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Feasibility of nut digestion using single reaction chamber for further trace element determination by ICP-OES
Feasibility of Nuts Digestion Using Single Reaction Chamber for Further Trace Elements Determination by ICP OES Cristiano C. M¨uller, Aline L.H. Muller, Camillo Pirola, Fabio A. Duarte, Erico M.M. Flores, Edson I. Muller PII: DOI: Reference:
S0026-265X(14)00072-1 doi: 10.1016/j.microc.2014.04.013 MICROC 1944
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
31 December 2013 23 April 2014 23 April 2014
Please cite this article as: Cristiano C. M¨ uller, Aline L.H. Muller, Camillo Pirola, Fabio A. Duarte, Erico M.M. Flores, Edson I. Muller, Feasibility of Nuts Digestion Using Single Reaction Chamber for Further Trace Elements Determination by ICP OES, Microchemical Journal (2014), doi: 10.1016/j.microc.2014.04.013
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ACCEPTED MANUSCRIPT Feasibility of Nuts Digestion Using Single Reaction Chamber for
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Further Trace Elements Determination by ICP OES
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Cristiano C. Müllera, Aline L. H. Mullera, Camillo Pirolab, Fabio A. Duartea, Erico M.
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Departamento de Química, Universidade Federal de Santa Maria, 97105-990, Santa
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Maria, Brazil b
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M. Floresa, Edson I. Mullera*
Milestone Srl, 24010, Sorisole, BG, Italy.
*E-mail address: [email protected]
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ACCEPTED MANUSCRIPT Abstract A method for nuts digestion using Single Reaction Chamber (SRC,
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UltraWave™ system) was proposed. High sample masses (up to 1.5 g) were used in
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order to improve the limits of detection for trace elements. Nuts with high fat content
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(Brazil nut, cashew nut and walnut) were used to evaluate the efficiency of the proposed method. Different oxidant conditions were evaluated (only concentrated HNO3 and mixtures of HNO3 and H2O2). Comparison with Multiwave™ system was also carried Efficiency of digestion for both systems was evaluated by determination of
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out.
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residual carbon content (RCC) and residual acidity of final digestates. Determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn was performed using inductively coupled plasma optical emission spectrometry (ICP OES). Carbon related effects on analytical
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response for emission lines used in ICP OES determination was also evaluated. The best efficiency of digests was obtained using UltraWave™ system with 1.5 g of nut,
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concentrated HNO3 (without the addition of H2O2) and temperature and pressure of 275 °C and 180 bar, respectively. In this condition the RCC was 4.6% (correspondent to
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2350 mg L-1 of C in digestates) and residual acidity was lower than 5% (equivalent to 0.3 mL of concentrated HNO3 diluted up to 25 mL). On the other hand, for Multiwave™ system only 0.5 g of nuts could be digested. Carbon related effects on analytical response on ICP OES determination were not observed in these conditions. Significant differences were not observed by the comparison between the results obtained by UltraWave™ system and certified reference values (t test, 95% confidence level). Therefore, using UltraWave™ system that allowed a relatively high sample mass to be digested it was possible to determine trace elements at low concentration even using ICP OES in conventional mode. Keywords: single reaction chamber (SRC), nuts, microwave digestion, ICP OES 2
ACCEPTED MANUSCRIPT 1. Introduction Methods using concentrated acids and microwave heating have been widely
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used for sample preparation [1-3]. Especially for organic matrices, concentrated HNO3
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acid is preferred to convert samples in a suitable solution for further determination of
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trace elements using plasma-based techniques [2,4]. However, some organic compounds are not completely oxidized using conventional low and medium pressure microwave systems and the residual carbon content (RCC, the remaining content of C
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after digestion related to the total C content in original sample) in the final digests can
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affect the analytical response of some emission lines during element determination by inductively coupled plasma optical emission spectrometry (ICP OES) [5-7]. Studies reported in literature have shown that carbon related effects become more severe when
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carbon concentration in digests is higher than 5000 mg L-1 [7]. In addition, the emission intensity of some atomic lines with excitation energies below 6 eV decreases (up to
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15%) whereas the emission intensity of atomic lines with higher excitation energies (e.g., As and Se) can be increased (up to 30%). It is important to mention that the
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emission intensity of ionic lines is not affected by residual carbon content in digests [7]. Moreover, sample preparation methods should produce digests containing the lowest RCC as possible [8]. Additionally, nuts are considered as difficult matrix and it is not easy to digest due to the high content of polyunsaturated fatty acids. These compounds are hardly oxidized by HNO3 and a complete digestion is achieved only by using concentrated HNO3 and temperatures higher than 180 ºC [9]. In order to minimize this problem, many methods for nuts digestion have been reported in literature [10-12]. The most simple and widely used is dry ashing method, that allows obtaining digests with very low RCC (lower than 1%) [13]. However, some drawbacks have been related, such
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ACCEPTED MANUSCRIPT as losses of volatile elements (e.g., Cd, Pb and Hg) and the possibility of contamination during sample digestion step [14].
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Microwave induced combustion (MIC) is a method that was developed in a few
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years ago and can be considered as suitable for complete digestion of organic samples,
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allowing digests with RCC lower than 1% that is well suitable for further analysis by several spectrometric or chromatographic techniques [15-21]. However, up to now only about 0.5 g of sample can be digested in order to prevent pressure overload of
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microwave system. In addition, a special vessel is required for pressurization with
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oxygen [11, 22]. In order to improve the sample mass introduced in the combustion system, focused microwave induced combustion (FMIC) was recently proposed [23]. This method allows the digestion of up to 3 g of sample and diluted HNO3 can be used
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as absorbing solution allowing lower LODs [23-24]. However, the instrumentation required to perform FMIC method is not commercially available up to now. On the
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other hand, wet digestion using focused microwave systems allows the digestion of high sample amounts (up to 10 g), specially because there is no overpressure and reagents
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can be added during the digestion [25]. However, one of the main drawbacks is related to the necessity of using sulfuric acid to improve the digestion temperature as digestion is performed in atmospheric pressure [25]. In addition to physical interferences caused by sulfuric acid, some elements such as Ag, Ba, Pb and others, may form insoluble compounds with sulfate [26]. Microwave assisted digestion (MAD) using closed vessels and concentrated HNO3 is preferred for organic sample digestion due to the reduced risk of contamination and high sample throughput [1]. This system has been used for digestion of nuts (babassu, sapucaia e cashew) using sample mass ranging from 0.15 to 0.25 g and a mixture of oxidants (2 mL de HNO3 and 1 mL of H2O2). However, even using relatively 4
ACCEPTED MANUSCRIPT high temperature RCC was up to 13.4% (m/m). One of the main limitation of MAD is the maximum sample mass that could be digested, which is limited by the maximum
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pressure of microwave system [12].
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In this regard, some manufactures have developed microwave systems that allow
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the use of very high pressure and high temperature to assure an efficient digestion of higher sample masses. Recently, it was proposed a new system called single reaction chamber (SRC, UltraWave™ system) that allows temperature and pressure up to 300 ºC
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and 199 bar, respectively. In the UltraWave™ system, the power source (1500 W) is
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located at the base of a chamber, allowing an efficient distribution of microwave radiation. In addition, prior to start the heating program, the chamber is pressurized with N2 which acts as a “cover” over the vials, preventing boiling and spitting, and
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eliminating any possibility of cross-contamination [8]. This system was used for the digestion of different organic matrices (milk powder, bovine liver, bovine kidney, etc.)
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in the same running batch, operating at maximum temperature and pressure of 250 ºC and 120 bar, respectively. Using this system, 0.5 g of sample were placed into
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fluoropolymer vessels containing diluted HNO3 (2 mol L-1) and H2O2. The RCC and residual acidity were lower than 1.5% and 1.04 mol L-1 of HNO3, respectively [8]. In the present study, the UltraWave™ system was evaluated for nuts digestion (Brazil nut, cashew nut and walnut samples) using HNO3. Hydrogen peroxide was also evaluated as auxiliary reagent for HNO3 regeneration in UltraWave™ system [27]. In order to evaluate the digestion efficiency of this system, sample masses up to 2 g were digested. Nuts digestion was also performed using a Multiwave™ system in order to compare the efficiency of both systems. The digestion efficiency was evaluated by determination of RCC in final digests as well as by determination of residual acidity. The follow elements were determined by ICP OES: Al, As, Cd, Co, Cu, Fe, Mn, Ni, Pb 5
ACCEPTED MANUSCRIPT and Zn. Finally, the accuracy of the proposed method using UltraWave™ system was
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evaluated using certified reference materials.
