ± 2.6 121.1 ± 3.8 100 293.6 ± 3.1 293.9 ± 2.6 293.1 ± 3.8 100

Average value ± standard deviation (N = 3).

The influence of extractants such as concentrated HCl, HNO3, a mixture of HCl–HNO3 and a mixture of HNO3– HClO4–HF was studied in a univariate way by fixing the other variables at their optimal values. The leaching results obtained with the use of a single acid or acid mixtures are shown in Fig. 4. It can be seen that high recoveries with UALM in concentrated HNO3–HClO4–HF mixture are obtained for metals, although there are some differences in recovery depending on the analyte. But, the recoveries with in a mixture of concentrated HNO3– HClO4–HF are in good agreement with the results found for a mixture of concentrated HNO3–HCl. The leaching results obtained with the use of a mixture of HNO3–HClO4–HF and a mixture of HNO3–HCl revealed that there is no significant difference between both solvent systems at 0.05 probability, except Zn and Mn. It is also clear that HNO3 gives less metal leaching than HCI. A concentrated HNO3–HClO4–HF mixture was chosen for leaching. 3.2. Analytical results using UALM, ABDM and CADM 3.2.1. Calibration and validation The detection and quantification limits were calculated for flame AAS determinations. Limits of detection [3 (s/m), N = 10] for the UALM were 2.76, 1.48, 1.74, 0.63 and 1.11 μg g− 1 for Pb, Cu, Ni, Zn and Mn, respectively, being similar to those attained with CADM and ABDM when a 0.3 g sample mass was used for leaching. Limits of quantification [10 (s/m), N = 10] were 9.1, 4.9, 5.3, 2.2 and 3.6 μg g− 1 for Pb, Cu, Ni, Zn and Mn, respectively. The equation for the linear range of the calibration graphs for all metals was found as: Absorbance ¼ 2ðF1Þ  10−3 þ 0:043F0:027½X ; ðX ¼ 0−7mgL−1 Þ; r ¼ 0:9999:

H. Güngör, A. Elik / Microchemical Journal 86 (2007) 65–70 Table 4 Recovery of metals by UALM and CADM using an internal standard method Metal

Pb Cu Ni Zn Mn a

Original a

Added a

UALM

CADM

21.1 ± 0.2 28.8 ± 0.4 157.6 ± 1.7 79.5 ± 0.4 544.2 ± 7.9

21.2 ± 0.3 29.2 ± 0.6 158.51 ± 2.2 79.1 ± 0.7 543.0 ± 3.8

20.2 20.2 30.5 20.2 70.6

Found a UALM

CADM

41.6 ± 0.5 48.6 ± 0.7 187.2 ± 1.7 100.0 ± 0.9 615.0 ± 8.9

40.9 ± 0.7 48.9 ± 1.0 187.8 ± 2.6 101.1 ± 0.8 617.1 ± 5.5

Average value ± standard deviation (N = 3) (μg g− 1).

The accuracy of UALM for the proposed method under the optimized leaching conditions was determined by comparing the results with those obtained using different digestion methods (CADM and ABDM) for all sediment samples , as well as by analyzing a pond sediment (CRM-2). Validation of the UALM is shown in Table 2. A good agreement between the found and certified metal contents can be observed for the all metals studied. The results, presented in Table 2, show that no statistical differences were observed at 0.05 probability, indicating that the UALM is applicable for this type of sample, opening the possibility of its application for other samples. UALM provided (+ 1.4) − (− 5.0) % relative error and 1.8–3.9% (N = 3) RSD %, depending on the analyte and sample, which are acceptable ranges for this kind of studies. Analytical results obtained by UALM, ABDM and CADM corresponding to the sediment samples analyzed are shown in Table 3. Average recoveries were 100 ± 1, 99 ± 1, 100 ± 0, 100 ± 0 and 99 ± 1% for Pb, Cu, Zn, Mn and Ni, respectively, thus indicating that there was a good agreement either between UALM and ABDM or UALM and CADM. When the average heavy metal values were compared using a significance statistical test it was concluded that there is no difference for the UALM and CADM at 0.05 probability. The main differences among the three methods lie in the time required to complete the digestion or leaching. In precision test, the average relative standard deviation (RSD) (N = 6) values for all metals varied in the range of 0.7–1.9%, 0.9–2.7% and 0.9– 3.4% for UALM, CADM and ABDM, respectively. RSDs were calculated from pooled data for method. The precision obtained from 6 replicate UALM yielded an average RSD of 1.32, 1.91, 0.74, 1.26 and 1.40% for Pb, Cu, Zn, Mn and Ni, respectively, depending on the analyte. Besides, the precision of the UALM was better than ABDM and CADM. An internal standard method was used on both methods (UALM and CADM) to verify the leaching recovery of metals. A river sediment sample was spiked with a known concentration of each metal before the digestion process by the two methods was carried out (Table 4). The results show that the recovery is quantitative for each metal and there are no interaction effects between the elements. 4. Conclusion Ultrasound-assisted leaching has described offers a fast, easy, reliable and efficient sample preparation for direct determination of Pb, Cu, Ni, Mn and Zn in sediment samples