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2. Material and methods
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2.1. Instrumentation
In this study two microwave sample preparation systems were used for nuts digestion. A digestion system named UltraWave™ (Milestone, Sorisole, Italy) equipped
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with five quartz vessel (total volume of 40 mL) was used. This system has a microwave
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cavity (1 L) made of stainless steel and covered with polytetrafluoroethylene (PTFE) vessel which was sealed and pressurized with 40 bar of argon 99.996% (White Martins, São Paulo, Brazil). All experiments using UltraWave™ were carried out using 1500 W
bar, respectively.
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of microwave irradiation and maximum temperature and pressure of 275 °C and 180
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A Multiwave™ 3000 (Anton Paar, Graz, Austria) system with eight quartz closed vessels (total volume of 80 mL) was used for sample digestion. Maximum
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temperature and pressure used were 280°C and 80 bar, respectively. The irradiation power of Multiwave™ system was 1400 W. An inductively coupled plasma optical emission spectrometer (Perkin Elmer, model Optima 4300DV, Shelton, USA) using axial view mode and equipped with cyclonic spray chamber and concentric nebulizer was used for determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn. For plasma generation, nebulization and auxiliary gas, argon with purity of 99.996 % was used. The conditions used for elements determination by ICP OES are summarized in Table 1.
2.2. Reagents, samples and standards 6
ACCEPTED MANUSCRIPT Water was purified by distillation and further submitted to a Milli-Q system (Millipore Corp. Bedford, USA). Purified water was used to prepare all reference
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solutions and reagents used in this study. Nitric acid (65%, Merck, Darmstadt,
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Germany) used for sample digestion was purified in a quartz sub-boiling system
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(Milestone, Duopur model, Bergamo, Italy). Hydrogen peroxide (30%, Merck, Darmstadt, Germany) was also used as oxidant agent. A multi-element stock solution (PlasmaCal SCP33MS, SCP Science, Quebec, Canada) containing 10 mg L-1 of all
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elements evaluated in this study was used to prepare the calibration curve for ICP OES determination in the range of 1 to 100 µg L-1. The multielement stock solution (SCP
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33MS 10 mg L-1) was also used for evaluation of carbon related effects on the response of emission lines used for analytes determination. Reference solutions used for RCC
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determination were prepared by sequential dilution of a carbon stock solution prepared after dissolution of citric acid (Merck) in water. Yttrium (1 mg L-1, Spex CertPrep,
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Metuchen, USA) was used as internal standard for RCC determination. Samples of Brazil nuts, cashew nuts and walnuts purchased at local market were
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used in this study. About 200 g of nuts were ground and homogenized in household food mill during 10 min. The evaluation of RCC and residual acidity were carried out using Brazil nuts. Accuracy was evaluated using certified reference materials (CRM) of oyster tissue (NIST 1566 b), skim milk powder (BCR 151) and dogfish liver (DOLT-4).
2.4. Microwave assisted digestion using UltraWave™ system Sample masses ranging from 0.25 to 2 g were directly weighed inside the quartz vessels. Three oxidant conditions were evaluated: 6 mL of concentrated HNO3 (Condition 1); 5 mL of concentrated HNO3 + 1 mL H2O2 (Condition 2) and 4 mL of concentrated HNO3 + 2 mL of H2O2 (Condition 3). About 120 mL of water and 5 mL 7
ACCEPTED MANUSCRIPT of H2O2 were inserted into the SRC chamber and was pressurized up to 40 bar to cover and keep the cap in the upper part of the quartz vessel. All digestions were carried out
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using maximum temperature and pressure of 275 °C and 180 bar, respectively. In spite
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of the SRC system allows higher pressure and temperature conditions, authors
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arbitrarily selected 275oC and 180 bar in order to assure a more safe operation condition. Microwave heating program was as follow: 10 min of ramp and hold for 20 min at 1500 W. After cooling down (65 ºC) the chamber was depressurized and the
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resultant solution was diluted in volumetric flasks. Final digests were analyzed by ICP
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OES (Figure 1).
2.5. Microwave assisted wet digestion using Multiwave™ system
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Sample masses ranging from 0.25 to 2 g were directly weighed inside the quartz vessels. Only concentrated HNO3 (6 mL) was used as oxidant agent. All digestions were
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performed using maximum temperature and pressure available in the microwave system (280 °C and 80 bar, respectively). The heating program was as follows: i) 10 min of
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ramp up to 1400 W; ii) 1400 W for 20 min and iii) 0 W for 20 min. Final digests were diluted in volumetric flask and analyzed by ICP OES (Figure 1).
2.6. Evaluation of digestion efficiency Determination of RCC and residual acidity in digests obtained by both microwave digestion systems were carried out in order to evaluate the digestion efficiency. Residual carbon content was determined by ICP OES [28] and expressed as mg of C per 100 mg of the original sample (or simply, %). Determination of residual acidity was performed using potentiometric titration with KOH solution and expressed as % of the original acid that remains in final digest. 8
ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Digestion efficiency of UltraWave™ and Multiwave™ systems
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Initial experiments showed that sample masses higher than 0.75 g were not
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completely digested using HNO3 and Multiwave™ system. RCC and residual acidity
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were not determined for digests obtained using Multiwave™ system. On the other hand, sample masses up to 2 g were efficiently digested using UltraWave™
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Condition 1 (only concentrated HNO3). Results for RCC are showed in Figure 2 for nuts
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digestion using both microwave systems (UltraWave™ and Multiwave™). For samples
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masses lower than 1.25 g, it was not observed a significant increase in the RCC values when UltraWave™ and Condition 1 were used (Figure 2, black bars). RCC for 0.25 g of sample was 0.86% (75 mg L-1 of C in final digest), while for 1.25 g of sample, the RCC
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was about 1.8% (630 mg L-1 of C in final digest). The RCC values up to 14.4% (9530 mg L-1 of C) were observed for the digestion of 2 g of sample in UltraWave™ system
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using concentrated HNO3 (Condition 1). It has been reported in literature that carbon related effects were observed in analytical response for emission lines for solutions
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containing carbon contents higher than 5000 mg L-1 [7]. In this sense, digests of spiked sample (final concentration of 100 g L-1 for all analytes), unspiked sample and standard solution (100 g L-1 of all analytes) were analyzed by ICP OES. No suppression (difference of original signal lower than 5%) was observed for all the analytes. It is important to point out that most of lines used in this study for trace element determination were ionic emission lines (except for Al, As, Cd and Cu) which were less affected by carbon effect on ICP OES determination. However, for As emission line (As I – 188.979) an increase of 20% in analytical response was observed and 2 g of sample could not be recommended for trace element determination in nuts. It is well noticed in literature that As emission line is more affected by residual carbon 9
ACCEPTED MANUSCRIPT content [7]. Similar enhancement of As signal was observed for sample mass equal to 1.75 g (7480 mg L-1 of C in final digest). Therefore, in order to minimize the carbon
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effects on analytical response, especially for As, a digestion of 1.5 g using UltraWave™
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system and concentrated HNO3 is recommended and this condition was used for the
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digestion of all nuts samples. Digests of 1.5 g of Brazil nut showed RCC of 4.6% (2350 mg L-1 of C).
Comparing the digests obtained by both digestion systems, the RCC was always
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higher for Multiwave™ system (Figure 2, white bars). It very probably occurred due to the lower temperature achieved by this system during the heating program (digestion
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temperature was never higher than 210 ºC even selecting a maximum value of 280 oC). Additionally, the temperature achieved using UltraWave™ system was close to 275 °C
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(maximum temperature) independent of the sample mass. Using 0.75 g of sample in Multiwave™ system with concentrated HNO3, RCC was about 20.8% (5200 mg L-1 of
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C). A significant increase or suppression (higher than 5%) of analytical response for emission lines was not observed when using 0.75 g of sample (Brazil nut) digested by
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Multiwave™ system with concentrated HNO3. However, the RCC (5200 mg L-1 of C in final digest) was very close to the value that can promote effects on analytical response for ICP OES measurements (5000 mg L-1). In this sense, the use of only 0.5 g of nut sample was recommended for digestion of all samples with Multiwave™ system. Comparing the digests using the three digestion conditions for UltraWave™ system with sample mass of 0.75 g, it was not observed significant difference in RCC values (Figure 2). For Conditions 2 and 3, sample masses up to 1.5 g could be digested. However, RCC values for Condition 3 were higher than 14% (6000 mg L-1 of C) and 24% (12000 mg L-1 of C) when 1.25 and 1.5 g of sample were used, respectively. Thus,
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ACCEPTED MANUSCRIPT for Condition 3, only samples masses lower or equal to 1 g can be used due to carbon effects on ICP OES determination.
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more efficient digestion in comparison to Multiwave™ system.
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Finally, UltraWave™ system allowed the digestion of higher sample masses and
3.2. Residual acidity of digests obtained using UltraWave™ and Multiwave™ systems Depending on the concentration of HNO3 in final digests (after dilution),
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suppression of analytical response can be observed [29]. Unfortunately, the exact matrix
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matching of standards is not possible. On this matter, digests with low residual acidity are preferred to obtain accurate results and allow external calibration with standard solutions containing the analytes in diluted HNO3 (generally 5% HNO3 solution).
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Residual acidity in the digests obtained using both systems are shown in Figure 3. It is possible to observe that the acidity is always lower in digests obtained by UltraWave™
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system. This probably occurs due to the higher temperature achieved by UltraWave™ which allows more efficient oxidant condition and more HNO3 was consumed for
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oxidation of organic matter. For example, comparing the acidity of both systems, when 0.75 g sample mass was used in combination with concentrated HNO3, the acidity was two times higher while the RCC value was about twenty fold higher when Multiwave™ system was emplyoed. In addition, the acidity of digests using 1.5 g in UltraWave™ system and concentrated HNO3 (optimized condition) was lower than 5% (equivalent to 0.3 mL of concentrated HNO3 diluted up to 25 mL). This amount of HNO3 did not caused the suppression of the signal in ICP OES determination. Acidity of digests containing H2O2 was always similar or lower that showed by digests obtained using only concentrated HNO3. This fact evidences that H2O2, contrarily to other systems does not results in HNO3 regeneration during digestion using 11
ACCEPTED MANUSCRIPT UltraWave™. It can be explained considering the higher free volume inside the reaction chamber in UltraWave™ system when compared to individual vessels (80 mL internal
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volume) used in Multiwave™ microwave oven.
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3.3 Comparison of proposed UltraWave method with other methods previously described in literature for nuts analysis
The proposed method using UltraWave system showed some advantages,
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particularly related to sample mass and RCC values. Sample masses used in the
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proposed method (1.5 g) is at least 3 times higher than the mass used in other works described in literature for nuts analysis [30-32]. In the same way, RCC value was better for digests obtained using UltraWave system in comparison with RCC values available
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in literature [12]. For example, Brazil nuts digests by the proposed method showed RCC values of 4.6% that is two times lower than the RCC values previously described for nut
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analysis [12]. It probably occurs due to the higher temperature and pressure achieved by the proposed method. Acidity of final digests was lower than 5% of HNO3 and did not
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cause changes in analytical signal in determination by ICP OES. The analytical throughput was lower in comparison with data available in literature regarding to nuts analysis [12, 30]. However, the UltraWave system also allows the use of a rotor with 15 vessels, instead of 5 vessels of 40 mL (used in the proposed method) and in this case the analytical throughput should be comparable to those reported in literature [12, 30].
3.4. Determination of trace elements Brazil nut, walnut and cashew nuts by ICP OES Brazil nut, walnut and cashew nuts were digested using 1.5 g and 0.5 g in UltraWave™ and Multiwave™ systems, respectively. Results obtained by ICP OES for Al, As, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn are shown in Table 2. For all elements, the 12
ACCEPTED MANUSCRIPT relative standard deviation (RSD) was lower than 12% after digestion using UltraWave™ and Multiwave™ systems. Comparing the analytes concentration, no
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significant differences (t test, 95% confidence level) were observed for both microwave
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digestion systems. Zinc showed the highest concentration in comparison with the other
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trace elements, whereas Al, As, Cd and Pb were not detected in nuts samples (LOD lower than 790 ng g-1). Concentration of Co, Fe, Ni and Zn were higher in Brazil nut when compared to the other nuts. On the other hand, the concentration of Cu and V was
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similar in all kinds of nuts. For other elements nuts showed concentration ranging from
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lower than 0.03 to 0.80 g g-1 for Co, and 9.7 to 26 g g-1 for Mn. Accuracy was evaluated by the analysis of three CRMs and no significant differences were observed when certified results were compared with those obtained
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using UltraWave™ system, as shown in Table 3. The LOD and LOQ values obtained by ICP OES after digestion using
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UltraWave™ were always lower than those obtained by Multiwave system (Table 4). Therefore, UltraWave™ system it was possible to determine trace elements at low
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concentration even using ICP OES, a technique with low sensitivity when compared to ICP-MS. Moreover, the sample mass that could be digested using UltraWave™ (1.5 g) was three times higher that this digested by Multiwave system.
4. Conclusions This study showed the feasibility of UltraWave system for digestion of nuts samples and subsequent determination of trace elements by ICP OES. The main advantages of this system are the possibility to digest relatively large sample masses (up to 1.5 g) that improves the LODs. Moreover, low RCC and residual acidity were obtained making the proposed procedure well suited to further analysis by ICP OES. In 13
ACCEPTED MANUSCRIPT addition, as a result of higher sample mass using UltraWave system, the LODs were
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better than those obtained for other methods.
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Acknowledgements
The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de
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Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do
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Sul (FAPERGS) for supporting this study.
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2155–2160.
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[24] J.S.F. Pereira, L.S.F. Pereira, L. Schmidt, C.M. Moreira, J. S. Barin, E.M.M. Flores, Metals determination in milk powder samples for adult and infant nutrition after focused-microwave induced combustion, Microchem. J. 109 (2013) 29–35.
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[25] J.A. Nobrega,, L.C. Trevizan, G.C.L. Araujo, A.R.A. Nogueira, Review. Focused-
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Microwave-assisted strategies for sample preparation. Spectrochim. Acta Part B 57 (2002) 1855–1876.
[26] M. Hoenig, Critical discussion of trace element analysis of plant matrices, Sci.
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Total Environ. 176 (1995) 85-91.
[27] C.A. Bizzi, E.L.M. Flores, J.A. Nobrega, J.S. S. Oliveira, L. Schmidt S.R. Mortari,
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Evaluation of a digestion procedure based on the use of diluted HNO3 solutions and H2O2 for the multielement determination of whole milk powder and bovine liver by
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ICP-based techniques, , J. Anal. At. Spectrom., 2013, In press, DOI: 10.1039/c3ja50330e. [28] S.T. Gouveia, F.V. Silva, L.M. Costa, A.R.A. Nogueira, J.A. Nobrega, Determination of residual carbon by inductively-coupled plasma optical emission spectrometry with axial and radial view configurations, Anal. Chim. Acta, 445 (2001) 269-275. [29] Jose-Luis Todoli, Jean-Michel Mermet, Review. Acid interferences in atomic spectrometry: analyte signal effects and subsequent reduction, Spectrochim. Acta Part B 54 (1999) 895-929.
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ACCEPTED MANUSCRIPT [30] K. Phan-Thien, G.C. Wright, N. A. Lee. Inductively coupled plasma-mass spectrometry (ICP-MS) and –optical emission spectroscopy (ICP-OES) for
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determination of essential minerals in closed acid digestates of peanuts (Arachis
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hypogaea L.), Food Chem. 134 (2012) 453-460.
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[31] E. P Nardi, F.S. Evangelista, L. Tormen, T.D. SaintPierre, A.J. Curtius, S. S. Souza, F. Barbosa Jr.. The use of inductively coupled plasma mass spectrometry (ICP-MS) for the determination of toxic elements and essential elements in
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different types of food samples, Food Chem. 112 (2009) 727-732.
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[32] A.A. Momen, G.A. Zachariadis, A.N. Anthemidis, J. A. Stratis, Use of fractional factorial design for optimization of digestion procedures followed by multi-element determination of essential and non-essential elements in nuts using ICP-OES
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technique, Talanta 71 (2007) 443-451.