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by flame AAS. All parameters studied (sonication time, sample amount, particle size and extractant) influence the leaching efficiency. Under optimum conditions, quantitative recoveries for all metals are reached, and the results obtained are comparable to the obtained ones by means of classical sample pre-treatment based on acid digestion. The advantages of UALM over ABDM and CADM are the following: i) UALM is faster and easier than ABDM and CADM. The use of the UALM allowed the leaching of the target analyte in a shorter time than required by the ABDM and CADM that were providing similar results. This method reduces the time required for all treatments (leaching or digestion, heating to dryness, cooling and separation) with ABDM (from ∼ 8 h to ∼ 1.5 h) and CADM (from ∼ 14 h to ∼1.5 h). ii) The UALM is safer than acid digestion as neither high temperature nor pressure are present during the leaching. iii) The consumption of reagents is diminished. iv) The whole procedure is simpler since a lesser number of operations is involved that minimizes contamination risks. It is clear that the UALM is a rapid, inexpensive, easy, reproducible and selective technique for the total determination of Pb, Cu, Ni, Mn and Zn in sediment samples which are important in monitoring environmental pollution. References [1] Z. Sulcek, P. Povondra, “Sample Preparation” in Methods of Decomposition in Inorganic Analysis, CRC Press, Boca Raton, FL, 1989. [2] H.M. Kingston, L.B. Jassie, “Microwave Digestion” in Introduction to Microwave Sample Preparation: Theory and Practice, ACS, Washington, DC, 1988. [3] L.B. Fischer, Anal. Chem. 58 (1986) 261. [4] H. Matusiewicz, “Wet Digestion Methods” in Sample Preparation for Trace Element Analysis, Elsevier, Amsterdam, 2003 Chapter 6. [5] K. Ashley, R.N. Andrews, L. Cavazos, M. Demange, J. Anal. At. Spectrom. 16 (2001) 1147. [6] M.D. Luque de Castro, M.P. da Silva, Trends Anal. Chem. 16 (1997) 16. [7] A. Elik, M. Akçay, Int. J. Environ. Anal. Chem. 80 (2001) 257. [8] C. Bendicho, I. Lavilla, Ultrasound-assisted extraction, in: I.D. Wilson (Ed.), Encyclopedia of Separation Science, Academic Press, London, 2000. [9] C. Bendicho, I. Lavilla, Applications of ultrasound-assisted metal extractions, in: I.D. Wilson (Ed.), Encyclopedia of Separation Science, Academic Press, London, 2000. [10] J.L. Gomez-Ariza, E. Morales, R. Beltran, I. Giraldez, M. Ruiz-Benitez, Analyst 120 (1995) 1171. [11] M. Akçay, A. Elik, Ş. Savaşcı, Analyst 114 (1989) 1079. [12] K.S. Suslick, Ultrasound: Its Chemical, Physical and Biological Effects, VHC Publishers, Weinheim, Germany, 1988. [13] J.L. Luque-Garcia, M.D. Luque de Castro, Analyst 127 (2002) 1115. [14] J. Mierzwa, Y.C. Sun, M.H. Yang, Anal. Chim. Acta 355 (1997) 277. [15] K. Ashley, Trends Anal. Chem. 17 (1998) 366. [16] S. Morales-Munoz, J.L. Luque-Garcia, M.D. Luque de Castro, Crit. Rev. Environ. Sci. Technol. 33 (2003) 391. [17] J. Grimalt, C. Marfil, J. Albaiges, Int. J. Environ. Anal. Chem. 18 (1984) 183. [18] T.S. Koh, Anal. Chem. 55 (1983) 1814. [19] S.L. Harper, J.F. Walling, D.M. Holland, L.J. Prongler, Anal. Chem. 55 (1983) 1553. [20] A. Marin, A. Lopez-Gonzalvez, C. Barbas, Anal. Chim. Acta 442 (2001) 305. [21] A.V. Filgueiras, J.L. Capelo, I. Lavilla, C. Bendicho, Talanta 53 (2000) 433.

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