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ACCEPTED MANUSCRIPT
NUTS
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Grinding
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Sample (Brazil nut, cashew nut and walnut)
Sample mass: 0.25 to 2 g Digestion mixture:
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Heating program:
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Condition 1: 6 mL HNO3 Condition 2: 5 ml HNO3 + 1 mL H2O2 Condition 3: 4 ml HNO3 + 2 mL H2O2
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ULTRAWAVE
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(i) 30 min at 275 ºC (10 min ramp) (ii) 20 min for cooling Max. pressure: 180 bar Max. temperature: 275 ºC
MULTIWAVE Sample mass: 0.25 to 2 g Digestion mixture: (i) 6 ml HNO3
Heating program: (i) 30 min at 1400 W (10 min ramp) (ii) 20 min for cooling Max. pressure: 80 bar Max. temperature: 280 ºC Dilution up to 25 ml
Dilution up to 25 ml
Determination of RCC and residual acidity Determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn by ICP OES Figure 1. Flowchart with conditions used for UltraWave and Multiwave methods 19
ACCEPTED MANUSCRIPT
28
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16
12
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RCC (%)
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8
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4 0 0.50
0.75
1.00
1.25
1.50
1.75 1.75
2.00 2.00
Sample mass (g)
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0.25
Figure 2. Residual carbon content after digestion of Brazil nuts using UltraWave
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(UW) and Multiwave (MW) systems. For MW system digestion was performed using
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only concentrated HNO3. Error bars represent the standard deviation (n = 3).
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ACCEPTED MANUSCRIPT
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80 70
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50
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40 30 20
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Residual acidity (%)
60
10
0.5
0.75
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0.25
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0
1
1.25
1.5
1.75 1.75
2.002
Sample mass, Sample mass (g)g
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Figure 3. Results of residual acidity obtained by potentiometric titration after digestion of Brazil nuts using UltraWave (UW) and Multiwave (MW) systems. For MW
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system digestion was performed using only concentrated HNO3. Error bars represent the standard deviation (n = 3).
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ACCEPTED MANUSCRIPT Table 1. Operational conditions utilized for determination of elements using ICP OES Parameter 1400
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RF power, W
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Gas flow rate, L min-1
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Plasma Auxiliary
0.20 0.70
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Nebulizer
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Element wavelength (nm)
Al(I) = 396.153 As(I) = 188.979 C*(I) =193.091 Cd(I) = 228.802 Co(II) = 228.616 Cu(I) = 327.393 Fe(II) = 238.204 Mn(II) = 257.610 Ni(II) = 231.604 Pb(II) = 220.353 V(II) = 290.880 Zn(II) = 206.200 Y(II)* = 371.029
* Used for RCC determination. (I) Atomic line. (II) ionic line.
22
ACCEPTED MANUSCRIPT Table 2. Concentrations of trace elements (g g-1) in nuts (mean ± standard deviation, n = 5) Brazil nut
Cashewnut
Walnut
Multiwave
Ultrawave
Multiwave
Ultrawave
Multiwave
Al
< 0.05
< 0.145
< 0.05
< 0.145
< 0.05
< 0.145
As
< 0.79
< 2.38
< 0.79
< 2.38
< 0.79
< 2.38
Cd
< 0.03
< 0.09
< 0.03
< 0.09
< 0.03
< 0.09
Co
0.80 ± 0.06
0.81 ± 0.08
0.12 ± 0.01
0.13 ± 0.02
< 0.03
< 0.08
Cu
18.2 ± 1.10
18.7 ± 2.08
12.5 ± 1.33
12.0 ± 1.08
13.5 ± 1.28
12.4 ± 1.08
Fe
0.94 ± 0.06
0.90 ± 0.08
0.23 ± 0.02
0.25 ± 0.03
0.69 ± 0.04
0.70 ± 0.06
Mn
9.72 ± 0.51
9.63 ± 0.71
26.4 ± 2.28
24.5 ± 2.31
26.2 ± 1.98
26.3 ± 2.34
Ni
4.23 ± 0.24
4.40 ± 0.40
1.31 ± 0.10
1.30 ± 0.11
1.75 ± 0.14
1.80 ± 0.11
Pb
< 0.180
< 0.540
< 0.180
< 0.540
< 0.180
< 0.540
V
0.26 ± 0.02
0.251 ± 0.022
0.27 ± 0.03
0.26 ± 0.03
0.35 ± 0.02
0.34 ± 0.02
Zn
41.2 ± 2.11
42.4 ± 2.20
26.3 ± 1.76
28.2 ± 2.17
25.7 ± 2.41
26.0 ± 2.15
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Ultrawave
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Element
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ACCEPTED MANUSCRIPT
Oyster tissue (NIST 1566 b)
Skim milk powder (BCR 151)
Certified
Ultrawave
Certified
Al
201 ± 10
197.2 ± 6.0
---
---
As
7.50 ± 1.05
7.65 ± 0.65
---
---
Cd
2.61 ± 0.10
2.48 ± 0.08
0.105 ± 0.021
Co
0.390 ± 0.040
0.371 ± 0.009
---
Cu
73.2 ± 4.18
71.6 ± 1.6
Fe
208 ± 10
Mn
Dogfish liver (DOLT 4)
Ultrawave
Certified
---
---
9.50 ± 0.38
9.66 ± 0.62
0.101 ± 0.008
25.1 ± 1.05
24.3 ± 0.81
---
---
---
5.21 ± 0.11
5.23 ± 0.08
32.2 ± 0.72
31.2 ± 1.1
205.8 ± 6.8
52.4 ± 2.05
50.1 ± 1.3
1799 ± 58
1833 ± 75
19.1 ± 2.03
18.5 ± 0.2
---
---
---
---
Ni
0.970 ± 0.050
1.04 ± 0.09
---
---
0.950 ± 0.110
0.970 ± 0.110
Pb
< 0.599
0.308 ± 0.009
2.02 ± 0.10
2.002 ± 0.026
< 0.18
0.160 ± 0.040
V
0.591 ± 0.080
0.577 ± 0.023
---
---
---
---
Zn
1405 ± 78
1424 ± 46
---
---
119 ± 4
116 ± 6
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Ultrawave
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Element
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Table 3. Concentration of trace elements (g g-1) in certified reference materials (mean ± standard deviation, n = 5).
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ACCEPTED MANUSCRIPT Table 4. Limits of detection and quantification (ng g-1) obtained by ICP-OES after digestion using UltraWave (1.5 g, maximum sample mass) and Multiwave (0.5 g,
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maximum sample mass).
Multiwave
Al
49
163
As
790
2631
Cd
29
97
Co
27
90
Cu
51
Fe
12
Mn
8
Ni
50
Pb
180
145
483
2380
7925
87
290
81
270
170
153
509
40
36
120
27
24
80
167
150
500
599
540
1798
14
47
42
140
36
120
108
360
D TE
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Zn
LOQ
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V
LOD
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LOQ
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LOD
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UltraWave
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ACCEPTED MANUSCRIPT Highlights
1. It was possible to determine trace elements in low concentration in nuts
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3. Low RCC and residual acidity in final digests
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2. High sample mass (up to 1.5 g) can be used
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4. Carbon related effects were not observed in ICP OES determination
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5. Low LODs were obtained
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S0026-265X(14)00072-1 doi: 10.1016/j.microc.2014.04.013 MICROC 1944
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
31 December 2013 23 April 2014 23 April 2014
Please cite this article as: Cristiano C. M¨ uller, Aline L.H. Muller, Camillo Pirola, Fabio A. Duarte, Erico M.M. Flores, Edson I. Muller, Feasibility of Nuts Digestion Using Single Reaction Chamber for Further Trace Elements Determination by ICP OES, Microchemical Journal (2014), doi: 10.1016/j.microc.2014.04.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Feasibility of Nuts Digestion Using Single Reaction Chamber for
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Further Trace Elements Determination by ICP OES
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Cristiano C. Müllera, Aline L. H. Mullera, Camillo Pirolab, Fabio A. Duartea, Erico M.
a
Departamento de Química, Universidade Federal de Santa Maria, 97105-990, Santa
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Maria, Brazil b
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M. Floresa, Edson I. Mullera*
Milestone Srl, 24010, Sorisole, BG, Italy.
*E-mail address: [email protected]
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ACCEPTED MANUSCRIPT Abstract A method for nuts digestion using Single Reaction Chamber (SRC,
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UltraWave™ system) was proposed. High sample masses (up to 1.5 g) were used in
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order to improve the limits of detection for trace elements. Nuts with high fat content
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(Brazil nut, cashew nut and walnut) were used to evaluate the efficiency of the proposed method. Different oxidant conditions were evaluated (only concentrated HNO3 and mixtures of HNO3 and H2O2). Comparison with Multiwave™ system was also carried Efficiency of digestion for both systems was evaluated by determination of
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out.
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residual carbon content (RCC) and residual acidity of final digestates. Determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn was performed using inductively coupled plasma optical emission spectrometry (ICP OES). Carbon related effects on analytical
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response for emission lines used in ICP OES determination was also evaluated. The best efficiency of digests was obtained using UltraWave™ system with 1.5 g of nut,
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concentrated HNO3 (without the addition of H2O2) and temperature and pressure of 275 °C and 180 bar, respectively. In this condition the RCC was 4.6% (correspondent to
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2350 mg L-1 of C in digestates) and residual acidity was lower than 5% (equivalent to 0.3 mL of concentrated HNO3 diluted up to 25 mL). On the other hand, for Multiwave™ system only 0.5 g of nuts could be digested. Carbon related effects on analytical response on ICP OES determination were not observed in these conditions. Significant differences were not observed by the comparison between the results obtained by UltraWave™ system and certified reference values (t test, 95% confidence level). Therefore, using UltraWave™ system that allowed a relatively high sample mass to be digested it was possible to determine trace elements at low concentration even using ICP OES in conventional mode. Keywords: single reaction chamber (SRC), nuts, microwave digestion, ICP OES 2
ACCEPTED MANUSCRIPT 1. Introduction Methods using concentrated acids and microwave heating have been widely
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used for sample preparation [1-3]. Especially for organic matrices, concentrated HNO3
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acid is preferred to convert samples in a suitable solution for further determination of
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trace elements using plasma-based techniques [2,4]. However, some organic compounds are not completely oxidized using conventional low and medium pressure microwave systems and the residual carbon content (RCC, the remaining content of C
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after digestion related to the total C content in original sample) in the final digests can
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affect the analytical response of some emission lines during element determination by inductively coupled plasma optical emission spectrometry (ICP OES) [5-7]. Studies reported in literature have shown that carbon related effects become more severe when
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carbon concentration in digests is higher than 5000 mg L-1 [7]. In addition, the emission intensity of some atomic lines with excitation energies below 6 eV decreases (up to
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15%) whereas the emission intensity of atomic lines with higher excitation energies (e.g., As and Se) can be increased (up to 30%). It is important to mention that the
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emission intensity of ionic lines is not affected by residual carbon content in digests [7]. Moreover, sample preparation methods should produce digests containing the lowest RCC as possible [8]. Additionally, nuts are considered as difficult matrix and it is not easy to digest due to the high content of polyunsaturated fatty acids. These compounds are hardly oxidized by HNO3 and a complete digestion is achieved only by using concentrated HNO3 and temperatures higher than 180 ºC [9]. In order to minimize this problem, many methods for nuts digestion have been reported in literature [10-12]. The most simple and widely used is dry ashing method, that allows obtaining digests with very low RCC (lower than 1%) [13]. However, some drawbacks have been related, such
3
ACCEPTED MANUSCRIPT as losses of volatile elements (e.g., Cd, Pb and Hg) and the possibility of contamination during sample digestion step [14].
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Microwave induced combustion (MIC) is a method that was developed in a few
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years ago and can be considered as suitable for complete digestion of organic samples,
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allowing digests with RCC lower than 1% that is well suitable for further analysis by several spectrometric or chromatographic techniques [15-21]. However, up to now only about 0.5 g of sample can be digested in order to prevent pressure overload of
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microwave system. In addition, a special vessel is required for pressurization with
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oxygen [11, 22]. In order to improve the sample mass introduced in the combustion system, focused microwave induced combustion (FMIC) was recently proposed [23]. This method allows the digestion of up to 3 g of sample and diluted HNO3 can be used
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as absorbing solution allowing lower LODs [23-24]. However, the instrumentation required to perform FMIC method is not commercially available up to now. On the
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other hand, wet digestion using focused microwave systems allows the digestion of high sample amounts (up to 10 g), specially because there is no overpressure and reagents
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can be added during the digestion [25]. However, one of the main drawbacks is related to the necessity of using sulfuric acid to improve the digestion temperature as digestion is performed in atmospheric pressure [25]. In addition to physical interferences caused by sulfuric acid, some elements such as Ag, Ba, Pb and others, may form insoluble compounds with sulfate [26]. Microwave assisted digestion (MAD) using closed vessels and concentrated HNO3 is preferred for organic sample digestion due to the reduced risk of contamination and high sample throughput [1]. This system has been used for digestion of nuts (babassu, sapucaia e cashew) using sample mass ranging from 0.15 to 0.25 g and a mixture of oxidants (2 mL de HNO3 and 1 mL of H2O2). However, even using relatively 4
ACCEPTED MANUSCRIPT high temperature RCC was up to 13.4% (m/m). One of the main limitation of MAD is the maximum sample mass that could be digested, which is limited by the maximum
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pressure of microwave system [12].
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In this regard, some manufactures have developed microwave systems that allow
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the use of very high pressure and high temperature to assure an efficient digestion of higher sample masses. Recently, it was proposed a new system called single reaction chamber (SRC, UltraWave™ system) that allows temperature and pressure up to 300 ºC
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and 199 bar, respectively. In the UltraWave™ system, the power source (1500 W) is
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located at the base of a chamber, allowing an efficient distribution of microwave radiation. In addition, prior to start the heating program, the chamber is pressurized with N2 which acts as a “cover” over the vials, preventing boiling and spitting, and
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eliminating any possibility of cross-contamination [8]. This system was used for the digestion of different organic matrices (milk powder, bovine liver, bovine kidney, etc.)
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in the same running batch, operating at maximum temperature and pressure of 250 ºC and 120 bar, respectively. Using this system, 0.5 g of sample were placed into
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fluoropolymer vessels containing diluted HNO3 (2 mol L-1) and H2O2. The RCC and residual acidity were lower than 1.5% and 1.04 mol L-1 of HNO3, respectively [8]. In the present study, the UltraWave™ system was evaluated for nuts digestion (Brazil nut, cashew nut and walnut samples) using HNO3. Hydrogen peroxide was also evaluated as auxiliary reagent for HNO3 regeneration in UltraWave™ system [27]. In order to evaluate the digestion efficiency of this system, sample masses up to 2 g were digested. Nuts digestion was also performed using a Multiwave™ system in order to compare the efficiency of both systems. The digestion efficiency was evaluated by determination of RCC in final digests as well as by determination of residual acidity. The follow elements were determined by ICP OES: Al, As, Cd, Co, Cu, Fe, Mn, Ni, Pb 5
ACCEPTED MANUSCRIPT and Zn. Finally, the accuracy of the proposed method using UltraWave™ system was
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evaluated using certified reference materials.
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2. Material and methods
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2.1. Instrumentation
In this study two microwave sample preparation systems were used for nuts digestion. A digestion system named UltraWave™ (Milestone, Sorisole, Italy) equipped
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with five quartz vessel (total volume of 40 mL) was used. This system has a microwave
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cavity (1 L) made of stainless steel and covered with polytetrafluoroethylene (PTFE) vessel which was sealed and pressurized with 40 bar of argon 99.996% (White Martins, São Paulo, Brazil). All experiments using UltraWave™ were carried out using 1500 W
bar, respectively.
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of microwave irradiation and maximum temperature and pressure of 275 °C and 180
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A Multiwave™ 3000 (Anton Paar, Graz, Austria) system with eight quartz closed vessels (total volume of 80 mL) was used for sample digestion. Maximum
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temperature and pressure used were 280°C and 80 bar, respectively. The irradiation power of Multiwave™ system was 1400 W. An inductively coupled plasma optical emission spectrometer (Perkin Elmer, model Optima 4300DV, Shelton, USA) using axial view mode and equipped with cyclonic spray chamber and concentric nebulizer was used for determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn. For plasma generation, nebulization and auxiliary gas, argon with purity of 99.996 % was used. The conditions used for elements determination by ICP OES are summarized in Table 1.
2.2. Reagents, samples and standards 6
ACCEPTED MANUSCRIPT Water was purified by distillation and further submitted to a Milli-Q system (Millipore Corp. Bedford, USA). Purified water was used to prepare all reference
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solutions and reagents used in this study. Nitric acid (65%, Merck, Darmstadt,
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Germany) used for sample digestion was purified in a quartz sub-boiling system
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(Milestone, Duopur model, Bergamo, Italy). Hydrogen peroxide (30%, Merck, Darmstadt, Germany) was also used as oxidant agent. A multi-element stock solution (PlasmaCal SCP33MS, SCP Science, Quebec, Canada) containing 10 mg L-1 of all
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elements evaluated in this study was used to prepare the calibration curve for ICP OES determination in the range of 1 to 100 µg L-1. The multielement stock solution (SCP
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33MS 10 mg L-1) was also used for evaluation of carbon related effects on the response of emission lines used for analytes determination. Reference solutions used for RCC
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determination were prepared by sequential dilution of a carbon stock solution prepared after dissolution of citric acid (Merck) in water. Yttrium (1 mg L-1, Spex CertPrep,
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Metuchen, USA) was used as internal standard for RCC determination. Samples of Brazil nuts, cashew nuts and walnuts purchased at local market were
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used in this study. About 200 g of nuts were ground and homogenized in household food mill during 10 min. The evaluation of RCC and residual acidity were carried out using Brazil nuts. Accuracy was evaluated using certified reference materials (CRM) of oyster tissue (NIST 1566 b), skim milk powder (BCR 151) and dogfish liver (DOLT-4).
2.4. Microwave assisted digestion using UltraWave™ system Sample masses ranging from 0.25 to 2 g were directly weighed inside the quartz vessels. Three oxidant conditions were evaluated: 6 mL of concentrated HNO3 (Condition 1); 5 mL of concentrated HNO3 + 1 mL H2O2 (Condition 2) and 4 mL of concentrated HNO3 + 2 mL of H2O2 (Condition 3). About 120 mL of water and 5 mL 7
ACCEPTED MANUSCRIPT of H2O2 were inserted into the SRC chamber and was pressurized up to 40 bar to cover and keep the cap in the upper part of the quartz vessel. All digestions were carried out
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using maximum temperature and pressure of 275 °C and 180 bar, respectively. In spite
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of the SRC system allows higher pressure and temperature conditions, authors
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arbitrarily selected 275oC and 180 bar in order to assure a more safe operation condition. Microwave heating program was as follow: 10 min of ramp and hold for 20 min at 1500 W. After cooling down (65 ºC) the chamber was depressurized and the
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resultant solution was diluted in volumetric flasks. Final digests were analyzed by ICP
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OES (Figure 1).
2.5. Microwave assisted wet digestion using Multiwave™ system
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Sample masses ranging from 0.25 to 2 g were directly weighed inside the quartz vessels. Only concentrated HNO3 (6 mL) was used as oxidant agent. All digestions were
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performed using maximum temperature and pressure available in the microwave system (280 °C and 80 bar, respectively). The heating program was as follows: i) 10 min of
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ramp up to 1400 W; ii) 1400 W for 20 min and iii) 0 W for 20 min. Final digests were diluted in volumetric flask and analyzed by ICP OES (Figure 1).
2.6. Evaluation of digestion efficiency Determination of RCC and residual acidity in digests obtained by both microwave digestion systems were carried out in order to evaluate the digestion efficiency. Residual carbon content was determined by ICP OES [28] and expressed as mg of C per 100 mg of the original sample (or simply, %). Determination of residual acidity was performed using potentiometric titration with KOH solution and expressed as % of the original acid that remains in final digest. 8
ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. Digestion efficiency of UltraWave™ and Multiwave™ systems
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Initial experiments showed that sample masses higher than 0.75 g were not
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completely digested using HNO3 and Multiwave™ system. RCC and residual acidity
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were not determined for digests obtained using Multiwave™ system. On the other hand, sample masses up to 2 g were efficiently digested using UltraWave™
and the
Condition 1 (only concentrated HNO3). Results for RCC are showed in Figure 2 for nuts
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digestion using both microwave systems (UltraWave™ and Multiwave™). For samples
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masses lower than 1.25 g, it was not observed a significant increase in the RCC values when UltraWave™ and Condition 1 were used (Figure 2, black bars). RCC for 0.25 g of sample was 0.86% (75 mg L-1 of C in final digest), while for 1.25 g of sample, the RCC
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was about 1.8% (630 mg L-1 of C in final digest). The RCC values up to 14.4% (9530 mg L-1 of C) were observed for the digestion of 2 g of sample in UltraWave™ system
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using concentrated HNO3 (Condition 1). It has been reported in literature that carbon related effects were observed in analytical response for emission lines for solutions
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containing carbon contents higher than 5000 mg L-1 [7]. In this sense, digests of spiked sample (final concentration of 100 g L-1 for all analytes), unspiked sample and standard solution (100 g L-1 of all analytes) were analyzed by ICP OES. No suppression (difference of original signal lower than 5%) was observed for all the analytes. It is important to point out that most of lines used in this study for trace element determination were ionic emission lines (except for Al, As, Cd and Cu) which were less affected by carbon effect on ICP OES determination. However, for As emission line (As I – 188.979) an increase of 20% in analytical response was observed and 2 g of sample could not be recommended for trace element determination in nuts. It is well noticed in literature that As emission line is more affected by residual carbon 9
ACCEPTED MANUSCRIPT content [7]. Similar enhancement of As signal was observed for sample mass equal to 1.75 g (7480 mg L-1 of C in final digest). Therefore, in order to minimize the carbon
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effects on analytical response, especially for As, a digestion of 1.5 g using UltraWave™
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system and concentrated HNO3 is recommended and this condition was used for the
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digestion of all nuts samples. Digests of 1.5 g of Brazil nut showed RCC of 4.6% (2350 mg L-1 of C).
Comparing the digests obtained by both digestion systems, the RCC was always
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higher for Multiwave™ system (Figure 2, white bars). It very probably occurred due to the lower temperature achieved by this system during the heating program (digestion
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temperature was never higher than 210 ºC even selecting a maximum value of 280 oC). Additionally, the temperature achieved using UltraWave™ system was close to 275 °C
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(maximum temperature) independent of the sample mass. Using 0.75 g of sample in Multiwave™ system with concentrated HNO3, RCC was about 20.8% (5200 mg L-1 of
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C). A significant increase or suppression (higher than 5%) of analytical response for emission lines was not observed when using 0.75 g of sample (Brazil nut) digested by
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Multiwave™ system with concentrated HNO3. However, the RCC (5200 mg L-1 of C in final digest) was very close to the value that can promote effects on analytical response for ICP OES measurements (5000 mg L-1). In this sense, the use of only 0.5 g of nut sample was recommended for digestion of all samples with Multiwave™ system. Comparing the digests using the three digestion conditions for UltraWave™ system with sample mass of 0.75 g, it was not observed significant difference in RCC values (Figure 2). For Conditions 2 and 3, sample masses up to 1.5 g could be digested. However, RCC values for Condition 3 were higher than 14% (6000 mg L-1 of C) and 24% (12000 mg L-1 of C) when 1.25 and 1.5 g of sample were used, respectively. Thus,
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ACCEPTED MANUSCRIPT for Condition 3, only samples masses lower or equal to 1 g can be used due to carbon effects on ICP OES determination.
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more efficient digestion in comparison to Multiwave™ system.
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Finally, UltraWave™ system allowed the digestion of higher sample masses and
3.2. Residual acidity of digests obtained using UltraWave™ and Multiwave™ systems Depending on the concentration of HNO3 in final digests (after dilution),
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suppression of analytical response can be observed [29]. Unfortunately, the exact matrix
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matching of standards is not possible. On this matter, digests with low residual acidity are preferred to obtain accurate results and allow external calibration with standard solutions containing the analytes in diluted HNO3 (generally 5% HNO3 solution).
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Residual acidity in the digests obtained using both systems are shown in Figure 3. It is possible to observe that the acidity is always lower in digests obtained by UltraWave™
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system. This probably occurs due to the higher temperature achieved by UltraWave™ which allows more efficient oxidant condition and more HNO3 was consumed for
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oxidation of organic matter. For example, comparing the acidity of both systems, when 0.75 g sample mass was used in combination with concentrated HNO3, the acidity was two times higher while the RCC value was about twenty fold higher when Multiwave™ system was emplyoed. In addition, the acidity of digests using 1.5 g in UltraWave™ system and concentrated HNO3 (optimized condition) was lower than 5% (equivalent to 0.3 mL of concentrated HNO3 diluted up to 25 mL). This amount of HNO3 did not caused the suppression of the signal in ICP OES determination. Acidity of digests containing H2O2 was always similar or lower that showed by digests obtained using only concentrated HNO3. This fact evidences that H2O2, contrarily to other systems does not results in HNO3 regeneration during digestion using 11
ACCEPTED MANUSCRIPT UltraWave™. It can be explained considering the higher free volume inside the reaction chamber in UltraWave™ system when compared to individual vessels (80 mL internal
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volume) used in Multiwave™ microwave oven.
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3.3 Comparison of proposed UltraWave method with other methods previously described in literature for nuts analysis
The proposed method using UltraWave system showed some advantages,
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particularly related to sample mass and RCC values. Sample masses used in the
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proposed method (1.5 g) is at least 3 times higher than the mass used in other works described in literature for nuts analysis [30-32]. In the same way, RCC value was better for digests obtained using UltraWave system in comparison with RCC values available
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in literature [12]. For example, Brazil nuts digests by the proposed method showed RCC values of 4.6% that is two times lower than the RCC values previously described for nut
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analysis [12]. It probably occurs due to the higher temperature and pressure achieved by the proposed method. Acidity of final digests was lower than 5% of HNO3 and did not
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cause changes in analytical signal in determination by ICP OES. The analytical throughput was lower in comparison with data available in literature regarding to nuts analysis [12, 30]. However, the UltraWave system also allows the use of a rotor with 15 vessels, instead of 5 vessels of 40 mL (used in the proposed method) and in this case the analytical throughput should be comparable to those reported in literature [12, 30].
3.4. Determination of trace elements Brazil nut, walnut and cashew nuts by ICP OES Brazil nut, walnut and cashew nuts were digested using 1.5 g and 0.5 g in UltraWave™ and Multiwave™ systems, respectively. Results obtained by ICP OES for Al, As, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn are shown in Table 2. For all elements, the 12
ACCEPTED MANUSCRIPT relative standard deviation (RSD) was lower than 12% after digestion using UltraWave™ and Multiwave™ systems. Comparing the analytes concentration, no
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significant differences (t test, 95% confidence level) were observed for both microwave
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digestion systems. Zinc showed the highest concentration in comparison with the other
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trace elements, whereas Al, As, Cd and Pb were not detected in nuts samples (LOD lower than 790 ng g-1). Concentration of Co, Fe, Ni and Zn were higher in Brazil nut when compared to the other nuts. On the other hand, the concentration of Cu and V was
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similar in all kinds of nuts. For other elements nuts showed concentration ranging from
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lower than 0.03 to 0.80 g g-1 for Co, and 9.7 to 26 g g-1 for Mn. Accuracy was evaluated by the analysis of three CRMs and no significant differences were observed when certified results were compared with those obtained
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using UltraWave™ system, as shown in Table 3. The LOD and LOQ values obtained by ICP OES after digestion using
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UltraWave™ were always lower than those obtained by Multiwave system (Table 4). Therefore, UltraWave™ system it was possible to determine trace elements at low
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concentration even using ICP OES, a technique with low sensitivity when compared to ICP-MS. Moreover, the sample mass that could be digested using UltraWave™ (1.5 g) was three times higher that this digested by Multiwave system.
4. Conclusions This study showed the feasibility of UltraWave system for digestion of nuts samples and subsequent determination of trace elements by ICP OES. The main advantages of this system are the possibility to digest relatively large sample masses (up to 1.5 g) that improves the LODs. Moreover, low RCC and residual acidity were obtained making the proposed procedure well suited to further analysis by ICP OES. In 13
ACCEPTED MANUSCRIPT addition, as a result of higher sample mass using UltraWave system, the LODs were
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better than those obtained for other methods.
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Acknowledgements
The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de
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Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do
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Sul (FAPERGS) for supporting this study.
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References
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[10] A. Şimşek, D. Korkmaz, Y.S.Velioǧ lu, O.Y. Ataman, Determination of boron in hazelnut (Corylus avellana L.) varieties by inductively coupled plasma optical emission spectrometry and spectrophotometry, Food Chem. 83 (2003) 293-296.
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[11] A.L.H. Muller, C.C. Muller, F. Lyra, P.A. Mello, M.F. Mesko, E.I. Muller, E.M. M. Flores, Determination of Toxic Elements in Nuts by Inductively Coupled
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[12] J. Naozuka, E.C. Vieira, A.N. Nascimento, P.V. Oliveira, Elemental analysis of nuts and seeds by axially viewed ICP OES, Food Chem. 124 (2011) 1667–1672. [13] Z. Mester, R. Sturgeon, Sample Preparation for Trace Element Analysis, Elsevier, 2003. [14] J.S. Barin, B. Tischer, R.S. Picoloto, F.G. Antes, F.E.B. Silva, F.R. Paula, E.M.M. Flores, J. Anal. At. Spectrom., 2013, In press, DOI: 10.1039/C3JA50334H. [15] J.V. Maciel, C.L. Knorr, E.M.M. Flores, E.I. Muller, M.F. Mesko, E.G. Primel, F.A. Duarte, Feasibility of microwave-induced combustion for trace element determination in Engraulis anchoita by ICP-MS Food Chem. 145 (2014) 927-931.
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(2010) 1125–1131.
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[18] M.F. Mesko, D.P. Moraes, J..S. Barin, V.L. Dressler, G. Knapp, E.M.M. Flores, Digestion of biological materials using the microwave-assisted sample combustion technique, Microchem. J. 82 (2006) 183–188.
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[19] J.S.F. Pereira, P.A. Mello, F.A. Duarte, M.F.P. Santos, R.C.L. Guimarães, G. Knapp, V.L. Dressler, E.M.M. Flores, Feasibility of Microwave-Induced
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Combustion for Digestion of Crude Oil Vacuum Distillation Residue for Chlorine Determination, Energy Fuels, 23 (2009) 6015–6019.
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[20] F.G Antes, F.A. Duarte, M.F. Mesko, M.A. Nunes, V.A. Pereira, E.I. Muller, V.L. Dressler, E.M.M. Flores, Determination of toxic elements in coal by ICP-MS after digestion using microwave-induced combustion, Talanta, 83 (2010) 364-369. [21] A.L.H. Muller, P.A. Mello, M.F. Mesko, F.A. Duarte, V.L. Dressler, E.I. Muller, E.M.M. Flores, Bromine and iodine determination in active pharmaceutical ingredients by ICP-MS, J. Anal. At. Spectrom. 27 (2012) 1889-1994. [22] J.T.P. Barbosa, C.M.M. Santos, L.S. Bispo, F.H. Lyra, J.M. David, M.G.A. Korn, E.M.M. Flores, Bromine, Chlorine, and Iodine Determination in Soybean and its Products by ICP-MS After Digestion Using Microwave-Induced Combustion, Food Anal. Methods, 6 (2013) 1065–1070. 16
ACCEPTED MANUSCRIPT [23] M.F. Mesko, J.S. F. Pereira, D.P. Moraes, J.S. Barin, P.A. Mello, J.N.G. Paniz, J. A. Nobrega, M.G.A. Korn, E.M. M. Flores, Focused Microwave-Induced
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[25] J.A. Nobrega,, L.C. Trevizan, G.C.L. Araujo, A.R.A. Nogueira, Review. Focused-
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Microwave-assisted strategies for sample preparation. Spectrochim. Acta Part B 57 (2002) 1855–1876.
[26] M. Hoenig, Critical discussion of trace element analysis of plant matrices, Sci.
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[27] C.A. Bizzi, E.L.M. Flores, J.A. Nobrega, J.S. S. Oliveira, L. Schmidt S.R. Mortari,
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Evaluation of a digestion procedure based on the use of diluted HNO3 solutions and H2O2 for the multielement determination of whole milk powder and bovine liver by
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ICP-based techniques, , J. Anal. At. Spectrom., 2013, In press, DOI: 10.1039/c3ja50330e. [28] S.T. Gouveia, F.V. Silva, L.M. Costa, A.R.A. Nogueira, J.A. Nobrega, Determination of residual carbon by inductively-coupled plasma optical emission spectrometry with axial and radial view configurations, Anal. Chim. Acta, 445 (2001) 269-275. [29] Jose-Luis Todoli, Jean-Michel Mermet, Review. Acid interferences in atomic spectrometry: analyte signal effects and subsequent reduction, Spectrochim. Acta Part B 54 (1999) 895-929.
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ACCEPTED MANUSCRIPT [30] K. Phan-Thien, G.C. Wright, N. A. Lee. Inductively coupled plasma-mass spectrometry (ICP-MS) and –optical emission spectroscopy (ICP-OES) for
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determination of essential minerals in closed acid digestates of peanuts (Arachis
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hypogaea L.), Food Chem. 134 (2012) 453-460.
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[31] E. P Nardi, F.S. Evangelista, L. Tormen, T.D. SaintPierre, A.J. Curtius, S. S. Souza, F. Barbosa Jr.. The use of inductively coupled plasma mass spectrometry (ICP-MS) for the determination of toxic elements and essential elements in
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different types of food samples, Food Chem. 112 (2009) 727-732.
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[32] A.A. Momen, G.A. Zachariadis, A.N. Anthemidis, J. A. Stratis, Use of fractional factorial design for optimization of digestion procedures followed by multi-element determination of essential and non-essential elements in nuts using ICP-OES
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technique, Talanta 71 (2007) 443-451.
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NUTS
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Grinding
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Sample (Brazil nut, cashew nut and walnut)
Sample mass: 0.25 to 2 g Digestion mixture:
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Heating program:
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Condition 1: 6 mL HNO3 Condition 2: 5 ml HNO3 + 1 mL H2O2 Condition 3: 4 ml HNO3 + 2 mL H2O2
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ULTRAWAVE
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(i) 30 min at 275 ºC (10 min ramp) (ii) 20 min for cooling Max. pressure: 180 bar Max. temperature: 275 ºC
MULTIWAVE Sample mass: 0.25 to 2 g Digestion mixture: (i) 6 ml HNO3
Heating program: (i) 30 min at 1400 W (10 min ramp) (ii) 20 min for cooling Max. pressure: 80 bar Max. temperature: 280 ºC Dilution up to 25 ml
Dilution up to 25 ml
Determination of RCC and residual acidity Determination of Al, As, C, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn by ICP OES Figure 1. Flowchart with conditions used for UltraWave and Multiwave methods 19
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28
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12
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RCC (%)
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4 0 0.50
0.75
1.00
1.25
1.50
1.75 1.75
2.00 2.00
Sample mass (g)
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0.25
Figure 2. Residual carbon content after digestion of Brazil nuts using UltraWave
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(UW) and Multiwave (MW) systems. For MW system digestion was performed using
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only concentrated HNO3. Error bars represent the standard deviation (n = 3).
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80 70
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50
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40 30 20
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Residual acidity (%)
60
10
0.5
0.75
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0
1
1.25
1.5
1.75 1.75
2.002
Sample mass, Sample mass (g)g
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Figure 3. Results of residual acidity obtained by potentiometric titration after digestion of Brazil nuts using UltraWave (UW) and Multiwave (MW) systems. For MW
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system digestion was performed using only concentrated HNO3. Error bars represent the standard deviation (n = 3).
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ACCEPTED MANUSCRIPT Table 1. Operational conditions utilized for determination of elements using ICP OES Parameter 1400
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RF power, W
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Gas flow rate, L min-1
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Plasma Auxiliary
0.20 0.70
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Nebulizer
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Element wavelength (nm)
Al(I) = 396.153 As(I) = 188.979 C*(I) =193.091 Cd(I) = 228.802 Co(II) = 228.616 Cu(I) = 327.393 Fe(II) = 238.204 Mn(II) = 257.610 Ni(II) = 231.604 Pb(II) = 220.353 V(II) = 290.880 Zn(II) = 206.200 Y(II)* = 371.029
* Used for RCC determination. (I) Atomic line. (II) ionic line.
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ACCEPTED MANUSCRIPT Table 2. Concentrations of trace elements (g g-1) in nuts (mean ± standard deviation, n = 5) Brazil nut
Cashewnut
Walnut
Multiwave
Ultrawave
Multiwave
Ultrawave
Multiwave
Al
< 0.05
< 0.145
< 0.05
< 0.145
< 0.05
< 0.145
As
< 0.79
< 2.38
< 0.79
< 2.38
< 0.79
< 2.38
Cd
< 0.03
< 0.09
< 0.03
< 0.09
< 0.03
< 0.09
Co
0.80 ± 0.06
0.81 ± 0.08
0.12 ± 0.01
0.13 ± 0.02
< 0.03
< 0.08
Cu
18.2 ± 1.10
18.7 ± 2.08
12.5 ± 1.33
12.0 ± 1.08
13.5 ± 1.28
12.4 ± 1.08
Fe
0.94 ± 0.06
0.90 ± 0.08
0.23 ± 0.02
0.25 ± 0.03
0.69 ± 0.04
0.70 ± 0.06
Mn
9.72 ± 0.51
9.63 ± 0.71
26.4 ± 2.28
24.5 ± 2.31
26.2 ± 1.98
26.3 ± 2.34
Ni
4.23 ± 0.24
4.40 ± 0.40
1.31 ± 0.10
1.30 ± 0.11
1.75 ± 0.14
1.80 ± 0.11
Pb
< 0.180
< 0.540
< 0.180
< 0.540
< 0.180
< 0.540
V
0.26 ± 0.02
0.251 ± 0.022
0.27 ± 0.03
0.26 ± 0.03
0.35 ± 0.02
0.34 ± 0.02
Zn
41.2 ± 2.11
42.4 ± 2.20
26.3 ± 1.76
28.2 ± 2.17
25.7 ± 2.41
26.0 ± 2.15
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Ultrawave
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Element
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Oyster tissue (NIST 1566 b)
Skim milk powder (BCR 151)
Certified
Ultrawave
Certified
Al
201 ± 10
197.2 ± 6.0
---
---
As
7.50 ± 1.05
7.65 ± 0.65
---
---
Cd
2.61 ± 0.10
2.48 ± 0.08
0.105 ± 0.021
Co
0.390 ± 0.040
0.371 ± 0.009
---
Cu
73.2 ± 4.18
71.6 ± 1.6
Fe
208 ± 10
Mn
Dogfish liver (DOLT 4)
Ultrawave
Certified
---
---
9.50 ± 0.38
9.66 ± 0.62
0.101 ± 0.008
25.1 ± 1.05
24.3 ± 0.81
---
---
---
5.21 ± 0.11
5.23 ± 0.08
32.2 ± 0.72
31.2 ± 1.1
205.8 ± 6.8
52.4 ± 2.05
50.1 ± 1.3
1799 ± 58
1833 ± 75
19.1 ± 2.03
18.5 ± 0.2
---
---
---
---
Ni
0.970 ± 0.050
1.04 ± 0.09
---
---
0.950 ± 0.110
0.970 ± 0.110
Pb
< 0.599
0.308 ± 0.009
2.02 ± 0.10
2.002 ± 0.026
< 0.18
0.160 ± 0.040
V
0.591 ± 0.080
0.577 ± 0.023
---
---
---
---
Zn
1405 ± 78
1424 ± 46
---
---
119 ± 4
116 ± 6
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Ultrawave
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Element
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Table 3. Concentration of trace elements (g g-1) in certified reference materials (mean ± standard deviation, n = 5).
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ACCEPTED MANUSCRIPT Table 4. Limits of detection and quantification (ng g-1) obtained by ICP-OES after digestion using UltraWave (1.5 g, maximum sample mass) and Multiwave (0.5 g,
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maximum sample mass).
Multiwave
Al
49
163
As
790
2631
Cd
29
97
Co
27
90
Cu
51
Fe
12
Mn
8
Ni
50
Pb
180
145
483
2380
7925
87
290
81
270
170
153
509
40
36
120
27
24
80
167
150
500
599
540
1798
14
47
42
140
36
120
108
360
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Zn
LOQ
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V
LOD
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LOQ
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LOD
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UltraWave
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ACCEPTED MANUSCRIPT Highlights
1. It was possible to determine trace elements in low concentration in nuts
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3. Low RCC and residual acidity in final digests
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2. High sample mass (up to 1.5 g) can be used
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4. Carbon related effects were not observed in ICP OES determination
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5. Low LODs were obtained
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